Chapter 6: FizzBuzz

Of course, the first thing that your job interview for that cushy new Crystal job will task you with is building FizzBuzz. Let’s do it!

If you’re not familiar, FizzBuzz is a simple programming problem:

“Write a program that prints the numbers from 1 to 100. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”.”

This will give us a good excuse to go over some basics of Crystal: Looping, tests, printing to standard output, and a host of other simple things.

First, let’s create our project.

$ crystal init app fizzbuzz

Let’s write our first failing test. Open up /spec/fizzbuzz_spec.cr

require "./spec_helper"

describe Fizzbuzz do
  it "shouldn't divide 1 by 3" do
    div_by_three(1).should eq(false)
  end
end

And run it:

$ crystal spec
Error in ./spec/fizzbuzz_spec.cr:7: undefined method 'div_by_three'

div_by_three(1).should eq(false)

This makes sense: We haven’t defined any method yet. Let’s define one:

require "./fizzbuzz/*"

def div_by_three(n)
  false
end

Akin to Ruby, the value of the last expression gets returned.

TDD means do the simplest thing! Now that we’ve defined our method, let’s compile and run our tests:

$  crystal spec
.

Finished in 0.82 milliseconds
1 examples, 0 failures, 0 errors, 0 pending

Awesome! We pass! Let’s write another test, and see what happens:

require "./spec_helper"

describe Fizzbuzz do
  it "shouldn't divide 1 by 3" do
    div_by_three(1).should eq(false)
  end

  it "should divide 3 by 3" do
    div_by_three(3).should eq(true)
  end
end

Run it!

$ crystal spec

.F

Failures:

  1) Fizzbuzz should divide 3 by 3
     Failure/Error: div_by_three(3).should eq(true)

       expected: true
            got: false

     # spec/fizzbuzz_spec.cr:9

Finished in 0.83 milliseconds
2 examples, 1 failures, 0 errors, 0 pending

Failed examples:

crystal spec ./spec/fizzbuzz_spec.cr:8 # Fizzbuzz should divide 3 by 3

We have 1 failure. Let’s make this pass.

require "./fizzbuzz/*"

def div_by_three(n)
  if n % 3 == 0
    true
  else
    false
  end
end

Run it.

$ crystal spec

..

Finished in 0.61 milliseconds
2 examples, 0 failures, 0 errors, 0 pending

Awesome! This shows off how else work, as well. It’s probably what you expected. Go ahead and try to refactor this into a one-liner.

Done? How’d you do? Here’s mine:

def div_by_three(n)
  n % 3 == 0
end

Remember, the value of the last expression gets returned.

Okay, now try to TDD out the div_by_five and div_by_fifteen methods. They should work the same way, but this will let you get practice actually writing it out. Once you see this, you’re ready to advance:

$ crystal spec -v

Fizzbuzz
  shouldn't divide 1 by 3
  should divide 3 by 3
  shouldn't divide 8 by 5
  should divide 5 by 5
  shouldn't divide 13 by 15
  should divide 15 by 15

Finished in 0.61 milliseconds
6 examples, 0 failures, 0 errors, 0 pending

Okay! Let’s talk about the main program now. We’ve got the tools to build FizzBuzz, let’s make it work. First thing we need to do is print out all the numbers from one to 100. It’s easy!

100.times do |num|
  puts num
end

Step one: print something 100 times. If you run this via crystal build src/fizzbuzz.cr && ./fizzbuzz you should see num printed 100 times. Note that our tests didn’t actually run. Not only are they not run, they’re actually not even in the executable:

Now we can put the two together:

100.times do |num|
  answer = ""

  if div_by_fifteen num
    answer = "FizzBuzz"
  elsif div_by_three num
    answer = "Fizz"
  elsif div_by_five num
    answer = "Buzz"
  else
    answer = num
  end

  puts answer
end

Because the if returns a value, we could also do something like this:

(1..100).each do |num|
  answer = if div_by_fifteen num
    "FizzBuzz"
  elsif div_by_three num
    "Fizz"
  elsif div_by_five num
    "Buzz"
  else
    num
  end

  puts answer
end

Notice that we also changed 100.times to (1..100).each, to make num go from 1 to 100 instead of from 0 to 99.

Try running it.

Awesome! We’ve conquered FizzBuzz.