Four Different Methods or Tricks to Swap Two Numbers Without Using a Third Variable

In this post, we will see four different methods in c to swap two variables without using a third variable. This type of questions are often asked in technical interviews. Here we will see four different C programs to swap two numbers without a third temporary variable. Using a third variable is not an overhead. But still this kinda questions are asked in technical interviews for programming jobs to test your knowledge in the language.

Trick 1

In the first c program to swap two numbers without a third variable, we swap the values of the two variables in 3 steps which are simple addition and subtractions.

#include<stdio.h>
void main(){
    int a=5,b=10;
    a=b+a; //now a =15
    b=a-b; //now b= 5
    a=a-b; // now a=10 (done)
    printf("a= %d\nb=  %d",a,b);
}

Trick 2

In the second c program to swap two numbers without a third variable, we swap them in a single statement. The statement contains an assignment, addition and subtraction.

#include<stdio.h>
void main(){
    int a=10,b=30;
    a=a+b-(b=a);
    //10+30-(10)  a becomes 30
    // b is assigned with 10 in the same line of code
    printf("a= %d\nb=  %d",a,b);
}

Trick 3

In the third c code, we swap the numbers using bitwise XOR operation. The bitwise operator ^ 
is used in the program. The binary representation of the numbers, the bitwise xor operation on them and the result are shown in the c program as comments. The program is as follows:

#include<stdio.h>
void main(){
    int a=8,b=12;

   /* a: 8 : 01000
       b:12 : 01100
    */
   
    a=a^b; 

    /*      01000 ^
            01100
       a=   00100 (4)
    */


    b=a^b;

    /*      00100 ^
            01100
       b=   01000 (8)
    */

a=b^a;

    /*      01000 ^
            00100
       a=   01100 (12)
    */



    printf("a= %d\nb=  %d",a,b);
}

Trick 4

In the fourth method to swap two numbers without a temporary variable, we swap the numbers using a combination of addition, subtraction and bitwise NOT (inversion) operation. The bitwise operator ~ 
is used in the program. The binary representation of the numbers, the bitwise NOT operation on them and the result are shown in the c program as comments. The program is as follows:

#include<stdio.h>
void main(){

    int a=14,b=4;
    
   /* a: 14 : 1110
      b:  4 : 0100
     ~a:   :  0001  
     ~b:   :  1011  
   */
   a=b-~a-1;
    /* b:   0100 -
      ~a:   0001
            0011 -
       1:   0001
            0010
     now a= 0010 
    */
    b=a+~b+1;
    /* a:   0010 +
      ~b:   1011
       1:   0001
    now b:  1110  (14)
       ~b:  0001
    */
    a=a+~b+1;
   /*  a:   0010 +
      ~b:   0001
       1:   0001
    now a:  0100 (4)
   */

    printf("a= %d\nb=  %d",a,b);
}