C programming language has logical operators for logical expressions. The output of a logical operation is a Boolean value – true or false. In a logical expression you have one or more operands and each has a truth value, depending on the truth value of each operand the logical operator decides the final truth value of the logical expression.

## Logical Operators

A list of logical operators in C language is given below. The minimum numbers of operands required by each of the operators is also given in the same table.

- AND (&&)
- OR (||)
- NOT (!)

We shall discuss each of them in more detail now.

## AND (&&) operator

The AND operation is denoted by two ‘ampersand’ symbol. To understand the AND operation, consider the following expression,

[cc lang=”c” tab_size=”4″ line_numbers=”off”]A && B

Where A = (3 < 4) and B = (10 < 20)[/cc]

The final output of the expression depends on following conditions.

A is **true** && B is **true** = A && B is **true**

A is **false** && B is **true** = A && B is **false**.

A is **true** && B is **false** = A && B is **false**.

A is **false** && B is **false** = A && B is **false**.

So, the AND is only **true** when both A and B are **true**.

## OR (||) operator

The OR operation is also useful logical operator in C language. The value of expression is true under following conditions.

A is **true** || B is **true** = A || B is **true**

A is **false** || B is **true** = A || B is **true**

A is **true** || B is **false** = A || B is **true**

A is **false** || B is **false** = A || B is **false**

From the above example, it is clear that the OR is only **true** when at least A or B is **true**.

## NOT (!) operator

The NOT is called the **negation** operation. It negates the truth value of the existing expression.

For example,

if A is **true** , then !A is **false**

If A is **false**, then !A is **true**.

Note:- The *!(!A) == A.*

## Example – AND, OR, NOT

[cc lang=”c” tab_size=”4″]/* C Program to demonstrate use of Logical Operators */

#include

int main() {

int A, B ;

A = (5 < 10); /* True */ B = (10 < 20); /* True */ printf("\n\n\n\n\n"); /* Working of AND operation - both A and B must be true */ if(A && B) { printf("\tOutput AND - both A and B are true\n\n"); } /* Operation of Not operator */ A = (2 < 10); /* False */ B = (4 < 8); /* True */ /* A && B is False */ if(!(A && B)) { printf("\tOutput NOT - (A && B) is False so negation !(A && B) is True\n\n"); } /* Working of OR operator - any one of A or B must be true */ if (A || B) { printf("\tOutput OR - Any one of A or B is true\n\n"); } getch(); return 0; }[/cc]

## Output – AND, OR, NOT

## Bibliography

Balagurusamy, E. 2000. *Programming in ANSI C.* Tata McGraw-Hill Education,.

Brian W. Kernighan, Dennis M. Ritchie. 1988. *C Programming Language, 2nd Edition.* Prentice Hall.

Kanetkar, Yashavant. 20 November 2002. *Let us C.* Bpb Publications.