Swap two variables in one line in C/C++, Python and Java

2.3

We have discussed different approaches to swap two integers without the temporary variable. How to swap in a single line without using library function?

Python: In Python, there is a simple and syntactically neat construct to swap variables, we just need to write “x, y = y, x”.

C/C++: Below is one generally provided classical solution

// Swap using bitwise XOR (Wrong Solution in C/C++)
x ^= y ^= x ^= y; 

The above solution is wrong in C/C++ as it causes undefined behaviour (compiler is free to behave in any way). The reason is, modifying a variable more than once in an expression causes undefined behaviour if there is no sequence point between the modifications.

However, we can use comma to introduce sequence points. So the modified solution is

// Swap using bitwise XOR (Correct Solution in C/C++)
// sequence point introduced using comma.
(x ^= y), (y ^= x), (x ^= y);

Java: In Java, rules for subexpression evaluations are clearly defined. The left hand operand is always evaluated before right hand operand (See this for more details). In Java, the expression “x ^= y ^= x ^= y;” doesn’t produce the correct result according to Java rules. It makes x = 0. However, we can use “x = x ^ y ^ (y = x);” Note the expressions are evaluated from left to right. If x = 5 and y = 10 initially, the expression is equivalent to “x = 5 ^ 10 ^ (y = 5);”. Note that we can’t use this in C/C++ as in C/C++, it is not defined whether left operand or right operand is executed for any operator (See this for more details)

C/C++

// C/C++ program to swap two variables in single line
#include <stdio.h>
int main()
{
    int x = 5, y = 10;
    (x ^= y), (y ^= x), (x ^= y); 
    printf("After Swapping values of x and y are %d %d",
            x, y);
    return 0;
}

Java

// Java program to swap two variables in single line
class GFG
{
   public static void main (String[] args)
   {
      int x = 5, y = 10;
      x = x ^ y ^ (y = x);
      System.out.println("After Swapping values of x and y are "
                          + x + " " + y);
   }
} 

Python

# Python program to swap two variables in single line
x = 5
y = 10
x, y = y, x
print "After Swapping values of x and y are", x, y

Output:

After Swapping values of x and y are 10 5

Alternate Solutions :

  • C++ also provides a library function swap()
  • b = (a + b) – (a = b); [Thanks to Rajat Mishra for this]
  • a += b – (b = a); [Thanks to Zoran Davidović for this]

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