Logical operators

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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

Contents

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 = {"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