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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

See Also

Operator precedence

- - - -
Common operators
assignment increment
decrement 		arithmetic logical comparison member
access 		other

a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b

-

++a --a a++ a--

-

+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b

-

!a a && b a || b

-

a == b a != b a < b a > b a <= b a >= b

-

a[b] *a &a a->b a.b

-

a(...) a, b (type) a a ? b : c sizeof

_Alignof
(since C11)

-

See also

-
C++ documentation for Logical operators
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/language/operator_logical -

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