Swap Two Numbers Without Temporary Variable

If you want to swap two variables, you would normally require a temporary third variable. But how to swap them without using a third variable? This is quite often a popular interview question for programming jobs.

Below is a sample code written in C to swap two numbers without any temporary variables. Integer arithmetic is used for the operation.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  int x, y;
  char *end;
    
  if(argc!=3) {
    printf("\nUsage: %s <x> <y>\n",argv[0]);
    exit(0);
  }
    
  x = strtol(argv[1],&end,10);  
  if (argv[1] == end) { printf ("\nx is invalid!"); exit(1); }
  
  y = strtol(argv[2],&end,10);
  if (argv[2] == end) { printf ("\ny is invalid!"); exit(2); }
  
  printf("\nbefore-swap: x=%-10d y=%-10d",x,y);
  
  // let x = sum of both x and y
  x=x+y; 
  
  // y = x-y 
  // substitute x=x+y
  // y = (x+y)-y
  // y = x+y-y
  // y = x
  y=x-y;
  
  // y now has the value of x and x is the sum of the 2 numbers
  // x = (x+y)-y
  // x = x+y-y
  // x = x
  x=x-y;
  
  printf("\nafter-swap : x=%-10d y=%-10d",x,y);
  
  return 0;
}

The actual swapping of the variables is done by the three lines highlighted above. The comments above the three lines provide the reasons behind the arithmetic steps.

Mohamed Ibrahim

ibrahim = { interested_in(unix, linux, android, open_source, reverse_engineering); coding(c, shell, php, python, java, javascript, nodejs, react); plays_on(xbox, ps4); linux_desktop_user(true); }