C Pocket Reference

Logical Operators

The logical operators, shown in Table 1-12, can be used to combine the results of several comparison expressions into one logical expression.

Table 1-12. The logical operators

Operator	Meaning	Example	Result: 1 (true) or 0 (false)
&&	logical AND	x && y	1 if both x and y are not equal to 0
||	logical OR	x || y	1 if either or both of x and y is not equal to 0
!	logical NOT	!x	1 if x equals 0. In all other cases, the expression yields 0.

The operands of logical operators may have any scalar (i. e., arithmetic or pointer) type. Any value except 0 is interpreted as “true”; 0 is “false.”

Like relational expressions, logical expressions yield the values “true” or “false”; that is, the int values 0 or 1:

!x || y  // "(not x) or y" yields 1 (true)
         // if x == 0 or y != 0

The operators && and || first evaluate the left operand. If the result of the operation is already known from the value of the left operand (i. e., the left operand of && is 0 or the left operand of || is not 0), then the right operand is not evaluated. For example:

i < max  &&  scanf("%d", &x) == 1

In this logical expression, the function scanf() is only called if i is less than max.