8.1 Logical Operators

The logical operators combine truth values, which are normally represented in C as numbers. Any expression with a numeric value is a valid truth value: zero means false, and any other value means true. A pointer type is also meaningful as a truth value; a null pointer (which is zero) means false, and a non-null pointer means true (see Pointer Types). The value of a logical operator is always 1 or 0 and has type int (see Integer Types).

The logical operators are used mainly in the condition of an if statement, or in the end test in a for statement or while statement (see Statements). However, they are valid in any context where an integer-valued expression is allowed.

‘! exp’

Unary operator for logical “not.” The value is 1 (true) if exp is 0 (false), and 0 (false) if exp is nonzero (true).

Warning: if exp is anything but an lvalue or a function call, you should write parentheses around it.

‘left && right’

The logical “and” binary operator computes left and, if necessary, right. If both of the operands are true, the ‘&&’ expression gives the value 1 (which is true). Otherwise, the ‘&&’ expression gives the value 0 (false). If left yields a false value, that determines the overall result, so right is not computed.

‘left || right’

The logical “or” binary operator computes left and, if necessary, right. If at least one of the operands is true, the ‘||’ expression gives the value 1 (which is true). Otherwise, the ‘||’ expression gives the value 0 (false). If left yields a true value, that determines the overall result, so right is not computed.

Warning: never rely on the relative precedence of ‘&&’ and ‘||’. When you use them together, always use parentheses to specify explicitly how they nest, as shown here:

if ((r != 0 && x % r == 0)
    ||
    (s != 0 && x % s == 0))