C Programming Tutorial - Logical Operators

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 are 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,

\begin{aligned}&A \hspace{2px} \&\& \hspace{2px} B\\ \\ &Where \hspace{2px}A = (3 < 4)\hspace{2px} and \hspace{2px} B = (10 < 20) \end{aligned}

The final output of the expression depends on the following conditions.

\begin{aligned}&A \hspace{5px} is \hspace{5px} \textbf{true} \hspace{5px} \&\& \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ true} = A \hspace{5px} \&\&\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{true}\\ &A \hspace{5px} is \hspace{5px} \textbf{false} \hspace{5px} \&\& \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ true} = A \hspace{5px} \&\&\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{false}\\ &A \hspace{5px} is \hspace{5px} \textbf{true} \hspace{5px} \&\& \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ false} = A \hspace{5px} \&\&\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{false}\\ &A \hspace{5px} is \hspace{5px} \textbf{false} \hspace{5px} \&\& \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ false} = A \hspace{5px} \&\&\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{false}\\ \end{aligned}

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

OR (||) operator

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

\begin{aligned}&A \hspace{5px} is \hspace{5px} \textbf{true} \hspace{5px} || \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ true} = A \hspace{5px} ||\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{true}\\ &A \hspace{5px} is \hspace{5px} \textbf{false} \hspace{5px} || \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ true} = A \hspace{5px} ||\hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{true}\\ &A \hspace{5px} is \hspace{5px} \textbf{true} \hspace{5px} || \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ false} = A \hspace{5px} || \hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{true}\\ &A \hspace{5px} is \hspace{5px} \textbf{false} \hspace{5px} || \hspace{5px} B \hspace{5px} is \hspace{5px}\textbf{ false} = A \hspace{5px} || \hspace{5px} B\hspace{5px} is \hspace{5px} \textbf{false}\\ \end{aligned}

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,

\begin{aligned}&If \hspace{5px}A \hspace{5px} is \hspace{5px} \textbf{true} , then\hspace{5px} !A \hspace{5px} is \hspace{5px} \textbf{false}\\ &If \hspace{5px}A \hspace{5px} is \hspace{5px} \textbf{false} , then\hspace{5px} !A \hspace{5px} is \hspace{5px} \textbf{true}\\ \end{aligned}

Note:- The $!(!A) == A$

Example – AND, OR, NOT

/* C Program to demonstrate use of Logical Operators */
#include <stdio.h>
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;
}

Output – AND, OR, NOT

Output AND - both A and B are true
Output NOT - (A && B) is False so negation !(A && B) is True
Output OR - Any one of A or B is true

References

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.