Suppose we number the bytes in a w-bit word from 0 (least significant) to w/8 − 1 (most significant). Write a code for the following C function, which will return an unsigned value in which byte i of argument x has been replaced by byte b:

unsigned replace_byte (unsigned x, int i, unsigned char b);

Answer and Explanation:

// header file

#include <stdio.h>

// function to replace value

unsigned replace_byte(unsigned x,int i,unsigned char b){

  char* p = (char*)(&x);

  p[i] = b;

  return x;

}

int main(){

  // declare variables

  unsigned n = 0x12345678;

  unsigned r1,r2;

  // display result

  printf("%x\n%x\n",replace_byte(n,2,0xAB),replace_byte(n,0,0xAB));

}

output

                                                                                                                                               12ab5678                                                                                                                                                    

123456ab