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 Data 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.
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.
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.
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))