| ALGOL | |
|---|---|
 A 1965 manual for ALGOL-20 | |
| Paradigm | Procedural,imperative,structured |
| Family | ALGOL |
| Designed by | Bauer,Bottenbruch,Rutishauser,Samelson,Backus,Katz,Perlis,Wegstein,Naur,Vauquois,van Wijngaarden,Woodger,Green,McCarthy |
| First appeared | 1958; 68 years ago (1958) |
| Typing discipline | Static,strong |
| Scope | Lexical |
| Influenced | |
| Most subsequent imperative languages (including so-calledALGOL-like languages) e.g.PL/I,Simula,Pascal,C andScheme | |
ALGOL (/ˈælɡɒl,-ɡɔːl/; short for "Algorithmic Language")[1] is a family ofimperative computerprogramming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method foralgorithm description used by theAssociation for Computing Machinery (ACM) in textbooks and academic sources for more than thirty years.[2]
In the sense that thesyntax of most modern languages is "Algol-like",[3] it was arguably more influential than three other high-level programming languages among which it was roughly contemporary:FORTRAN,Lisp, andCOBOL.[4] It was designed to avoid some of the perceived problems with FORTRAN and eventually gave rise to many other programming languages, includingPL/I,Simula,BCPL,B,Pascal,Ada, andC.
ALGOL introducedcode blocks and thebegin...end pairs for delimiting them. It was also the first language implementingnested function definitions withlexical scope. Moreover, it was the first programming language to give detailed attention to formal language definition and, through theAlgol 60 Report, it introducedBackus–Naur form, a principalformal grammar notation for language design.
There were three major specifications, named after the years they were first published:
ALGOL 68 is substantially different from ALGOL 60 and was not well received,[according to whom?] so reference to "Algol" is generally understood to mean ALGOL 60 and its dialects.[citation needed]
ALGOL was developed jointly by a committee of European and American computer scientists in a meeting in 1958 at theSwiss Federal Institute of Technology in Zurich (cf.ALGOL 58).[9] It specified three different syntaxes: a reference syntax, a publication syntax, and an implementation syntax, syntaxes that permitted it to use different keyword names and conventions for decimal points (commas vs periods) for different languages.[5]
ALGOL was used mostly by research computer scientists in the United States and in Europe; commercial applications were hindered by the absence of standardinput/output facilities in its description, and the lack of interest in the language by large computer vendors (other thanBurroughs Corporation).[10] ALGOL 60 did however become the standard for the publication of algorithms and had a profound effect on future language development.[10]
John Backus developed theBackus normal form method of describing programming languages specifically for ALGOL 58. It was revised and expanded byPeter Naur for ALGOL 60, and atDonald Knuth's suggestion renamedBackus–Naur form.[11]
Peter Naur: "As editor of theALGOL Bulletin I was drawn into the international discussions of the language and was selected to be member of the European language design group in November 1959. In this capacity I was the editor of the ALGOL 60 report, produced as the result of the ALGOL 60 meeting in Paris in January 1960."[12]
The following people attended the meeting in Paris (from 11 to 16 January):[5]
Alan Perlis gave a vivid description of the meeting: "The meetings were exhausting, interminable, and exhilarating. One became aggravated when one's good ideas were discarded along with the bad ones of others. Nevertheless, diligence persisted during the entire period. The chemistry of the 13 was excellent."[13]
A significant contribution of the ALGOL 58 Report was to provide standard terms for programming concepts: statement, declaration, type, label, primary, block, and others.[10]
ALGOL 60 inspired many languages that followed it.Tony Hoare remarked: "Here is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all its successors."[14] TheScheme programming language, a variant ofLisp that adopted the block structure and lexical scope of ALGOL, also adopted the wording "Revised Report on the Algorithmic Language Scheme" for its standards documents in homage to ALGOL.[15]
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ALGOL 60 as officially defined had noI/O facilities; implementations defined their own in ways that were rarely compatible with each other. In contrast, ALGOL 68 offered an extensive library oftransput (input/output) facilities.
ALGOL 60 allowed for twoevaluation strategies forparameter passing: the commoncall-by-value, andcall-by-name. Call-by-name has certain effects in contrast tocall-by-reference. For example, without specifying the parameters asvalue orreference, it is impossible to develop a procedure that will swap the values of two parameters if the actual parameters that are passed in are an integer variable and an array that is indexed by that same integer variable.[16] Think of passing a pointer to swap(i, A[i]) in to a function. Now that every time swap is referenced, it is reevaluated. Say i := 1 and A[i] := 2, so every time swap is referenced it will return the other combination of the values ([1,2], [2,1], [1,2] and so on). A similar situation occurs with a random function passed as actual argument.
Call-by-name is known by many compiler designers for the interesting "thunks" that are used to implement it.Donald Knuth devised the "man or boy test" to separate compilers that correctly implemented "recursion and non-local references." This test contains an example of call-by-name.
ALGOL 68 was defined using a two-level grammar formalism invented byAdriaan van Wijngaarden and which bears his name.Van Wijngaarden grammars use acontext-free grammar to generate an infinite set of productions that will recognize a particular ALGOL 68 program; notably, they are able to express the kind of requirements that in many other programming language standards are labelled "semantics" and have to be expressed in ambiguity-prone natural language prose, and then implemented in compilers asad hoc code attached to the formal language parser.
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(The way the bold text has to be written depends on the implementation, e.g. 'INTEGER'—quotation marks included—for integer. This is known asstropping.)
procedure Absmax(a) Size:(n, m) Result:(y) Subscripts:(i, k);value n, m;array a;integer n, m, i, k;real y;comment The absolute greatest element of the matrix a, of size n by m, is copied to y, and the subscripts of this element to i and k;begininteger p, q; y := 0; i := k := 1;for p := 1step 1until ndofor q := 1step 1until mdoif abs(a[p, q]) > ythenbegin y := abs(a[p, q]); i := p; k := qendend Absmax
Here is an example of how to produce atable using Elliott 803 ALGOL.[17]
FLOATING POINT ALGOL TEST' BEGIN REAL A,B,C,D' READ D' FOR A:= 0.0 STEP D UNTIL 6.3 DO BEGIN PRINTPUNCH(3),££L??' B := SIN(A)' C := COS(A)' PRINT PUNCH(3),SAMELINE,ALIGNED(1,6),A,B,C' END END'
The following code samples are ALGOL 68 versions of the above ALGOL 60 code samples.
ALGOL 68 implementations used ALGOL 60's approaches tostropping. In ALGOL 68's case tokens with the bold typeface are reserved words, types (modes) or operators.
proc abs max = ([,]real a,refreal y,refint i, k)real:comment The absolute greatest element of the matrix a, of size ⌈a by 2⌈ais transferred to y, and the subscripts of this element to i and k;commentbeginreal y := 0; i := ⌊a; k := 2⌊a;for pfrom ⌊ato ⌈adofor qfrom 2⌊ato 2⌈adoifabs a[p, q] > ythen y :=abs a[p, q]; i := p; k := qfiodod; yend # abs max #
Note: lower (⌊) and upper (⌈) bounds of an array, and array slicing, are directly available to the programmer.
floating point algol68 test:(real a,b,c,d; #printf – sends output to thefilestand out. # #printf($p$); – selects anew page # printf(($pg$,"Enter d:")); read(d); for stepfrom 0while a:=step*d; a <= 2*pido printf($l$); #$l$ - selects anew line. # b := sin(a); c := cos(a); printf(($z-d.6d$,a,b,c)) # formats output with 1 digit before and 6 after the decimal point. #od)
The variations and lack of portability of the programs from one implementation to another is easily demonstrated by the classichello world program.[citation needed]
ALGOL 58 had no I/O facilities.
Since ALGOL 60 had no I/O facilities, there is no portablehello world program in ALGOL.The next three examples are in Burroughs Extended Algol. The first two direct output at the interactive terminal they are run on. The first uses a character array, similar to C. The language allows the array identifier to be used as a pointer to the array, and hence in a REPLACE statement.
BEGINFILEF(KIND=REMOTE);EBCDICARRAYE[0:11];REPLACEEBY"HELLO WORLD!";WRITE(F,*,E);END.
A simpler program using an inline format:
BEGINFILEF(KIND=REMOTE);WRITE(F,<"HELLO WORLD!">);END.
An even simpler program using the Display statement. Note that its output would end up at the system console ('SPO'):
BEGINDISPLAY("HELLO WORLD!")END.
An alternative example, using Elliott Algol I/O is as follows. Elliott Algol used different characters for "open-string-quote" and "close-string-quote", represented here by ‘ and ’ .
programHiFolks;beginprint‘Helloworld’end;
Below is a version from Elliott 803 Algol (A104). The standard Elliott 803 used five-hole paper tape and thus only had upper case. The code lacked any quote characters so £ (UK Pound Sign) was used for open quote and ? (Question Mark) for close quote. Special sequences were placed in double quotes (e.g£. £L?? produced a new line on the teleprinter).
HIFOLKS' BEGIN PRINT £HELLO WORLD£L??' END'
TheICT 1900 series Algol I/O version allowed input from paper tape or punched card. Paper tape 'full' mode allowed lower case. Output was to a line printer. The open and close quote characters were represented using '(' and ')' and spaces by %.[18]
'BEGIN' WRITE TEXT('('HELLO%WORLD')'); 'END'ALGOL 68 code was published with reserved words typically in lowercase, but bolded or underlined.
begin printf(($gl$,"Hello, world!"))end
In the language of the "Algol 68 Report" theinput/output facilities were collectively called the "Transput".
The ALGOLs were conceived at a time when character sets were diverse and evolving rapidly; also, the ALGOLs were defined so that onlyuppercase letters were required.
1960:IFIP – The Algol 60 language and report included several mathematical symbols which are available on modern computers and operating systems, but, unfortunately, were unsupported on most computing systems at the time. For instance: ×, ÷, ≤, ≥, ≠, ¬, ∨, ∧, ⊂, ≡, ␣ and ⏨.
1961 September: ASCII – TheASCII character set, then in an early stage of development, had the\ (Back slash) character added to it in order to support ALGOL'sBoolean operators/\ and\/.[19]
1962:ALCOR – This character set included the unusual "᛭" runic cross[20] character for multiplication and the "⏨" Decimal Exponent Symbol[21] for floating point notation.[22][23][24]
1964:GOST – The 1964 Soviet standardGOST 10859 allowed the encoding of 4-bit, 5-bit, 6-bit and 7-bit characters in ALGOL.[25]
1968: The "Algol 68 Report" – used extant ALGOL characters, and further adopted →, ↓, ↑, □, ⌊, ⌈, ⎩, ⎧, ○, ⊥, and ¢ characters which can be found on theIBM 2741 keyboard withtypeball (orgolf ball)print heads inserted (such as theAPL golf ball). These became available in the mid-1960s while ALGOL 68 was being drafted. The report was translated into Russian, German, French, and Bulgarian, and allowed programming in languages with larger character sets, e.g.,Cyrillic alphabet of the SovietBESM-4. All ALGOL's characters are also part of theUnicode standard and most of them are available in several popularfonts.
2009 October:Unicode – The⏨ (Decimal Exponent Symbol) for floating-point notation was added to Unicode 5.2 for backward compatibility with historicBuran programme ALGOL software.[26]
To date there have been at least 70 augmentations, extensions, derivations and sublanguages of Algol 60.[27]
| Name | Year | Author | Country | Description | Target CPU |
|---|---|---|---|---|---|
| ZMMD-implementation | 1958 | Friedrich L. Bauer, Heinz Rutishauser, Klaus Samelson, Hermann Bottenbruch |  Germany | implementation ofALGOL 58 | Z22 (laterZuse'sZ23 was delivered with an Algol 60 compiler)[28] |
| X1 ALGOL 60 | 1960 August[29] | Edsger W. Dijkstra andJaap A. Zonneveld |  Netherlands | First implementation of ALGOL 60[30] | Electrologica X1 |
| Elliott ALGOL | 1960s | C. A. R. Hoare |  UK | Subject of the 1980Turing Award Lecture[31] | Elliott 803, Elliott 503, Elliott 4100 series |
| JOVIAL | 1960 | Jules Schwartz |  US | ADODHOL prior toAda | Various (see article) |
| Burroughs Algol (Several variants) | 1961 | Burroughs Corporation (with participation by Hoare,Dijkstra, and others) | US | Basis of theBurroughs (and nowUnisysMCP based) computers | Burroughs Large Systems and their midrange also. |
| Case ALGOL | 1961 | Case Institute of Technology[32] | US | Simula was originally contracted as a simulation extension of the Case ALGOL | UNIVAC 1107 |
| GOGOL | 1961 | William M. McKeeman | US | For ODIN time-sharing system[33] | PDP-1 |
| RegneCentralen ALGOL | 1961 | Peter Naur,Jørn Jensen |  Denmark | Implementation of full Algol 60 | DASK at Regnecentralen |
| Dartmouth ALGOL 30 | 1962 | Thomas Eugene Kurtz et al. | US | LGP-30 | |
| USS 90 Algol | 1962 | L. Petrone |  Italy | ||
| ALGOL 60 | 1962 | Bernard Vauquois, Louis Bolliet[34] |  France | Institut d'Informatique et Mathématiques Appliquées de Grenoble (IMAG) and Compagnie des Machines Bull | Bull Gamma 60 |
| Algol Translator | 1962 | G. van der Mey andW.L. van der Poel | Netherlands | Staatsbedrijf der Posterijen, Telegrafie en Telefonie | ZEBRA |
| Kidsgrove Algol | 1963 | F. G. Duncan | UK | English Electric CompanyKDF9 | |
| VALGOL | 1963 | Val Schorre | US | A test of theMETA II compiler compiler | |
| Whetstone | 1964 | Brian Randell and L. J. Russell | UK | Atomic Power Division of English Electric Company. Precursor toFerranti Pegasus, National Physical LaboratoriesACE andEnglish Electric DEUCE implementations. | English Electric CompanyKDF9 |
| NU ALGOL | 1965 |  Norway | UNIVAC | ||
| ALGEK | 1965 |  Soviet Union | АЛГЭК, based on ALGOL-60 andCOBOL support, for economical tasks | Minsk-22 | |
| ALGOL W | 1966 | Niklaus Wirth | US | Proposed successor to ALGOL 60 | IBM System/360 |
| MALGOL | 1966 | publ. A. Viil, M Kotli & M. Rakhendi, |  Estonian SSR | Minsk-22 | |
| ALGAMS | 1967 | GAMS group (ГАМС, группа автоматизации программирования для машин среднего класса), cooperation of Comecon Academies of Science | Comecon | Minsk-22, laterES EVM,BESM | |
| ALGOL/ZAM | 1967 |  Poland | PolishZAM computer | ||
| Simula 67 | 1967 | Ole-Johan Dahl andKristen Nygaard | Norway | Algol 60 with classes | UNIVAC 1107 |
| Triplex-ALGOL Karlsruhe | 1967/1968 | Karlsruhe,Germany | ALGOL 60 (1963) withtriplex numbers forinterval arithmetic | [35] | |
| Chinese Algol | 1972 |  China | Chinese characters, expressed via the Symbol system | ||
| DG/L | 1972 | US | DGEclipse family of Computers | ||
| S-algol | 1979 | Ron Morrison | UK | Addition of orthogonal datatypes with intended use as a teaching language | PDP-11 with a subsequent implementation on theJava VM |
The Burroughs dialects included special Bootstrapping dialects such asESPOL andNEWP. The latter is still used for Unisys MCP system software.
This means that the need to transcode GOST-based software and documentation can still arise: legacy numerical algorithms (some of which may be of interest, e.g. for the automatic landing of the Buran shuttle ...) optimized for the non-IEEE floating point representation of BESM-6 cannot be simply recompiled and be expected to work reliably, and some human intervention may be necessary.
