Archive for the 'Design Exercise' Category

Exercise: a Pesky Error Message

Tuesday, August 28th, 2012

We have some files containing sequences of integers, one number per line. We want to compute the sum of the numbers. The catch is that the numbers are written as roman numerals.

This code works, but there is a problem: every time we run the tests, a nasty error message appears on standard error. We’d like to be able to turn off the error message when we run the tests. The error message should still appear when the code is run in production.

Your job: make it so that the error message does not appear when the tests are run.

See here the starting code. Copy it in a file pesky.rb and run it with the commmand ruby pesky.rb.

class RomanNumeral
  def initialize(string)
    @value = string

  def to_i
    case @value
    when "I"; 1
    when "II"; 2
    when "III"; 3
    when "IV"; 4
      STDERR.puts("Invalid numeral '#{@value}'")

class RomanSequence
  def initialize(input)
    @input = input

  def sum
    result = 0
    @input.lines.each do |line|
      roman =
      result += roman.to_i
    return result

require "minitest/autorun"
require "stringio"

class RomanSequenceTest < MiniTest::Unit::TestCase
  def test_adds_roman_numerals
    input ="I\nII\nIII\n")
    assert_equal 6,

  def test_skips_invalid_numerals
    input ="I\nY\nIII\n")
    assert_equal 4,


Solve the exercise before continuing.

*            *


Anti-FOR tips from the Yahtzee Kata

Saturday, May 14th, 2011

Again on the Kata Yahtzee, that I blogged about some time ago.

If you have not solved the kata at least once, please stop reading this! Get back when you have.

*         *

Good to see you again! Now that you solved it, you probably know that the naive solution takes many “for” loops. Let D be the player dice, represented as an array of die results, e.g., D=(1,6,1,6,4). The naive rules for sixes would be

    def sixes_score 
      sum = 0
      for d in D
        if d == 6
          sum += 6
      return sum

This solution involves searching for sixes and adding up. Why do we need to search? We need to search because there are many different D that are worth exactly the same for the sixes rule. For instance, both D=(1,2,3,6,6) and D=(6,6,1,2,3) are worth 12.


The geometry lab — an exercise

Tuesday, June 15th, 2010

Last week I was traning a team on XP techniques. We tried the following exercise:

I want you people to build me a Swing application that computes the area of a square with a given side length.

I asked for an estimate. The devs were nervous, someone said “impossible!” :-) someone said 5 hours. I played the part of the project-manager-who-was-once-a-developer and said “come on, five hours?? I could do that in 10 minutes in my sleep. What’s so difficult about it”? Then I reasoned with them that if we keep our estimates too comfortable, our business opportunities may fly out of the window. They agreed on a 2 hours estimate.

They proceeded to implement the feature. The three devs rotated every 7 minutes. This was a good slot size; everyone was involved, even the junior one who is rarely given the keyboard. The feature was done in one and a half hour. Then I said

Cool. Now we need to compute the area of a triangle of a given base and height. How much time for this?

The devs estimated 1.5 hours. It was delivered on time. Now the fun part started. The team wrote the application in the “usual” way, by writing new code for the new window. No effort was spent, at this time, to reduce duplication. I pointed out that

We’re going to need to implement many more of these geometry formulae. Make it so that it is trivial to add others.

The team came up with a design where the Swing window object is generic and can be customized to support the input for any formula that requires a variable number of inputs with different names. They thought they could do it in 2 hours. It took 4. At some point we wasted a lot of time on Swing layouts, trying to fathom the mysteries of GroupLayout. I gave some help here. Then we were done! Stepping again in my role of customer I said

Very well. The next feature we need is to compute the area of a circle from the radius.

It was done in 10 minutes. The customer was very satisfied, and so were the devs.

What have we learned?

  • I have learned the power of letting the team come up with their own design. It’s difficult for me, an xp-trainer-who-was-once-a-developer, to give up giving guidance on design. But time and again, I have seen the damage of doing so: the team follows my design, gets bogged down, does not learn.
  • We have learned how hard it is to make the code easy to change. It would have been easy to declare we were “done” after the area of triangle was working. But we were not really “done” from the point of view of TDD. Remember, the cycle is red-green-REFACTOR, and by “refactor” what is really meant is “remove duplication”.
  • Once you get to clean, refactored code, the cost of changes drops. And it’s a pleasure to work with!
  • The decision to invest time in making the code generic might seem difficult. After all, you can get skilled at copy-pasting Swing code and writing many copies of the Swing form class. But then you are left with gobs of code. And good luck applying a different graphic layout to them all! My answer is that we should get skilled at writing flexible code. It took us 4 hours to make the code generic. Next time they have to do something similar, it will take less.

    Copying-and-pasting is a dead end; there is a limit at how skilled you can become at it, and there is certainly a big problem in the quality of the code you deliver. Learning to do good, clean, flexible code never ends. It’s a path where you can get to write better and better code. Which path would you rather be on?

Very cool, guys. This geometry app rocks!!

Design problem #2

Monday, June 14th, 2010

This is a subset of the Back to the Checkout kata by Dave Thomas, which I used many times as a TDD training exercise.

Suppose you have a PriceRules object that knows that the prices of items. Its responsibility is to know the following table:

Item Unit Price Special Price
A 50 3 for 130
B 30 2 for 45
C 20
D 15

Then you have a Cart object that knows which items a customer is trying to buy. For instance, a given cart could contain the list [A, A, C, A, D].

The problem is to compute the total that the customer has to pay, out of a collaboration between (at least) the cart and the pricerRules objects. It seems easy, but there is a catch: you are forbidden to use getters. All methods must return “void”, in Java terms. Design the messages that are exchanged between the objects and produce the desired result. (In the example, it would be 165). Have fun!

Software Design problems, anyone?

Sunday, June 13th, 2010

You learn math by solving problems. Problems frame the way you learn, give you a tangible proof that you’re progressing, give you a sense of meaning and achievement. How do you learn physics? By studying the books of course, but then solving physics problems is very important. How do you learn to play deep games such as chess or go? By playing, mostly. And then by pondering and solving problems. Electronics? Chemistry? Building science? Genetics? The books on these subjects are full of problems.

How do you learn good software design? I don’t know. The books that I’ve read explain principles, and provide examples. Rarely I’ve seen books that contain problems, exercises, or challenges. (Notable exceptions: William Wake’s Refactoring Workbook and Ka Iok Tong’s Essential Skills for Agile Development.)

I propose that we assemble a collection of problems meant to develop and discuss software design. A good problem for this goal would problem *not* have a single correct answer, for design and engineering are always a matter of compromises. A good problem should be a means to discuss the various choices and tradeoff, and worse and better ways to solve it. A good problem should be a small framework.

Let’s start! Here is a problem that I find interesting. The good old Fizz-Buzz problem goes like this:

Write a program that prints the numbers in order from 1 to 100, with the exception that when a number is a multiple of 3, it prints “Fizz”. When a number is a multiple of 5, it prints “Buzz”. And when a number is multiple of both, it prints “FizzBuzz”. In all other cases, it just prints the decimal representation of the number.

There is an obvious way to solve this exercise, of course. It’s a very simple problem, from the point of view of programming. I would have the student solve it however they like. Most solutions contain a 3-way IF. I would then ask students to remove duplication. Early XP books were strong on removing duplication, for a good reason. It takes a bit of training to see how much duplication can creep in even such a small bit of programming.

The usual objection I get at this point is that it makes no sense to go this deep in removing duplication for such a small and trivial example. They also will say that the 3-IFs version is more readable than any version where duplication is removed. This is the crux of the matter.

I then continue the exercise by adding the requirement that

For multiples of 7, the program prints “Bang”.

Easy, they say. Add a fourth IF. Not so fast, I say :-)

For multiples of 7 and 3, the program prints “FizzBang”. For multiples of 5 and 7, the program prints “BuzzBang”. For multiples of 3, 5, 7, the program prints “FizzBuzzBang”!

Now we have an exploding number of IFs. If the next requirement is of the same sort as this one, we see how the IF-chain solution becomes untenable :-) Now solve this!


  • I got the idea of using FizzBuzz as a design example from Giordano Scalzo, who presented it at the Milano XPUG and posted a solution on slideshare
  • Other sources of problems, in no particular order: the Refactoring to patterns book by Joshua Kerievsky. The list of katas by Dave Thomas. The Refactoring in Ruby book by William Wake and Mike Rutherford. The Ruby Quiz site. I’m not merely looking for programming problems. I’m looking for design problems. The difference is that I don’t just want a problem that requires a correct or efficient solution. I want a problem that requires a solution that is easy to understand and change.