Logical operators

Logical operators apply standard boolean algebra operations to their operands.

Logical NOT

The logical NOT expression has the form

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'); // 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
}

!(2+2==4) = false
nonspace

Logical AND

The logical AND expression has the form

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
    }
}

Logical OR

The logical OR expression has the form

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));
}

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

Operator precedence

See also