Source code for a computer program written in theJavaScript language. It demonstrates theappendChild method. The method adds a new child node to an existing parent node. It is commonly used to dynamically modify the structure of an HTML document.
Acomputer program in itshuman-readable form is calledsource code. Source code needs another computer program toexecute because computers can only execute their nativemachine instructions. Therefore, source code may betranslated to machine instructions using acompiler written for the language. (Assembly language programs are translated using anassembler.) The resulting file is called anexecutable. Alternatively, source code may execute within aninterpreter written for the language.[2]
If the source code is requested for execution, then the operating system loads the corresponding interpreter into memory and starts a process. The interpreter then loads the source code into memory to translate and execute eachstatement. Running the source code is slower than running anexecutable.[5][b] Moreover, the interpreter must be installed on the computer.
The"Hello, World!" program is used to illustrate a language's basicsyntax. The syntax of the languageBASIC (1964) was intentionally limited to make the language easy to learn.[6] For example,variables are notdeclared before being used.[7] Also, variables are automatically initialized to zero.[7] Here is an example computer program, in Basic, toaverage a list of numbers:[8]
10INPUT"How many numbers to average?",A20FORI=1TOA30INPUT"Enter number:",B40LETC=C+B50NEXTI60LETD=C/A70PRINT"The average is",D80END
Once the mechanics of basic computer programming are learned, more sophisticated and powerful languages are available to build large computer systems.[9]
In 1837,Jacquard's loom inspiredCharles Babbage to attempt to build theAnalytical Engine.[10]The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled. The device had astore which consisted of memory to hold 1,000 numbers of 50 decimal digits each.[11] Numbers from thestore were transferred to themill for processing. The engine was programmed using two sets of perforated cards. One set directed the operation and the other set inputted the variables.[10][12] However, the thousands of cogged wheels and gears never fully worked together.[13]
Ada Lovelace worked for Charles Babbage to create a description of the Analytical Engine (1843).[14] The description contained Note G which completely detailed a method for calculatingBernoulli numbers using the Analytical Engine. This note is recognized by some historians as the world's firstcomputer program.[13]
In 1936,Alan Turing introduced theUniversal Turing machine, a theoretical device that can model every computation.[15]It is afinite-state machine that has an infinitely long read/write tape. The machine can move the tape back and forth, changing its contents as it performs analgorithm. The machine starts in the initial state, goes through a sequence of steps, and halts when it encounters the halt state.[16] All present-day computers areTuring complete.[17]
TheElectronic Numerical Integrator And Computer (ENIAC) was built between July 1943 and Fall 1945. It was aTuring complete, general-purpose computer that used 17,468vacuum tubes to create thecircuits. At its core, it was a series ofPascalines wired together.[18] Its 40 units weighed 30 tons, occupied 1,800 square feet (167 m2), and consumed $650 per hour (in 1940s currency) in electricity when idle.[18] It had 20base-10accumulators. Programming the ENIAC took up to two months.[18] Three function tables were on wheels and needed to be rolled to fixed function panels. Function tables were connected to function panels by plugging heavy black cables intoplugboards. Each function table had 728 rotating knobs. Programming the ENIAC also involved setting some of the 3,000 switches. Debugging a program took a week.[19] It ran from 1947 until 1955 atAberdeen Proving Ground, calculating hydrogen bomb parameters, predicting weather patterns, and producing firing tables to aim artillery guns.[20]
Instead of plugging in cords and turning switches, astored-program computer loads its instructions intomemory just like it loads its data into memory.[21] As a result, the computer could be programmed quickly and perform calculations at very fast speeds.[22]Presper Eckert andJohn Mauchly built the ENIAC. The two engineers introduced thestored-program concept in a three-page memo dated February 1944.[23] Later, in September 1944,John von Neumann began working on the ENIAC project. On June 30, 1945, von Neumann published theFirst Draft of a Report on the EDVAC, which equated the structures of the computer with the structures of the human brain.[22] The design became known as thevon Neumann architecture. The architecture was simultaneously deployed in the constructions of theEDVAC andEDSAC computers in 1949.[24]
IBM planned for each model to be programmed usingPL/1.[26] A committee was formed that includedCOBOL,Fortran andALGOL programmers. The purpose was to develop a language that was comprehensive, easy to use, extendible, and would replace Cobol and Fortran.[26] The result was a large and complex language that took a long time tocompile.[27]
Switches for manual input on aData General Nova 3, manufactured in the mid-1970s
Computers manufactured until the 1970s had front-panel switches for manual programming.[28] The computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed. This process was then repeated. Computer programs also were automatically inputted viapaper tape,punched cards ormagnetic-tape. After the medium was loaded, the starting address was set via switches, and the execute button was pressed.[28]
Originally, integrated circuit chips had their function set during manufacturing. During the 1960s, controlling the electrical flow migrated to programming amatrix ofread-only memory (ROM). The matrix resembled a two-dimensional array of fuses.[29] The process to embed instructions onto the matrix was to burn out the unneeded connections.[29] There were so many connections,firmware programmers wrote acomputer program on another chip to oversee the burning.[29] The technology became known asProgrammable ROM. In 1971, Intelinstalled the computer program onto the chip and named it theIntel 4004microprocessor.[35]
IBM's System/360 (1964) CPU was not a microprocessor.
Artist's depiction of Sacramento State University's Intel 8008 microcomputer (1972)
The Intel 4004 (1971) was a 4-bit microprocessor designed to run theBusicom calculator. Five months after its release, Intel released theIntel 8008, an 8-bit microprocessor. Bill Pentz led a team atSacramento State to build the firstmicrocomputer using the Intel 8008: theSac State 8008 (1972).[37] Its purpose was to store patient medical records. The computer supported adisk operating system to run aMemorex, 3-megabyte,hard disk drive.[29] It had a color display and keyboard that was packaged in a single console. The disk operating system was programmed usingIBM's Basic Assembly Language (BAL). The medical records application was programmed using aBASIC interpreter.[29] However, the computer was an evolutionary dead-end because it was extremely expensive. Also, it was built at a public university lab for a specific purpose.[37] Nonetheless, the project contributed to the development of theIntel 8080 (1974)instruction set.[29]
In 1978, the modernsoftware development environment began when Intel upgraded theIntel 8080 to theIntel 8086. Intel simplified the Intel 8086 to manufacture the cheaperIntel 8088.[38]IBM embraced the Intel 8088 when they entered thepersonal computer market (1981). Asconsumerdemand for personal computers increased, so did Intel's microprocessor development. The succession of development is known as thex86 series. Thex86 assembly language is a family ofbackward-compatiblemachine instructions. Machine instructions created in earlier microprocessors were retained throughout microprocessor upgrades. This enabled consumers to purchase new computers without having to purchase newapplication software. The major categories of instructions are:[c]
Programming language features exist to provide building blocks to be combined to express programming ideals.[39] Ideally, a programming language should:[39]
express ideas directly in the code.
express independent ideas independently.
express relationships among ideas directly in the code.
combine ideas freely.
combine ideas only where combinations make sense.
express simple ideas simply.
Theprogramming style of a programming language to provide these building blocks may be categorized intoprogramming paradigms.[40] For example, different paradigms may differentiate:[40]
Each of these programming styles has contributed to the synthesis of differentprogramming languages.[40]
Aprogramming language is a set ofkeywords,symbols,identifiers, and rules by which programmers can communicate instructions to the computer.[41] They follow a set of rules called asyntax.[41]
Programming languages get their basis fromformal languages.[42] The purpose of defining a solution in terms of itsformal language is to generate analgorithm to solve the underlining problem.[42] Analgorithm is a sequence of simple instructions that solve a problem.[43]
Thesecond generation of programming language isassembly language.[45]Assembly language allows the programmer to usemnemonicinstructions instead of remembering instruction numbers. Anassembler translates each assembly language mnemonic into its machine language number. For example, on the PDP-11, the operation 24576 can be referenced as ADD in the source code.[46] The four basic arithmetic operations have assembly instructions like ADD, SUB, MUL, and DIV.[46] Computers also have instructions like DW (DefineWord) to reservememory cells. Then the MOV instruction can copyintegers betweenregisters and memory.
The basic structure of an assembly language statement is a label,operation,operand, and comment.[47]
Labels allow the programmer to work withvariable names. The assembler will later translate labels into physicalmemory addresses.
Operations allow the programmer to work with mnemonics. The assembler will later translate mnemonics into instruction numbers.
Operands tell the assembler which data the operation will process.
Comments allow the programmer to articulate a narrative because the instructions alone are vague.
The key characteristic of an assembly language program is it forms a one-to-one mapping to its corresponding machine language target.[48]
Thethird generation of programming language usescompilers andinterpreters to execute computer programs. The distinguishing feature of athird generation language is its independence from particular hardware.[49] Early languages includeFortran (1958),COBOL (1959),ALGOL (1960), andBASIC (1964).[45] In 1973, theC programming language emerged as ahigh-level language that produced efficient machine language instructions.[50] Whereasthird-generation languages historically generated many machine instructions for each statement,[51] C has statements that may generate a single machine instruction.[e] Moreover, anoptimizing compiler might overrule the programmer and produce fewer machine instructions than statements. Today, an entireparadigm of languages fill theimperative,third generation spectrum.
Thefourth generation of programming language emphasizes what output results are desired, rather than how programming statements should be constructed.[45]Declarative languages attempt to limitside effects and allow programmers to write code with relatively few errors.[45] One popularfourth generation language is calledStructured Query Language (SQL).[45]Database developers no longer need to process each database record one at a time. Also, a simpleselect statement can generate output records without having to understand how they are retrieved.
Adeclaration introduces avariable name to thecomputer program and assigns it to adatatype[53] – for example:var x: integer;
Anexpression yields a value – for example:2 + 2 yields 4
Astatement mightassign an expression to a variable or use the value of a variable to alter the program'scontrol flow – for example:x := 2 + 2;if x = 4 then do_something();
FORTRAN (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system". It was designed for scientific calculations, withoutstring handling facilities. Along withdeclarations,expressions, andstatements, it supported:
However, non-IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler.[54] TheAmerican National Standards Institute (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:
COBOL (1959) stands for "COmmon Business Oriented Language". Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, sostrings were introduced.[55] TheUS Department of Defense influenced COBOL's development, withGrace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal.[56]
COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, likeobject-oriented programming.[56]
ALGOL (1960) stands for "ALGOrithmic Language". It had a profound influence on programming language design.[57] Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable, structured design. Algol was first to define itssyntax using theBackus–Naur form.[57] This led tosyntax-directed compilers. It added features like:
BASIC (1964) stands for "Beginner's All-Purpose Symbolic Instruction Code". It was developed atDartmouth College for all of their students to learn.[8] If a student did not go on to a more powerful language, the student would still remember Basic.[8] A Basic interpreter was installed in themicrocomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language.[8]
Statements could be programmed by preceding them with line numbers.[f]
The 'list' command displayed the program.
The 'run' command executed the program.
However, the Basic syntax was too simple for large programs.[8] Recent dialects added structure and object-oriented extensions.Microsoft'sVisual Basic is still widely used and produces agraphical user interface.[7]
C programming language (1973) got its name because the languageBCPL was replaced withB, andAT&T Bell Labs called the next version "C". Its purpose was to write theUNIXoperating system.[50] C is a relatively small language, making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s.[50] Its growth also was because it has the facilities ofassembly language, but uses ahigh-level syntax. It added advanced features like:
Theglobal and static data region is located just above theprogram region. (The program region is technically called thetext region. It is where machine instructions are stored.)
The global and static data region is technically two regions.[58] One region is called theinitializeddata segment, where variables declared with default values are stored. The other region is called theblock started by segment, where variables declared without default values are stored.
Variables stored in theglobal and static data region have theiraddresses set at compile-time. They retain their values throughout the life of the process.
The global and static region stores theglobal variables that are declared on top of (outside) themain() function.[59] Global variables are visible tomain() and every other function in the source code.
On the other hand, variable declarations inside ofmain(), other functions, or within{}block delimiters arelocal variables. Local variables also includeformal parameter variables. Parameter variables are enclosed within the parenthesis of a function definition.[60] Parameters provide aninterface to the function.
Local variables declared using thestatic prefix are also stored in theglobal and static data region.[58] Unlike global variables, static variables are only visible within the function or block. Static variables always retain their value. An example usage would be the functionint increment_counter(){static int counter = 0; counter++; return counter;}[g]
Thestack region is a contiguous block of memory located near the top memory address.[61] Variables placed in the stack are populated from top to bottom.[h][61] Astack pointer is a special-purposeregister that keeps track of the last memory address populated.[61] Variables are placed into the stack via theassembly language PUSH instruction. Therefore, the addresses of these variables are set duringruntime. The method for stack variables to lose theirscope is via the POP instruction.
Local variables declared without thestatic prefix, including formal parameter variables,[62] are calledautomatic variables[59] and are stored in the stack.[58] They are visible inside the function or block and lose their scope upon exiting the function or block.
Theheap region is located below the stack.[58] It is populated from the bottom to the top. Theoperating system manages the heap using aheap pointer and a list of allocated memory blocks.[63] Like the stack, the addresses of heap variables are set during runtime. Anout of memory error occurs when the heap pointer and the stack pointer meet.
C provides themalloc() library function toallocate heap memory.[i][64] Populating the heap with data is an additional copy function.[j] Variables stored in the heap are economically passed to functions using pointers. Without pointers, the entire block of data would have to be passed to the function via the stack.
In the 1970s,software engineers needed language support to break large projects down intomodules.[65] One obvious feature was to decompose large projectsphysically into separatefiles. A less obvious feature was to decompose large projectslogically intoabstract data types.[65] At the time, languages supportedconcrete (scalar) datatypes likeinteger numbers,floating-point numbers, andstrings ofcharacters. Abstract datatypes arestructures of concrete datatypes, with a new name assigned. For example, alist of integers could be calledinteger_list.
In object-oriented jargon, abstract datatypes are calledclasses. However, aclass is only a definition; no memory is allocated. When memory is allocated to a class andbound to anidentifier, it is called anobject.[66]
Object-oriented languages support a syntax to modelsubset/superset relationships. Inset theory, anelement of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other people do not have.Object-oriented languages modelsubset/superset relationships usinginheritance.[69]Object-oriented programming became the dominant language paradigm by the late 1990s.[65]
C++ (1985) was originally called "C with Classes".[70] It was designed to expandC's capabilities by adding the object-oriented facilities of the languageSimula.[71]
An object-oriented module is composed of two files. The definitions file is called theheader file. Here is a C++header file for theGRADE class in a simple school application:
// grade.h// -------// Used to allow multiple source files to include// this header file without duplication errors.// ----------------------------------------------#ifndef GRADE_H#define GRADE_HclassGRADE{public:// This is the constructor operation.// ----------------------------------GRADE(constcharletter);// This is a class variable.// -------------------------charletter;// This is a member operation.// ---------------------------intgrade_numeric(constcharletter);// This is a class variable.// -------------------------intnumeric;};#endif
Aconstructor operation is a function with the same name as the class name.[72] It is executed when the calling operation executes thenew statement.
A module's other file is thesource file. Here is a C++ source file for theGRADE class in a simple school application:
// grade.cpp// ---------#include"grade.h"GRADE::GRADE(constcharletter){// Reference the object using the keyword 'this'.// ----------------------------------------------this->letter=letter;// This is Temporal Cohesion// -------------------------this->numeric=grade_numeric(letter);}intGRADE::grade_numeric(constcharletter){if((letter=='A'||letter=='a'))return4;elseif((letter=='B'||letter=='b'))return3;elseif((letter=='C'||letter=='c'))return2;elseif((letter=='D'||letter=='d'))return1;elseif((letter=='F'||letter=='f'))return0;elsereturn-1;}
Here is a C++header file for thePERSON class in a simple school application:
Here is a C++header file for theSTUDENT class in a simple school application:
// student.h// ---------#ifndef STUDENT_H#define STUDENT_H#include"person.h"#include"grade.h"// A STUDENT is a subset of PERSON.// --------------------------------classSTUDENT:publicPERSON{public:STUDENT(constchar*name);GRADE*grade;};#endif
Here is a C++source file for theSTUDENT class in a simple school application:
// student.cpp// -----------#include"student.h"#include"person.h"STUDENT::STUDENT(constchar*name):// Execute the constructor of the PERSON superclass.// -------------------------------------------------PERSON(name){// Nothing else to do.// -------------------}
Imperative languages have one major criticism: assigning an expression to anon-local variable may produce an unintendedside effect.[73]Declarative languages generally omit the assignment statement and the control flow. They describewhat computation should be performed and nothow to compute it. Two broad categories of declarative languages arefunctional languages andlogical languages.
The principle behind afunctional language is to uselambda calculus as a guide for a well definedsemantic.[74] In mathematics, a function is a rule that maps elements from anexpression to a range ofvalues. Consider the function:
times_10(x) = 10 * x
Theexpression10 * x is mapped by the functiontimes_10() to a range ofvalues. Onevalue happens to be 20. This occurs when x is 2. So, the application of the function is mathematically written as:
times_10(2) = 20
Afunctional language compiler will not store this value in a variable. Instead, it willpush the value onto the computer'sstack before setting theprogram counter back to the calling function. The calling function will thenpop the value from the stack.[75]
Imperative languages do support functions. Therefore,functional programming can be achieved in an imperative language, if the programmer uses discipline. However, afunctional language will force this discipline onto the programmer through its syntax. Functional languages have a syntax tailored to emphasize thewhat.[76]
A functional program is developed with a set of primitive functions followed by a single driver function.[73] Consider thesnippet:
function max( a, b ){/* code omitted */}
function min( a, b ){/* code omitted */}
function range( a, b, c ) {
return max( a, max( b, c ) ) - min( a, min( b, c ) );
}
The primitives aremax() andmin(). The driver function isrange(). Executing:
Lisp (1958) stands for "LISt Processor".[79] It is tailored to processlists. A full structure of the data is formed by building lists of lists. In memory, atree data structure is built. Internally, the tree structure lends nicely forrecursive functions.[80] The syntax to build a tree is to enclose the space-separatedelements within parenthesis. The following is alist of three elements. The first two elements are themselves lists of two elements:
((A B) (HELLO WORLD) 94)
Lisp has functions to extract and reconstruct elements.[81] The functionhead() returns a list containing the first element in the list. The functiontail() returns a list containing everything but the first element. The functioncons() returns a list that is the concatenation of other lists. Therefore, the following expression will return the listx:
cons(head(x), tail(x))
One drawback of Lisp is when many functions are nested, the parentheses may look confusing.[76] Modern Lispenvironments help ensure parenthesis match. As an aside, Lisp does support theimperative language operations of the assignment statement and goto loops.[82] Also,Lisp is not concerned with thedatatype of the elements at compile time.[83] Instead, it assigns (and may reassign) the datatypes atruntime. Assigning the datatype at runtime is calleddynamic binding.[84] Whereas dynamic binding increases the language's flexibility, programming errors may linger until late in thesoftware development process.[84]
Writing large, reliable, and readable Lisp programs requires forethought. If properly planned, the program may be much shorter than an equivalentimperative language program.[76]Lisp is widely used inartificial intelligence. However, its usage has been accepted only because it hasimperative language operations, making unintended side-effects possible.[78]
ML (1973)[85] stands for "Meta Language". ML checks to make sure only data of the same type are compared with one another.[86] For example, this function has one input parameter (an integer) and returns an integer:
funtimes_10(n:int):int=10*n;
ML is not parenthesis-eccentric likeLisp. The following is an application oftimes_10():
times_10 2
It returns "20 : int". (Both the results and the datatype are returned.)
LikeLisp,ML is tailored to process lists. UnlikeLisp, each element is the same datatype.[87] Moreover,ML assigns the datatype of an element atcompile-time. Assigning the datatype at compile-time is calledstatic binding. Static binding increases reliability because the compiler checks the context of variables before they are used.[88]
The building blocks of a Prolog program arefacts andrules. Here is a simple example:
cat(tom).% tom is a catmouse(jerry).% jerry is a mouseanimal(X):-cat(X).% each cat is an animalanimal(X):-mouse(X).% each mouse is an animalbig(X):-cat(X).% each cat is bigsmall(X):-mouse(X).% each mouse is smalleat(X,Y):-mouse(X),cheese(Y).% each mouse eats each cheeseeat(X,Y):-big(X),small(Y).% each big animal eats each small animal
After all the facts and rules are entered, then a question can be asked:
Will Tom eat Jerry?
?-eat(tom,jerry).true
The following example shows how Prolog will convert a letter grade to its numeric value:
Rule (2) is arecursive (inductive) definition. It can be understood declaratively, without the need to understand how it is executed.
Rule (3) shows howfunctions are represented by using relations. Here, the mother and father functions ensure that every individual has only one mother and only one father.
Prolog is an untyped language. Nonetheless,inheritance can be represented by using predicates. Rule (4) asserts that a creature is a superclass of a dragon.
Object-oriented programming is a programming method to executeoperations (functions) onobjects.[91] The basic idea is to group the characteristics of aphenomenon into an objectcontainer and give the container a name. Theoperations on the phenomenon are also grouped into the container.[91]Object-oriented programming developed by combining the need for containers and the need for safefunctional programming.[92] This programming method need not be confined to anobject-oriented language.[93] In an object-oriented language, an object container is called aclass. In a non-object-oriented language, adata structure (which is also known as arecord) may become an object container. To turn a data structure into an object container, operations need to be written specifically for the structure. The resulting structure is called anabstract datatype.[94] However,inheritance will be missing. Nonetheless, this shortcoming can be overcome.
Here is aC programming languageheader file for theGRADE abstract datatype in a simple school application:
/* grade.h *//* ------- *//* Used to allow multiple source files to include *//* this header file without duplication errors. *//* ---------------------------------------------- */#ifndef GRADE_H#define GRADE_Htypedefstruct{charletter;}GRADE;/* Constructor *//* ----------- */GRADE*grade_new(charletter);intgrade_numeric(charletter);#endif
Thegrade_new() function performs the same algorithm as the C++constructor operation.
Here is a C programming languagesource file for theGRADE abstract datatype in a simple school application:
/* grade.c *//* ------- */#include"grade.h"GRADE*grade_new(charletter){GRADE*grade;/* Allocate heap memory *//* -------------------- */if(!(grade=calloc(1,sizeof(GRADE)))){fprintf(stderr,"ERROR in %s/%s/%d: calloc() returned empty.\n",__FILE__,__FUNCTION__,__LINE__);exit(1);}grade->letter=letter;returngrade;}intgrade_numeric(charletter){if((letter=='A'||letter=='a'))return4;elseif((letter=='B'||letter=='b'))return3;elseif((letter=='C'||letter=='c'))return2;elseif((letter=='D'||letter=='d'))return1;elseif((letter=='F'||letter=='f'))return0;elsereturn-1;}
In the constructor, the functioncalloc() is used instead ofmalloc() because each memory cell will be set to zero.
Here is a C programming languageheader file for thePERSON abstract datatype in a simple school application:
Here is a C programming languagesource file for thePERSON abstract datatype in a simple school application:
/* person.c *//* -------- */#include"person.h"PERSON*person_new(char*name){PERSON*person;if(!(person=calloc(1,sizeof(PERSON)))){fprintf(stderr,"ERROR in %s/%s/%d: calloc() returned empty.\n",__FILE__,__FUNCTION__,__LINE__);exit(1);}person->name=name;returnperson;}
Here is a C programming languageheader file for theSTUDENT abstract datatype in a simple school application:
/* student.h *//* --------- */#ifndef STUDENT_H#define STUDENT_H#include"person.h"#include"grade.h"typedefstruct{/* A STUDENT is a subset of PERSON. *//* -------------------------------- */PERSON*person;GRADE*grade;}STUDENT;/* Constructor *//* ----------- */STUDENT*student_new(char*name);#endif
Here is a C programming languagesource file for theSTUDENT abstract datatype in a simple school application:
/* student.c *//* --------- */#include"student.h"#include"person.h"STUDENT*student_new(char*name){STUDENT*student;if(!(student=calloc(1,sizeof(STUDENT)))){fprintf(stderr,"ERROR in %s/%s/%d: calloc() returned empty.\n",__FILE__,__FUNCTION__,__LINE__);exit(1);}/* Execute the constructor of the PERSON superclass. *//* ------------------------------------------------- */student->person=person_new(name);returnstudent;}
Here is a driver program for demonstration:
/* student_dvr.c *//* ------------- */#include<stdio.h>#include"student.h"intmain(void){STUDENT*student=student_new("The Student");student->grade=grade_new('a');printf("%s: Numeric grade = %d\n",/* Whereas a subset exists, inheritance does not. */student->person->name,/* Functional programming is executing functions just-in-time (JIT) */grade_numeric(student->grade->letter));return0;}
Production rules consist of a set of terminals and non-terminals.
Thesyntax of acomputer program is alist ofproduction rules which form itsgrammar.[96] A programming language's grammar correctly places itsdeclarations,expressions, andstatements.[97] Complementing thesyntax of a language are itssemantics. Thesemantics describe the meanings attached to various syntactic constructs.[98] A syntactic construct may need a semantic description because a production rule may have an invalid interpretation.[99] Also, different languages might have the same syntax; however, their behaviors may be different.
The syntax of a language is formally described by listing the production rules. Whereas the syntax of anatural language is extremely complicated, a subset of the English language can have this production rule listing:[100]
asentence is made up of anoun-phrase followed by averb-phrase;
anoun-phrase is made up of anarticle followed by anadjective followed by anoun;
averb-phrase is made up of averb followed by anoun-phrase;
anarticle is 'the';
anadjective is 'big' or
anadjective is 'small';
anoun is 'cat' or
anoun is 'mouse';
averb is 'eats';
The words inbold-face are known asnon-terminals. The words in 'single quotes' are known asterminals.[101]
From this production rule listing, complete sentences may be formed using a series of replacements.[102] The process is to replacenon-terminals with either a validnon-terminal or a validterminal. The replacement process repeats until onlyterminals remain. One valid sentence is:
sentence
noun-phraseverb-phrase
articleadjectivenounverb-phrase
theadjectivenounverb-phrase
thebignounverb-phrase
thebigcatverb-phrase
thebigcatverbnoun-phrase
thebigcateatsnoun-phrase
thebigcateatsarticleadjectivenoun
thebigcateatstheadjectivenoun
thebigcateatsthesmallnoun
thebigcateatsthesmallmouse
However, another combination results in an invalid sentence:
thesmallmouseeatsthebigcat
Therefore, asemantic is necessary to correctly describe the meaning of aneat activity.
Oneproduction rule listing method is called theBackus–Naur form (BNF).[103] BNF describes the syntax of a language and itself has asyntax. This recursive definition is an example of ameta-language.[98] Thesyntax of BNF includes:
::= which translates tois made up of a[n] when a non-terminal is to its right. It translates tois when a terminal is to its right.
| which translates toor.
< and> which surroundnon-terminals.
Using BNF, a subset of the English language can have thisproduction rule listing:
<sentence>::=<noun-phrase><verb-phrase><noun-phrase>::=<article><adjective><noun><verb-phrase>::=<verb><noun-phrase><article>::= the<adjective>::= big | small<noun>::= cat | mouse<verb>::= eats
Using BNF, a signed-integer has theproduction rule listing:[104]
Prior to programming languages,Betty Jennings andFran Bilas programmed theENIAC by moving cables and setting switches.
Software engineering is a variety of techniques to producequalitycomputer programs.[106]Computer programming is the process of writing or editingsource code. In a formal environment, asystems analyst will gather information from managers about all the organization's processes to automate. This professional then prepares adetailed plan for the new or modified system.[107] The plan is analogous to an architect's blueprint.[107]
The systems analyst has the objective to deliver the right information to the right person at the right time.[108] The critical factors to achieve this objective are:[108]
The quality of the output. Is the output useful for decision-making?
The accuracy of the output. Does it reflect the true situation?
The format of the output. Is the output easily understood?
The speed of the output. Time-sensitive information is important when communicating with the customer in real time.
Achieving performance objectives should be balanced with all of the costs, including:[109]
Development costs.
Uniqueness costs. A reusable system may be expensive. However, it might be preferred over a limited-use system.
Hardware costs.
Operating costs.
Applying asystems development process will mitigate the axiom: the later in the process an error is detected, the more expensive it is to correct.[110]
Theimplementation phase is to program the best solution.
Themaintenance phase lasts throughout the life of the system. Changes to the system after it is deployed may be necessary.[113] Faults may exist, including specification faults, design faults, or coding faults. Improvements may be necessary. Adaption may be necessary to react to a changing environment.
Acomputer programmer is a specialist responsible for writing or modifying the source code to implement the detailed plan.[107] A programming team is likely to be needed because most systems are too large to be completed by a single programmer.[114] However, adding programmers to a project may not shorten the completion time. Instead, it may lower the quality of the system.[114] To be effective, program modules need to be defined and distributed to team members.[114] Also, team members must interact with one another in a meaningful and effective way.[114]
Computer programmers may beprogramming in the small: programming within a single module.[115] Chances are a module will execute modules located in other source code files. Therefore, computer programmers may beprogramming in the large: programming modules so they will effectively couple with each other.[115] Programming-in-the-large includes contributing to theapplication programming interface (API).
Modular programming is a technique to refineimperative language programs. Refined programs may reduce the software size, separate responsibilities, and thereby mitigatesoftware aging. Aprogram module is a sequence of statements that are bounded within ablock and together identified by a name.[116] Modules have afunction,context, andlogic:[117]
Thefunction of a module is what it does.
Thecontext of a module are the elements being performed upon.
Thelogic of a module is how it performs the function.
The module's name should be derived first by itsfunction, then by itscontext. Itslogic should not be part of the name.[117] For example,function compute_square_root( x ) orfunction compute_square_root_integer( i : integer ) are appropriate module names. However,function compute_square_root_by_division( x ) is not.
The degree of interactionwithin a module is its level ofcohesion.[117]Cohesion is a judgment of the relationship between a module's name and itsfunction. The degree of interactionbetween modules is the level ofcoupling.[118]Coupling is a judgement of the relationship between a module'scontext and the elements being performed upon.
The levels of cohesion from worst to best are:[119]
Coincidental Cohesion: A module has coincidental cohesion if it performs multiple functions, and the functions are completely unrelated. For example,function read_sales_record_print_next_line_convert_to_float(). Coincidental cohesion occurs in practice if management enforces silly rules. For example, "Every module will have between 35 and 50 executable statements."[119]
Logical Cohesion: A module has logical cohesion if it has available a series of functions, but only one of them is executed. For example,function perform_arithmetic( perform_addition, a, b ).
Temporal Cohesion: A module has temporal cohesion if it performs functions related to time. One example,function initialize_variables_and_open_files(). Another example,stage_one(),stage_two(), ...
Procedural Cohesion: A module has procedural cohesion if it performs multiple loosely related functions. For example,function read_part_number_update_employee_record().
Communicational Cohesion: A module has communicational cohesion if it performs multiple closely related functions. For example,function read_part_number_update_sales_record().
Informational Cohesion: A module has informational cohesion if it performs multiple functions, but each function has its own entry and exit points. Moreover, the functions share the same data structure. Object-oriented classes work at this level.
Functional Cohesion: a module has functional cohesion if it achieves a single goal working only on local variables. Moreover, it may be reusable in other contexts.
The levels of coupling from worst to best are:[118]
Content Coupling: A module has content coupling if it modifies a local variable of another function. COBOL used to do this with thealter verb.
Common Coupling: A module has common coupling if it modifies a global variable.
Control Coupling: A module has control coupling if another module can modify itscontrol flow. For example,perform_arithmetic( perform_addition, a, b ). Instead, control should be on the makeup of the returned object.
Stamp Coupling: A module has stamp coupling if an element of a data structure passed as a parameter is modified. Object-oriented classes work at this level.
Data Coupling: A module has data coupling if all of its input parameters are needed and none of them are modified. Moreover, the result of the function is returned as a single object.
Data flow analysis is a design method used to achieve modules offunctional cohesion anddata coupling.[120] The input to the method is adata-flow diagram. A data-flow diagram is a set of ovals representing modules. Each module's name is displayed inside its oval. Modules may be at the executable level or the function level.
The diagram also has arrows connecting modules to each other. Arrows pointing into modules represent a set of inputs. Each module should have only one arrow pointing out from it to represent its single output object. (Optionally, an additional exception arrow points out.) Adaisy chain of ovals will convey an entirealgorithm. The input modules should start the diagram. The input modules should connect to the transform modules. The transform modules should connect to the output modules.[121]
Computer programs may be categorized along functional lines. The main functional categories areapplication software andsystem software. System software includes theoperating system, which couplescomputer hardware with application software.[122] The purpose of the operating system is to provide an environment where application software executes in a convenient and efficient manner.[122] Both application software and system software executeutility programs. At the hardware level, amicrocode program controls the circuits throughout thecentral processing unit.
Enterprise applications may be developed in-house as a one-of-a-kindproprietary software.[124] Alternatively, they may be purchased asoff-the-shelf software. Purchased software may be modified to providecustom software. If the application is customized, then either the company's resources are used or the resources are outsourced. Outsourced software development may be from the original software vendor or a third-party developer.[125]
The potential advantages of in-house software are features and reports may be developed exactly to specification.[126] Management may also be involved in the development process and offer a level of control.[127] Management may decide to counteract a competitor's new initiative or implement a customer or vendor requirement.[128] A merger or acquisition may necessitate enterprise software changes. The potential disadvantages of in-house software are time and resource costs may be extensive.[124] Furthermore, risks concerning features and performance may be looming.
The potential advantages of off-the-shelf software are upfront costs are identifiable, the basic needs should be fulfilled, and its performance and reliability have a track record.[124] The potential disadvantages of off-the-shelf software are it may have unnecessary features that confuse end users, it may lack features the enterprise needs, and the data flow may not match the enterprise's work processes.[124]
One approach to economically obtaining a customized enterprise application is through anapplication service provider.[129] Specialty companies provide hardware, custom software, and end-user support. They may speed the development of new applications because they possess skilled information system staff. The biggest advantage is it frees in-house resources from staffing and managing complex computer projects.[129] Many application service providers target small, fast-growing companies with limited information system resources.[129] On the other hand, larger companies with major systems will likely have their technical infrastructure in place. One risk is having to trust an external organization with sensitive information. Another risk is having to trust the provider's infrastructure reliability.[129]
In the 1950s, the programmer, who was also the operator, would write a program and run it. After the program finished executing, the output may have been printed, or it may have been punched onto paper tape or cards for later processing.[28] More often than not the program did not work. The programmer then looked at the console lights and fiddled with the console switches. If less fortunate, a memory printout was made for further study. In the 1960s, programmers reduced the amount of wasted time by automating the operator's job. A program called anoperating system was kept in the computer at all times.[130]
When the kernel initiallyloads an executable into memory, it divides the address space logically intoregions.[133] The kernel maintains a master-region table and many per-process-region (pregion) tables—one for each runningprocess.[133] These tables constitute thevirtual address space. The master-region table is used to determine where its contents are located inphysical memory. The pregion tables allow each process to have its own program (text) pregion, data pregion, and stack pregion.
The program pregion stores machine instructions. Since machine instructions do not change, the program pregion may be shared by many processes of the same executable.[133]
To save time and memory, the kernel may load only blocks of execution instructions from the disk drive, not the entire execution file completely.[132]
The kernel allocates memory from theheap upon request by a process.[64] When the process is finished with the memory, the process may request for it to befreed. If the process exits without requesting all allocated memory to be freed, then the kernel performsgarbage collection to free the memory.
The kernel also ensures that a process only accesses its own memory, and not that of the kernel or other processes.[132]
The kernel program should performfile system management.[132] The kernel has instructions to create, retrieve, update, and delete files.
The kernel program should performdevice management.[132] The kernel provides programs to standardize and simplify the interface to the mouse, keyboard, disk drives, printers, and other devices. Moreover, the kernel should arbitrate access to a device if two processes request it at the same time.
The kernel program should performnetwork management.[136] The kernel transmits and receivespackets on behalf of processes. One key service is to find an efficientroute to the target system.
Programmers access files through a relatively simple interface that in turn executes a relatively complicated low-level I/O interface. The low-level interface includes file creation,file descriptors, file seeking, physical reading, and physical writing.
Programmers create processes through a relatively simple interface that in turn executes a relatively complicated low-level interface.
Programmers perform date/time arithmetic through a relatively simple interface that in turn executes a relatively complicated low-level time interface.[138]
The kernel program should provide acommunication channel between executing processes.[139] For a large software system, it may be desirable toengineer the system into smaller processes. Processes may communicate with one another by sending and receivingsignals.
Originally, operating systems were programmed inassembly; however, modern operating systems are typically written in higher-level languages likeC,Objective-C, andSwift.[k]
Autility program is designed to aid system administration and software execution. Operating systems execute hardware utility programs to check the status of disk drives, memory, speakers, and printers.[140] A utility program may optimize the placement of a file on a crowded disk. System utility programs monitor hardware and network performance. When a metric is outside an acceptable range, a trigger alert is generated.[141]
Utility programs include compression programs so data files are stored on less disk space.[140] Compressed programs also save time when data files are transmitted over the network.[140] Utility programs can sort and merge data sets.[141] Utility programs detectcomputer viruses.[141]
Connecting two transistors in series forms theNAND gate.
Connecting two transistors in parallel forms theNOR gate.
Connecting a NOT gate to a NAND gate forms theAND gate.
Connecting a NOT gate to a NOR gate forms theOR gate.
These five gates form the building blocks ofbinary algebra—the digital logic functions of the computer.
Microcode instructions aremnemonics programmers may use to execute digital logic functions instead of forming them in binary algebra. They are stored in acentral processing unit's (CPU)control store.[146]These hardware-level instructions move data throughout thedata path.
The micro-instruction cycle begins when themicrosequencer uses its microprogram counter tofetch the nextmachine instruction fromrandom-access memory.[147] The next step is todecode the machine instruction by selecting the proper output line to the hardware module.[148]The final step is toexecute the instruction using the hardware module's set of gates.
A symbolic representation of an ALU
Instructions to perform arithmetic are passed through anarithmetic logic unit (ALU).[149] The ALU has circuits to perform elementary operations to add, shift, and compare integers. By combining and looping the elementary operations through the ALU, the CPU performs its complex arithmetic.
^Operators likex++ will usually compile to a single instruction.
^The line numbers were typically incremented by 10 to leave room if additional statements were added later.
^This function could be written more concisely asint increment_counter(){ static int counter; return ++counter;}. 1) Static variables are automatically initialized to zero. 2)++counter is a prefixincrement operator.
^This is despite the metaphor of astack, which normally grows from bottom to top.
^C also provides thecalloc() function to allocate heap memory. It provides two additional services: 1) It allows the programmer to create anarray of arbitrary size. 2) It sets eachmemory cell to zero.
^Forstring variables,C provides thestrdup() function. It executes both the allocation function and the copy function.
^TheUNIX operating system was written in C,macOS was written in Objective-C, and Swift replaced Objective-C.
^"ISO/IEC 2382:2015".ISO. 2020-09-03.Archived from the original on 2016-06-17. Retrieved2022-05-26.[Software includes] all or part of the programs, procedures, rules, and associated documentation of an information processing system.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 7.ISBN0-201-71012-9.An alternative to compiling a source program is to use an interpreter. An interpreter can directly execute a source program[.]
^Silberschatz, Abraham (1994).Operating System Concepts, Fourth Edition. Addison-Wesley. p. 98.ISBN978-0-201-50480-4.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 7.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 30.ISBN0-201-71012-9.Their intention was to produce a language that was very simple for students to learn[.]
^abcWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 31.ISBN0-201-71012-9.
^abcdefWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 30.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 30.ISBN0-201-71012-9.The idea was that students could be merely casual users or go on from Basic to more sophisticated and powerful languages[.]
^J. Fuegi; J. Francis (October–December 2003), "Lovelace & Babbage and the creation of the 1843 'notes'",Annals of the History of Computing,25 (4): 16, 19, 25,doi:10.1109/MAHC.2003.1253887
^Linz, Peter (1990).An Introduction to Formal Languages and Automata. D. C. Heath and Company. p. 234.ISBN978-0-669-17342-0.
^Linz, Peter (1990).An Introduction to Formal Languages and Automata. D. C. Heath and Company. p. 243.ISBN978-0-669-17342-0.[A]ll the common mathematical functions, no matter how complicated, are Turing-computable.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 26.ISBN0-201-71012-9.
^abcdWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 37.ISBN0-201-71012-9.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 160.ISBN0-619-06489-7.With third-generation and higher-level programming languages, each statement in the language translates into several instructions in machine language.
^Wilson, Leslie B. (1993).Comparative Programming Languages, Second Edition. Addison-Wesley. p. 75.ISBN978-0-201-56885-1.
^Stroustrup, Bjarne (2013).The C++ Programming Language, Fourth Edition. Addison-Wesley. p. 40.ISBN978-0-321-56384-2.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 16.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 24.ISBN0-201-71012-9.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 25.ISBN0-201-71012-9.
^abcdWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 19.ISBN0-201-71012-9.
^abKernighan, Brian W.; Ritchie, Dennis M. (1988).The C Programming Language Second Edition. Prentice Hall. p. 31.ISBN0-13-110362-8.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 128.ISBN0-201-71012-9.
^abcKerrisk, Michael (2010).The Linux Programming Interface. No Starch Press. p. 121.ISBN978-1-59327-220-3.
^Kerrisk, Michael (2010).The Linux Programming Interface. No Starch Press. p. 122.ISBN978-1-59327-220-3.
^Kernighan, Brian W.; Ritchie, Dennis M. (1988).The C Programming Language Second Edition. Prentice Hall. p. 185.ISBN0-13-110362-8.
^abKernighan, Brian W.; Ritchie, Dennis M. (1988).The C Programming Language Second Edition. Prentice Hall. p. 187.ISBN0-13-110362-8.
^abcWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 38.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 193.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 39.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 35.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 192.ISBN0-201-71012-9.
^Stroustrup, Bjarne (2013).The C++ Programming Language, Fourth Edition. Addison-Wesley. p. 22.ISBN978-0-321-56384-2.
^Stroustrup, Bjarne (2013).The C++ Programming Language, Fourth Edition. Addison-Wesley. p. 21.ISBN978-0-321-56384-2.
^Stroustrup, Bjarne (2013).The C++ Programming Language, Fourth Edition. Addison-Wesley. p. 49.ISBN978-0-321-56384-2.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 218.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 217.ISBN0-201-71012-9.
^Weiss, Mark Allen (1994).Data Structures and Algorithm Analysis in C++. Benjamin/Cummings Publishing Company, Inc. p. 103.ISBN0-8053-5443-3.When there is a function call, all the important information needs to be saved, such as register values (corresponding to variable names) and the return address (which can be obtained from the program counter)[.] ... When the function wants to return, it ... restores all the registers. It then makes the return jump. Clearly, all of this work can be done using a stack, and that is exactly what happens in virtually every programming language that implements recursion.
^abcWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 230.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 240.ISBN0-201-71012-9.
^abcWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 241.ISBN0-201-71012-9.
^Jones, Robin; Maynard, Clive; Stewart, Ian (December 6, 2012).The Art of Lisp Programming. Springer Science & Business Media. p. 2.ISBN9781447117193.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 220.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 221.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 229.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 227.ISBN0-201-71012-9.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 222.ISBN0-201-71012-9.
^Kowalski, R., Dávila, J., Sartor, G. and Calejo, M., 2023. Logical English for law and education. In Prolog: The Next 50 Years (pp. 287-299). Cham: Springer Nature Switzerland.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 35.ISBN0-201-71012-9.Simula was based on Algol 60 with one very important addition — the class concept. ... The basic idea was that the data (or data structure) and the operations performed on it belong together[.]
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 39.ISBN0-201-71012-9.Originally, a large number of experimental languages were designed, many of which combined object-oriented and functional programming.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 284.ISBN0-256-08515-3.While it is true that OOD [(object oriented design)] as such is not supported by the majority of popular languages, a large subset of OOD can be used.
^Weiss, Mark Allen (1994).Data Structures and Algorithm Analysis in C++. Benjamin/Cummings Publishing Company, Inc. p. 57.ISBN0-8053-5443-3.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 285.ISBN0-256-08515-3.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 290.ISBN0-201-71012-9.The syntax (or grammar) of a programming language describes the correct form in which programs may be written[.]
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 78.ISBN0-201-71012-9.The main components of an imperative language are declarations, expressions, and statements.
^abWilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 290.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 294.ISBN0-201-71012-9.
^Wilson, Leslie B. (2001).Comparative Programming Languages, Third Edition. Addison-Wesley. p. 297.ISBN0-201-71012-9.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. Preface.ISBN0-256-08515-3.
^abcStair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 507.ISBN0-619-06489-7.
^abStair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 513.ISBN0-619-06489-7.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 514.ISBN0-619-06489-7.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 516.ISBN0-619-06489-7.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 8.ISBN0-256-08515-3.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 517.ISBN0-619-06489-7.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 345.ISBN0-256-08515-3.
^abcdSchach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 319.ISBN0-256-08515-3.
^abSchach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 331.ISBN0-256-08515-3.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 216.ISBN0-256-08515-3.
^abcSchach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 219.ISBN0-256-08515-3.
^abSchach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 226.ISBN0-256-08515-3.
^abSchach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 220.ISBN0-256-08515-3.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 258.ISBN0-256-08515-3.
^Schach, Stephen R. (1990).Software Engineering. Aksen Associates Incorporated Publishers. p. 259.ISBN0-256-08515-3.
^abcSilberschatz, Abraham (1994).Operating System Concepts, Fourth Edition. Addison-Wesley. p. 1.ISBN978-0-201-50480-4.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 147.ISBN0-619-06489-7.The key to unlocking the potential of any computer system is application software.
^abcdStair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 147.ISBN0-619-06489-7.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 147.ISBN0-619-06489-7.[A] third-party software firm, often called a value-added software vendor, may develop or modify a software program to meet the needs of a particular industry or company.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 148.ISBN0-619-06489-7.Heading: Proprietary Software; Subheading: Advantages; Quote: You can get exactly what you need in terms of features, reports, and so on.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 148.ISBN0-619-06489-7.Heading: Proprietary Software; Subheading: Advantages; Quote: Being involved in the development offers a further level of control over the results.
^Stair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 147.ISBN0-619-06489-7.Heading: Proprietary Software; Subheading: Advantages; Quote: There is more flexibility in making modifications that may be required to counteract a new initiative by one of your competitors or to meet new supplier and/or customer requirements.
^abcdStair, Ralph M. (2003).Principles of Information Systems, Sixth Edition. Thomson. p. 149.ISBN0-619-06489-7.
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