Logical operators apply standard boolean algebra operations to their operands.
| Operator | Operator name | Example | Result |
|---|---|---|---|
| ! | logical NOT | !a | the logical negation ofa |
| && | logical AND | a&& b | the logical AND ofa andb |
| || | logical OR | a|| b | the logical OR ofa andb |
Contents |
The logical NOT expression has the form
!expression | |||||||||
where
| expression | - | an expression of anyscalar type |
The logical NOT operator has typeint. Its value is0 ifexpression evaluates to a value that compares unequal to zero. Its value is1 ifexpression evaluates to a value that compares equal to zero. (so!E is the same as(0==E))
#include <stdbool.h>#include <stdio.h>#include <ctype.h>int main(void){ bool b=!(2+2==4);// not trueprintf("!(2+2==4) = %s\n", b?"true":"false"); int n=isspace('a');// non-zero if 'a' is a space, zero otherwiseint x=!!n;// "bang-bang", common C idiom for mapping integers to [0,1]// (all non-zero values become 1)char*a[2]={"non-space","space"};puts(a[x]);// now x can be safely used as an index to array of 2 strings}
Output:
!(2+2==4) = falsenon-space
The logical AND expression has the form
lhs&&rhs | |||||||||
where
| lhs | - | an expression of any scalar type |
| rhs | - | an expression of any scalar type, which is only evaluated iflhs does not compare equal to0 |
The logical-AND operator has typeint and the value1 if bothlhs andrhs compare unequal to zero. It has the value0 otherwise (if eitherlhs orrhs or both compare equal to zero).
There is asequence point after the evaluation oflhs. If the result oflhs compares equal to zero, thenrhs is not evaluated at all (so-calledshort-circuit evaluation)
#include <stdbool.h>#include <stdio.h>int main(void){ bool b=2+2==4&&2*2==4;// b == true 1>2&&puts("this won't print"); char*p="abc";if(p&&*p)// common C idiom: if p is not null// AND if p does not point at the end of the string{// (note that thanks to short-circuit evaluation, this// will not attempt to dereference a null pointer)// ... // ... then do some string processing}}
The logical OR expression has the form
lhs||rhs | |||||||||
where
| lhs | - | an expression of any scalar type |
| rhs | - | an expression of any scalar type, which is only evaluated iflhs compares equal to0 |
The logical-OR operator has typeint and the value1 if eitherlhs orrhs compare unequal to zero. It has value0 otherwise (if bothlhs andrhs compare equal to zero).
There is asequence point after the evaluation oflhs. If the result oflhs compares unequal to zero, thenrhs is not evaluated at all (so-calledshort-circuit evaluation)
#include <stdbool.h>#include <stdio.h>#include <string.h>#include <errno.h>int main(void){ bool b=2+2==4||2+2==5;// trueprintf("true or false = %s\n", b?"true":"false"); // logical OR can be used simialar to perl's "or die", as long as rhs has scalar typefopen("test.txt","r")||printf("could not open test.txt: %s\n",strerror(errno));}
Possible output:
true or false = truecould not open test.txt: No such file or directory
| Common operators | ||||||
|---|---|---|---|---|---|---|
| assignment | increment decrement | arithmetic | logical | comparison | member access | other |
a= b | ++a | +a | !a | a== b | a[b] | a(...) |
C++ documentation forLogical operators |
