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Assign other value to a variable from two possible values

  • Difficulty Level : Easy
  • Last Updated : 05 May, 2021

Suppose a variable x can have only two possible values a and b, and you wish to assign to x the value other than its current one. Do it efficiently without using any conditional operator.
Note: We are not allowed to check current value of x. 
Examples: 
 

Input : a = 10, b = 15, x = a 
Output : x = 15 
Explanation x = 10, currently x has value of a (which is 10), we need to change it to 15.
Input : a = 9, b = 11, x = b 
Output : x = 9

We could solve this problem using if condition, but we are not allowed to do that.

if (x == a) 
   x = b;
else x = a;

We could have used Ternary operator, it also checks the current value of x and based on that it assigns new value. So we cannot use this approach as well 

x = x == a ? b : a;

But, we are not allowed to check value of x, so none of the above solutions work.
Solution 1: Using arithmetic operators only we can perform this operation 
 

x = a + b - x

This way the content of x will alternate between a and b every time it gets executed
 

C++




// CPP program to change value of x
// according to its current value.
#include <bits/stdc++.h>
using namespace std;
 
// Function to alternate the values
void alternate(int& a, int& b, int& x)
{
    x = a + b - x;
}
 
// Main function
int main()
{
    int a = -10;
    int b = 15;
    int x = a;
    cout << "x is : " << x;
 
    alternate(a, b, x);
 
    cout << "\nAfter change ";
    cout << "\nx is : " << x;
}

Java




// Java program to change value of x
// according to its current value.
import java.util.*;
 
class solution
{
 
// Function to alternate the values
static void alternate(int a, int b, int x)
{
    x = a + b - x;
    System.out.println("After change"+"\n"+" x is : "+x);
}
 
// Main function
public static void main(String args[])
{
    int a = -10;
    int b = 15;
    int x = a;
    System.out.println("x is : "+x);
    alternate(a, b, x);
}
}

Python3




# Python3 program to change value
# of x according to its current value.
 
# Function to alternate the values
def alternate(a,b,x):
    x = a+b-x
    print("After change x is:",x)
 
 
# Driver code
if __name__=='__main__':
    a = -10
    b = 15
    x = a
    print("x is:",x)
    alternate(a,b,x)
 
# This code is contributed by
# Shrikant13

C#




// C# program to change value of x
// according to its current value.
 
using System;
class gfg
{
 // Function to alternate the values
 public void alternate(ref int a, ref int b, ref int x)
   //'ref' indicates the references
 {
    x = a + b - x;
 }
}
 
// Main function
class geek
{
 public static int Main()
 {
    gfg g = new gfg();
    int a = -10;
    int b = 15;
    int x = a;
    Console.WriteLine("x is : {0}" , x);
 
    g.alternate(ref a, ref b, ref x);
 
    Console.WriteLine ("After change ");
    Console.WriteLine("x is : {0}", x);
    return 0;
 }
}
//This code is contributed by Soumik

PHP




<?php
// PHP program to change value of x
// according to its current value.
 
// Function to alternate the values
function alternate (&$a, &$b, &$x)
{
    $x = $a + $b - $x;
}
 
// Driver Code
$a = -10;
$b = 15;
$x = $a;
echo "x is : ", $x;
 
alternate($a, $b, $x);
 
echo "\nAfter change ";
echo "\nx is : ", $x;
 
// This code is contributed by ajit.
?>

Javascript




<script>
// javascript program to change value of x
// according to its current value.
 
    // Function to alternate the values
    function alternate(a , b , x) {
        x = a + b - x;
        document.write("After change" + "<br/>" + " x is : " + x);
    }
 
    // Main function
     
        var a = -10;
        var b = 15;
        var x = a;
        document.write("x is : " + x+"<br/>");
        alternate(a, b, x);
 
// This code is contributed by todaysgaurav
</script>
Output: 
x is : -10
After change 
x is : 15

 

Time Complexity: The time complexity of this approach is O(1) 
Space Complexity: The space complexity of this approach is O(1)
Solution 2: A better and efficient approach is using the bitwise XOR operation.
x = a^b^x
 

C++




// CPP program to change value of x
// according to its current value.
#include <bits/stdc++.h>
using namespace std;
 
// Function to alternate the values
void alternate(int& a, int& b, int& x)
{
    x = a ^ b ^ x;
}
 
// Main function
int main()
{
    int a = -10;
    int b = 15;
    int x = a;
    cout << "x is : " << x;
 
    alternate(a, b, x);
 
    cout << "\nAfter exchange ";
    cout << "\nx is : " << x;
     
    return 0;
}

Java




// Java program to change value of x
// according to its current value.
 
class GFG {
// Function to alternate the values
 
    static int alternate(int a, int b, int x) {
        return x = a ^ b ^ x;
    }
 
// Main function
    public static void main(String[] args) {
        int a = -10;
        int b = 15;
        int x = a;
        System.out.print("x is : " + x);
 
        x = alternate(a, b, x);
 
        System.out.print("\nAfter exchange ");
        System.out.print("\nx is : " + x);
 
    }
}
 
// This code is contributed by 29AjayKumar

Python3




# Python3 program to change value of x
# according to its current value.
 
# Function to alternate the values
def alternate(a, b, x):
    x = a ^ b ^ x
    print("After exchange")
    print("x is", x)
 
# Driver code
a = -10
b = 15
x = a
print("x is", x)
alternate(a, b, x)
 
# This code is contributed
# by Shrikant13

C#




     
// C# program to change value of x
// according to its current value.
using System;
public class GFG {
// Function to alternate the values
 
    static int alternate(int a, int b, int x) {
        return x = a ^ b ^ x;
    }
 
// Main function
    public static void Main() {
        int a = -10;
        int b = 15;
        int x = a;
        Console.Write("x is : " + x);
 
        x = alternate(a, b, x);
 
        Console.Write("\nAfter exchange ");
        Console.Write("\nx is : " + x);
 
    }
}
/*This code is contributed by Rajput-Ji*/

PHP




<?php
// PHP program to change value of x
// according to its current value.
 
// Function to alternate the values
function alternate(&$a, &$b, &$x)
{
    $x = $a ^ $b ^ $x;
}
 
// Driver Code
$a = -10;
$b = 15;
$x = $a;
echo "x is : ", $x;
 
alternate($a, $b, $x);
 
echo "\nAfter exchange ";
echo "\nx is : ", $x;
 
// This code is contributed
// by akt_mit
?>

Javascript




<script>
 
// Javascript program to change value of x
// according to its current value.
 
// Function to alternate the values
 
    function alternate(a , b , x) {
        return x = a ^ b ^ x;
    }
 
    // Main function
     
        var a = -10;
        var b = 15;
        var x = a;
        document.write("x is : " + x);
 
        x = alternate(a, b, x);
 
        document.write("<br/>After exchange ");
        document.write("<br/>x is : " + x);
 
 
// This code contributed by Rajput-Ji
 
</script>
Output: 
x is : -10
After exchange 
x is : 15

 

Time Complexity: The time complexity of this approach is O(1) 
Space Complexity: The space complexity of this approach is O(1)
 


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