In this blog post, I will teach you to how to write C/C++ program to check whether the bit at given position is set or unset. After reading this detail guide post I believe you able to write an efficient program to check whether the bit at given position is set or not.

Consider the below examples,

Example-1:

Input : n = 5 //number
      : d = 2 //position want to check


Here's a simple ASCII representation of 5 (an 8-bit integer):

  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                      ^
                      |
                      Want to check bit wether set or unset

Output: Bit is set

Input : n = 9 //number
      : d = 1 //position want to check


Here's a simple ASCII representation of 9 (an 8-bit integer):

  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 |  (Example bit values)
+---+---+---+---+---+---+---+---+
                          ^
                          |
                          Want to check bit wether set or unset

Output: Bit is unset

Let’s see the problem statements for this question.

Problem Statement: Given two positive number ‘n‘ and ‘d‘. You need to check whether the bit at position d from the right in the binary representation of n is set (‘1’) or unset (‘0’). That means you need check whether d^th bit in ‘n’ is set or unset. Note that d = 0 means the rightmost bit of ‘n’.

Disclaimer: Try to solve the problem yourself first otherwise it would be not worth solving this problem.

1. Using Bitwise And (&) with a Bit Mask:

Using the Bitwise And Operator (&) with a Mask you can easily check whether the bit at given position is set or unset.

Using the expression (1 << d), you can create a mask. This mask has only a set bit which you want to check.

After creating the mask, you need to perform the bitwise AND between given number ‘n’ and the mask. Now you know whether d^th bit of given number ‘n’ is set or unset.

Let’s take an example to understand the above steps, suppose n =5.

Now if you want to check its 2^nd bit (bit-position start from the 0), so here mask would be (1<<2).

Mask: (1 << 2)


  7   6   5   4   3   2   1   0    (Bit position)
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 |  (Example bit values)
+---+---+---+---+---+---+---+---+

Now ANDing this mask with original number ‘n’.

Original number n: 5
                   00000101 (5 in binary)
                    &
Mask:              00000100


Result: =        Non-Zero result, so Bit is set

C Program to check whether the bit at given position is set or unset:

/*
C Program to check whether the
bit at given position is set or unset:
*/
#include <stdio.h>

//If d valid,function to check whether the bit
// at given position is set or unset
int checkDthBits(unsigned int n, int d)
{
    unsigned int MASK = 0;
    int ret = -1;

    //d must be greater than 0
    if(d > 0)
    {
        //Create MASK
        MASK = (1 << d);

        // if it results to '1' then bit is set,
        // else it results to '0' bit is unset
        ret = (n & MASK)? 1 : 0;
    }

    return ret;
}


int main()
{
    int n = 5;
    int d = 2;

    int v = checkDthBits(n, d);
    switch(v)
    {
    case 0:
        printf("Bit is Unset");
        break;
    case 1:
        printf("Bit is Set");
        break;
    default:
        printf("Enter valid value of position\n");
        break;
    }

    return 0;
}

Output: Bit is Set

C++ Program to check whether the bit at given position is set or unset:

/*
C++ Program to check whether the
bit at given position is set or unset:
*/
#include <iostream>
#include <cassert>

// Use (void) to silence unused warnings.
#define assertm(exp, msg) assert(((void)msg, exp))



//If d valid,function to check whether the bit
// at given position is set or unset
bool checkDthBits(unsigned int n, int d)
{
    //d must be greater than 0
    assert((void("Enter valid value of position"), d>0));

    //Create MASK
    const unsigned int MASK = (1 << d);

    // if it results to '1' then bit is set,
    // else it results to '0' bit is unset
    const bool isBitSet = (n & MASK)? true : false;

    return isBitSet;
}


int main()
{
    int n = 5;
    int d = 2;
    const bool hasDthBitSet = checkDthBits(n, d);
    if(hasDthBitSet)
    {
        std::cout<<"Bit is Set"<<std::endl;
    }
    else
    {
        std::cout << "Bit is Unset"<<std::endl;
    }

    return 0;
}

Output: Bit is Set

Recommended Post

• How to Set a Particular Bit in a number.
• turn off a particular bit.
• Count Set bits in an Integer.
• Turn off the rightmost set bit.
• Rotate bits of a number.
• Macros for Bit Manipulation in C/C++.
• 5 Ways to Check if Two Integers have Opposite Signs.
• Operator Precedence And Associativity In C.
• Operators in C language