## A Clever Programming Trick…

If you need to swap the values of two variables, this usually requires a third temporary variable (that is, if you’re not using a language like Python that supports the a, b = b, a syntax.) It looks something like this:

```temp = a; a = b; b = temp;```

But if these are integer variables, there’s a nifty trick to save yourself a little bit of memory. You can use arithmetic instead of a temporary variable:

```a = a + b; b = a - b; a = a - b;```

If the integers on your platform are 32-bits, your new swap will save four bytes of memory.

NOBODY CARES ABOUT FOUR BYTES OF MEMORY. More… »

### Recursion Explained with the Flood Fill Algorithm (and Zombies and Cats)

This is a programming tutorial for beginner and intermediate programmers who want to learn what recursion is. The programming language used for the examples is Python, but you can probably follow along if you know programming in some other language such as PHP or JavaScript. There’s a lot more information about recursion on the Wikipedia article: http://en.wikipedia.org/wiki/Recursion_(computer_science) But this guide is meant to be a more practical guide to show how handy recursion is.

Consider the Lazy Zombie

This is a cat:

This is a normal human:

This is a normal human who has been turned into an ungodly, flesh-eating zombie of the undead:

Zombies are lazy and will only bite things that are next to them. Humans that are bitten will then turn into zombies:

There is an interesting recursive principle here, because the humans that have turned into zombies will start to bite other humans that are next to them, which will make more zombies, who bite more adjacent humans, which will make more zombies, and so on and so on in a chain reaction:

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