CategoryPython

Dinner Party Coding Interview Problem

You’ll find the text-form code from the second episode here. Following some very good critical feedback I got from friends and viewers, this episode is much, much shorter and focuses entirely on a coding challenge. The full video is here:

With this coding problem, you’re main challenge is generating combinations. There are some really fancy ways to achieve this. My favourite approach involves using bitmasks and Hamming Weights. Such an approach would just count up from 0 to 2^n looking for any number with a Hamming Weight of table_size. While that’s super snazzy, it’s also not super representative of what a person not familiar with these types of problems might come up with, so I didn’t take that approach for this video.

Instead, this video focuses on recursion. Reasoning about recursion can be a real challenge for our poor human brains. To ease this, in the video I draw the recursion chart shown here:

A diagram of a 4 choose 3 combination result created via a recursive function call.

A diagram of a 4 choose 3 combination result created via a recursive function call.

From this, you can get a basic idea of how a recursive approach to this problem might be solved. The really tricky thing for someone tackling such a problem for the first time is that you have to recurse twice in every function execution. Many people think of recursion as an approach where the function calls itself. The exciting thing about this algorithm is that the function calls itself not once, but twice!

One flaw with this code I noticed only after finishing the video and uploading it is that my groups argument is actually a bug. Because the default arguments are static across all calls to a function, this list will continue to grow and grow every time the find_dinner_parties function is invoked. It’s an easy thing to fix, but I ran out of whiteboard anyway. For those playing along at home, please add a list for groups in the call to combine_friends inside find_dinner_parties.

Generating a Color Picker Style Color Wheel in Python

color gamut generated from Python

This color wheel was generated with Python. Check out the code on GitHub.

For the record, I am very bad at Python. Yet, it has become more and more my go-to language for doing little things that aren’t web. For instance, I recently undertook to create a color picker/color wheel in Python.

For the uninitiated, a color wheel or color spectrum is a series of hues that run from red to indigo (roughly purple to our eyes). It’s the rainbow. The color wheel is this same thing but wrapped around in a way that shows all the hues the human eye can see. In reality, indigo doesn’t converge on red as you go up in light frequency, but the hues that you would see if it did are colors we know well — magenta/pink.

So, generating a color wheel is an interesting challenge. The primary colors can be created from the hex colors 0xff0000 (red), 0x00ff00 (green), and 0x0000ff (blue). You probably see the pattern easily. A color breaks down into three channels, each of which has up to 0xff or 256 values from 0, completely absent, to 0xff, completely displayed. Blending from blue to green implies moving from 0x00ff00 to 0x0000ff by decrementing the green channel monotonically as you monotonically increment the blue channel. So, you can probably start to see how the logic for generating the color spectrum graphic will have to work. Of course I’m ignoring a few colors that are important. Red, green, and blue are only half the colors that are necessary for the color wheel to look complete. The others are 0x00ffff (cyan), magenta (0xff00ff), and yellow (0xffff00). These colors appear in color spectrum interwoven between the primary colors. If you’re wondering why red and green make yellow, you’ll probably want to read about Color by Addition. Though, from a numbers perspective it’s easy to see a relationship between 0xff0000 -> 0xffff00 -> 0x00ff00.

Creating the gamut requires calculating how much of each color to use in the blending. One important aspect of a color wheel is that any color you can select is a blend of at most two colors. So, one way to generate a color wheel is to go pixel-by-pixel through the image you want to create figuring out what sextant you’re in, deciding which color you’re closest to, then figuring out how much of the other color to blend into it. That’s the approach I took. It’s true that you can do this other ways. In fact, I’m pretty sure there is a slick algorithm for this that I didn’t realize. I was on a plane when I wrote this and consider it more about learning Python than about making the most compact implementation of color spectrum generation.

Anyway, here’s the code (using the Python Image Library) on GitHub.