Nobody Cares About a Few Million Nanoseconds

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.


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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: But this guide is meant to be a more practical guide to show how handy recursion is.

The source code of everything in this article can be downloaded here:

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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Call for help to review games & code for a new "Invent with Python" book.

UPDATE: Thanks for everyone who emailed their help. I'll leave this post up here for now, but the code seems fairly solid now.

I'm currently looking for help editing the source code for the games that will go into my next book. This book will also be released under a Creative Commons license and be freely available.

Download the game source. (Requires Python & Pygame)

UPDATE: I've fixed a problem where the midi files for the Tetris game were left out of this zip. Redownload the zip file to get them.


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New Game Source Code: Gemgem (A Bejeweled clone)

Here's the source code for a Bejeweled clone called Gemgem, written in Python with the Pygame library. You’ll need Python (2 or 3) and Pygame installed to run it. I've tried to keep the source code simple so it's easy to follow and learn programming and Pygame from it. It comes in under 540 lines of code, including whitespace & comments.

Download the Gemgem source code and graphics.

To play, click on (or drag over) two adjacent gems to swap their position. You need to swap them so that there are three or more gems of the same type in a row. This causes the gems to disappear and new gems to fall in their place. You get more points for chain reactions or more-than-three matches. The score constantly trickles down, so solve as fast as possible. The game ends when no more moves can be made.

The backspace key will reset the board, the escape key quits.

The graphics were from Osmic on

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New Game Source Code: Star Pusher (Sokoban clone)

Here's a Sokoban ("box pusher") clone called Star Pusher. I've used the graphics from the Planet Cute collection. You'll need Python (2 or 3) and Pygame installed to run it. Just download and unzip the files to the same directory. It comes with 201 levels from David W. Skinner.

Download the source code and graphics.

(UPDATE: I've updated the code so that it works on Linux. It was due to a \r\n newline issue. I've tested the game in Ubuntu and it works.)

The source code is designed to be readable so that new programmers can understand and modify it without much effort.

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