Because experience
is the only teacher



How do you get to be a great musician? It helps to know the theory, and to understand the mechanics of your instrument. It helps to have talent. But ultimately, greatness comes from practicing; applying the theory over and over again, using feedback to get better every time.

How do you get to be an All-Star sports person? Obviously fitness and talent help. But the great athletes spend hours and hours every day, practicing.

But in the software industry we take developers trained in the theory and throw them straight in to the deep-end, working on a project. It’s like taking a group of fit kids and telling them that they have four quarters to beat the Redskins (hey, we manage by objectives, right?). In software we do our practicing on the job, and that’s why we make mistakes on the job. We need to find ways of splitting the practice from the profession. We need practice sessions.

The Kata

What makes a good practice session? You need time without interruptions, and a simple thing you want to try. You need to try it as many times as it takes, and be comfortable making mistakes. You need to look for feedback each time so you can work to improve. There needs to be no pressure: this is why it is hard to practice in a project environment. it helps to keep it fun: make small steps forward when you can. Finally, you’ll recognize a good practice session because you’ll came out of it knowing more than when you went in.

Code Kata is an attempt to bring this element of practice to software development. A kata is an exercise in karate where you repeat a form many, many times, making little improvements in each. The intent behind code kata is similar. Each is a short exercise (perhaps 30 minutes to an hour long). Some involve programming, and can be coded in many different ways. Some are open ended, and involve thinking about the issues behind programming. These are unlikely to have a single correct answer. I add a new kata every week or so. Invest some time in your craft and try them.

Remember that the point of the kata is not arriving at a correct answer. The point is the stuff you learn along the way. The goal is the practice, not the solution.


The Kata

  • Kata 1: Supermarket pricing. Pricing looks easy, but scratch the surface and there are some interesting issues to consider.

  • Kata 2: Karate Chop. A binary chop algorithm is fairly boring. Until you have to implement it using five totally different techniques.

  • Kata 3: How Big, How Fast? Quick estimation is invaluable when it comes to making design and implementation decisions. Here are some questions to make you turn over the envelope.

  • Kata 4: Data Munging. Implement two simple data extraction routines, and see how much they have in common.

  • Kata 5: Bloom Filters. Implement a simple hash-based lookup mechanism and explore its characteristics.

  • Kata 6: Anagrams. Find all the anagram combinations in a dictionary.

  • Kata 7: Reviewing. What does our code look like through critical eyes, and how can we make our eyes more critical?

  • Kata 8: Objectives. What effects do our objectives have on the way we write code?

  • Kata 9: Checkout. Back to the supermarket. This week, we’ll implement the code for a checkout system that handles pricing schemes such as “apples cost 50 cents, three apples cost $1.30.”

  • Kata 10: Hash vs. Class. Is it always correct to use (for example) classes and objects to structure complex business objects, or couple simpler structures (hash as Hashes) do the job?

  • Kata 11: Sorting it Out. Just because we need to sort something doesn’t necessarily mean we need to use a conventional sorting algorithm.

  • Kata 12: Best Sellers. Consider the implementation of a top-ten best sellers list for a high volume web store.

  • Kata 13: Counting Lines. Counting lines of code in Java source is not quite as simple as it seems.

  • Kata 14: Trigrams. Generating text using trigram analysis lets us experiment with different heuristics. (And it let’s us create new, original Tom Swift stories…)

  • Kata 15: Playing with bits. A diversion to discover the pattern in some bit sequences.

  • Kata 16: Business Rules. How can you tame a wild (and changing) set of business rules?

  • Kata 17: More Business Rules. The rules that specify the overall processing of an order can be complex too, particularly as they often involve waiting around for things to happen.

  • Kata 18: Dependencies. Let’s write some code that calculates how dependencies propagate between things such as classes in a program.

  • Kata 19: Word chains. Write a program that solves word chain puzzles (cat → cot → dot → dog).

  • Kata 20: Klondike. Experiment with various heuristics for playing the game Klondike.

  • Kata 21: Simple Lists. Play with different implementations of a simple list.

I have to admit that I’m nervous doing this. My hope is that folks will work on the kata for a while before discussing them; much of the benefit comes from the little “a-ha!” moments along the way. So, it’ll be interesting to see how (and if) the discussion develops.

CodeKata: How It Started


(This is a long one. It explains how I discovered that something I do almost every day to improve my coding is actually a little ritual that has much in common with practice in the martial arts…)

Kata, Kumite, Koan, and Dreyfus


A week or so ago I posted a piece called CodeKata, suggesting that as developers we need to spend more time just practicing: writing throwaway code just to get the experience of writing it. I followed this up with a first exercise, an experiment in supermarket pricing.

Kata02: Karate Chop


A binary chop (sometimes called the more prosaic binary search) finds the position of value in a sorted array of values. It achieves some efficiency by halving the number of items under consideration each time it probes the values: in the first pass it determines whether the required value is in the top or the bottom half of the list of values. In the second pass in considers only this half, again dividing it in to two. It stops when it finds the value it is looking for, or when it runs out of array to search. Binary searches are a favorite of CS lecturers.

Kata03: How Big? How Fast?


Rough estimation is a useful talent to possess. As you’re coding away, you may suddenly need to work out approximately how big a data structure will be, or how fast some loop will run. The faster you can do this, the less the coding flow will be disturbed.

Kata04: Data Munging


Martin Fowler gave me a hard time for Kata02, complaining that it was yet another single-function, academic exercise. Which, or course, it was. So this week let’s mix things up a bit.

Here’s an exercise in three parts to do with real world data. Try hard not to read ahead—do each part in turn.

Kata05: Bloom Filters


There are many circumstances where we need to find out if something is a member of a set, and many algorithms for doing it. If the set is small, you can use bitmaps. When they get larger, hashes are a useful technique. But when the sets get big, we start bumping in to limitations. Holding 250,000 words in memory for a spell checker might be too big an overhead if your target environment is a PDA or cell phone. Keeping a list of web-pages visited might be extravagant when you get up to tens of millions of pages. Fortunately, there’s a technique that can help.