How to swap the content of two variables without using an intermediate storage location?

Posted by Holmez on 20 Apr 2011. Filed under Programming Tips. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry

Traditionally most programmers use an extra variable when it’s come to swap the content of two variable. well, some programmers believe that there are no other ways to swap two variables with out using another temporary variable. The professor who taught me programming when I was in first year college said “You have two cups, one filled with water and the other filled with cola. Can you swap the content of them without using an extra cup?!!”

The tradition way looks like:

int a=10,b=35;
int temp;
temp = a;
a = b;
b = temp;

Now with out using the temp variable:

int a=10,b=35;
a = a + b; //a = 10+35 = 45
b = a - b; //b = 45-35 = 10
a = a - b; //a = 45-10 = 35

Very simple, isn’t it? but it doesn’t work with the water and cola example

You must first make sure that the sum a+b in line 2 doesn’t  exceed the int limit

Another way is to use XOR but this one doesn’t work with floats (only integers)

int a=10; //a = 001010
int b=35; //b = 100011
a ^= b; // a = 001010 ^ 100011 = 101001
b ^= a; // b = 100011 ^ 101001 = 001010
a ^= b; // a = 101001 ^ 001010 = 100011