Logical operators - cppreference.com (original) (raw)

Logical operators apply standard boolean algebra operations to their operands.

[edit] Logical NOT

The logical NOT expression has the form

| | | | | ---------------- | | | | ! expression | | | | | | |

where

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'); // non-zero if 'a' is a space, zero otherwise int 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) = false non-space

[edit] Logical AND

The logical AND expression has the form

| | | | | -------------- | | | | lhs && rhs | | | | | | |

where

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 } }

[edit] Logical OR

The logical OR expression has the form

| | | | | --------------- | | | | lhs || rhs | | | | | | |

where

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

[edit] 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

[edit] See Also

Operator precedence

[edit] See also