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C program to replace bit in an integer at a specified position from another integer

In this article, we are going to learn how to replace a bit of a number at a specified position from another number?
Submitted by Radib Kar, on December 30, 2018

Problem statement

Write a C program to replace specified bit of a number from another number.

Solution

We can use bitwise operator here to solve the problem.

In this solution we have considered 8 bits representation which can be extended up to 32 bits using the same idea.

    Let first no whose bit is to be replaced is:
    7 //0000 0111

    Second no from whom bit is to be replaced
    8 //0000 1000

    Specified position: 3 (0-indxed)

First no: 7

Second no: 8

Algorithm

IF specified bit at second no is 1
    1)  Form a bit mask (0, d)
        0 - for all position except the bit specified
        d - 1 at specified position(dth)
    2)  Do bitwise OR between the first no and the mask 
ELSE //specified bit at second no 0
    1)  Form a bit mask (1, d)
        1 - For all position except the bit specified
        d - 0 at specified position (dth)
    2)  Do bitwise AND between the first no & the mask.

Forming the bitmask (0, d)

Let the specified position d.
Declare temp as (second>>d) & 1
Right shift second number d times to get the dth bit as LSB
Bitwise AND with 1
This results in 0000 000di(where diis the dth bit of second no)
Left shift temp d times. This results in our desired bitmask(0, d)

Forming the bitmask (1, d)
Declare flag 255;//FF, all bit set to 1(considering 8 bit)
Declare temp as 1<<d;
Do bitwise XOR between flag and temp
//this sets only the specified position bit 0 and others with 1
This results in our desired bitmask (1, d)

C program to replace bit in an integer at a specified position from another integer

#include <stdio.h>

int main() {
  int first, second, pos;

  printf("enter first & second no:\n");
  scanf("%d %d", & first, & second);

  printf("enter specified position(0-indexed)\n");
  scanf("%d", & pos);

  //collect corresponding bit of second no
  int temp = (second >> pos) & 1;

  //if bit at specified position is 1
  if (temp == 1) {
    temp = temp << pos;
    first |= temp;
  } else { //if bit at specified position is 0
    int flag = 255; //FF, all bit set to 1(considering 8 bit)
    temp = 1 << pos;
    //this set only the specified position bit 0 others 1
    flag = flag ^ temp;
    first &= flag;
  }

  printf("converted no %d\n", first);

  return 0;
}

Output

First run:
enter first & second no:
7 8
enter specified position(0-indexed)
3
converted no 15

Second run:
enter first & second no:
7 8
enter specified position(0-indexed)
2
converted no 3

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