Logical NOT

The ! (logical NOT) operator is a unary operator that performs logical negation. It yields an int value of 1 if its operand compares equal to zero, and 0 if its operand compares unequal to zero.

Syntax

!operand

Operand Constraints The operand must be of a scalar type. In C, scalar types include:

• Arithmetic types (integer and floating-point types)
• Pointer types

Evaluation and Type Semantics Regardless of the operand’s original data type, the result of the ! operator is strictly of type int.

The evaluation follows these rules:

• If operand == 0 (or NULL for pointers), !operand evaluates to 1.
• If operand != 0 (or non-NULL for pointers), !operand evaluates to 0.

Semantically, the expression !E is exactly equivalent to (0 == E).

Precedence and Associativity The ! operator belongs to the unary operators group (Precedence Level 2 in C).

• Associativity: Right-to-left.
• Precedence: It binds more tightly than binary arithmetic, relational, equality, and binary logical operators.

Because of its high precedence, an expression like !a == b is parsed as (!a) == b. To negate the result of an equality check, explicit grouping parentheses are required: !(a == b).

Double Negation (!!) Because ! forces any non-zero value to 0 and any zero value to 1, applying the operator twice sequentially evaluates to a normalized integer boolean representation of the original scalar:

!!operand

• If operand is 0, !!operand evaluates to 0.
• If operand is any non-zero value (e.g., 42, -3.14, or a valid memory address), !!operand evaluates to 1.

Master C with Deep Grasping Methodology!

Learn More