DEV Community 👩‍💻👨‍💻

Cover image for An Algorithm to Swap Two Variables Synchronously, Without Creating a Temporary Variable
Gabriel Romualdo
Gabriel Romualdo

Posted on • Originally published at xtrp.io

An Algorithm to Swap Two Variables Synchronously, Without Creating a Temporary Variable

Originally published here at xtrp.io, my blog about computer science and just about anything programming.

Introduction

Let's say you want to swap the values of two integer variables, a, and b. Simply setting a equal to b and then b equal to a does not work, since a and b would just end up both equaling the initial value of b.

# doesn't work (both a and b end up equaling b):
a = b
b = a
Enter fullscreen mode Exit fullscreen mode

So, typically, you'd create another variable — let's call it c — and set that variable to the value of a, and then do something like this:

# the simplest solution, with a temporary variable:
c = a
a = b
b = c
Enter fullscreen mode Exit fullscreen mode

This is very simple, but the extra variable is not actually necessary.

Note that some programming languages, for example Python, provide the ability to swap two variables in a single line, like a, b = b, a. I will explain how to swap two variables without a temporary variable without utilizing this feature.

The Algorithm

By summing the two variables and adding a few subtraction operations, this same swapping operation can be done without a temporary variable, like this:

a = a + b
b = a - b
a = a - b
Enter fullscreen mode Exit fullscreen mode

To see that this works, let's test it out with a = 100 and b = 5.

a = a + b # a = 100 + 5 = 105
b = a - b # b = 105 - 5 = 100
a = a - b # a = 105 - 100 = 5
Enter fullscreen mode Exit fullscreen mode

As we can see, after running these operations, a = 5 and b = 100, so the values were swapped, as expected.

This can also be similarly done with multiplication and division, like this:

a = a * b
b = a / b
a = a / b
Enter fullscreen mode Exit fullscreen mode

This version is generally less efficient because most (if not all) programming languages take longer to calculate products and quotients in comparison to sums and differences.

Conclusion

Overall, this is an interesting algorithm that can be used in programs where you are swapping two integer values. Since it only saves a very small amount of space by not creating the temporary variable, it is not particularly useful in smaller projects and programs. However, on a larger scale when working with millions of variables and values at a time, and you'd like to save as much space and storage as possible, this can be useful.

I hope you enjoyed this article and found this to be an interesting algorithm.

Thanks for scrolling.

This post is originally from my blog at xtrp.io.

— Gabriel Romualdo, March 23, 2021

Top comments (11)

Collapse
romanright profile image
Roman

Hello,
Nice article and correct algo. But while you are using python here, there is a trick about this. Python supports tuple assignments. it means you can swap the values with just:

a, b = b, a
Enter fullscreen mode Exit fullscreen mode
Collapse
ninofiliu profile image
Nino Filiu

js too

[a, b] = [b, a]
Enter fullscreen mode Exit fullscreen mode
Collapse
goodevilgenius profile image
Dan Jones

PHP, too.

[$a, $b] = [$b, $a];
Enter fullscreen mode Exit fullscreen mode
Collapse
moopet profile image
Ben Sinclair

I can't imagine this being useful even at an enormous scale unless you're talking about processing millions of threads, and if you're doing that, you presumably have a spare megabyte or two of RAM to cope with it.

All it really does is obfuscate the code a little. It's an interesting puzzle for people who are learning about algorithms but it has no role in the real world except for that one programmer your friend met one time.

Collapse
gdledsan profile image
Edmundo Sanchez

It might be some close to metal thing that needs to happen in metofs we use daily.

I can see the benefit of not needing to reservu, use and clean up some variable, remmber io ooerations are slower than just adding and subtracting.

Collapse
ninofiliu profile image
Nino Filiu

Defo. I only had to swap variables in exercises in my old uni, and to answer people on stack overflow. Since then I've been in the industry for years and I never had to do that.

Collapse
thecosmicsound1 profile image
thecosmicsound1

Interesting algo. Liked the creative approach - can be a good trick interview question. One case where it won't work, for example with ints in Java, a and b are very large ints and summing them would cause an overflow.

Collapse
romanright profile image
Roman • Edited on

Probably you are right. I decided, that it is good to show this too, as Python is in the article's tags.

Collapse
pekosong profile image
Peko

asdasdsad

🤔 Did you know?

 
DEV has a variety of tags to help you find the content you like. Find and follow your favorite tags.