Logical operators
Logical operators apply standard boolean algebra operations to their operands.
| Operator | Operator name | Example | Result |
|---|---|---|---|
| ! | logical NOT | !a | the logical negation of a |
| && | logical AND | a && b | the logical AND of a and b |
| || | logical OR | a || b | the logical OR of a and b |
Logical NOT
The logical NOT expression has the form
! expression
|
|||||||||
where
| expression | - | an expression of any scalar type |
The logical NOT operator has type int. Its value is 0 if expression evaluates to a value that compares unequal to zero. Its value is 1 if expression 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 true printf("!(2+2==4) = %s\n", b ? "true" : "false"); int n = isspace('a'); // zero if 'a' is a space, nonzero otherwise int x = !!n; // "bang-bang", common C idiom for mapping integers to [0,1] // (all non-zero values become 1) char *a[2] = {"nonspace", "space"}; printf("%s\n", a[x]); // now x can be safely used as an index to array of 2 ints }
Output:
!(2+2==4) = false nonspace
Logical AND
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 if lhs does not compare equal to 0 |
The logical-AND operator has type int and the value 1 if both lhs and rhs compare unequal to zero. It has the value 0 otherwise (if either lhs or rhs or both compare equal to zero).
There is a sequence point after the evaluation of lhs. If the result of lhs compares equal to zero, then rhs is not evaluated at all (so-called short-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 } }
Logical OR
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 if lhs compares equal to 0 |
The logical-OR operator has type int and the value 1 if either lhs or rhs compare unequal to zero. It has value 0 otherwise (if both lhs and rhs compare equal to zero).
There is a sequence point after the evaluation of lhs. If the result of lhs compares unequal to zero, then rhs is not evaluated at all (so-called short-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; // true printf("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 type fopen("test.txt", "r") || printf("could not open test.txt: %s\n", strerror(errno)); }
Possible output:
true or false = true could not open test.txt: No such file or directory
References
- C11 standard (ISO/IEC 9899:2011):
- 6.5.3.3 Unary arithmetic operators (p: 89)
- 6.5.13 Logical AND operator (p: 99)
- 6.5.14 Logical OR operator (p: 99)
- C99 standard (ISO/IEC 9899:1999):
- 6.5.3.3 Unary arithmetic operators (p: 79)
- 6.5.13 Logical AND operator (p: 89)
- 6.5.14 Logical OR operator (p: 89)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.3.3.3 Unary arithmetic operators
- 3.3.13 Logical AND operator
- 3.3.14 Logical OR operator
See Also
| Common operators | ||||||
|---|---|---|---|---|---|---|
| assignment | increment decrement |
arithmetic | logical | comparison | member access |
other |
|
a = b |
++a |
+a |
!a |
a == b |
a[b] |
a(...) |