Assign other value to a variable from two possible values

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++

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// 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;
}

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Java

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// 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); 
}
}

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Python3

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# 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

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C#

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// 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

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PHP

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<?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.
?>

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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++

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// 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;
}

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Java

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// 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

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Python3

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# 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

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C#

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// 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*/

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PHP

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<?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
?>

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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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