# 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

### 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))

Run this code

    #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)

Run this code

    #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)

Run this code

    #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

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 ? : sizeof _Alignof (since C11)

### See also

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