Implementing the *ect methods in terms of inject

April 18, 2008

Josh told me today that one way to see if someone is any good at Ruby is to have them implement the *ect methods in terms of inject. This isn't a fool-proof method, and I'm also not concerned about it's screening value as much as I am simply interested in how to do it. Most importantly, do I meet Josh's standards?

(As Nick pointed out, anyone who's spent more than 5 minutes functional programming would laugh at this. I'm a newbie.)

Before I show my code, I'm going to give you the opportunity to go try it for yourself. Write custom #detect, #select, #reject, and #collect methods in terms of Enumerable#inject.

Here's a little RSpec example that will let you automatically verify your implementation:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

describe Looper do
  before(:each) do
    @looper = Looper.new(1,2,3)
  end
  
  it "should detect" do
    @looper.detect {|i| i > 1 }.should == 2
  end

  it "should select" do
    @looper.select {|i| i % 2 == 1 }.should == [1,3]
  end

  it "should reject" do
    @looper.reject {|i| i % 2 == 1 }.should == [2]
  end

  it "should collect" do
    @looper.collect {|i| i * 2 }.should == [2,4,6]
  end
end

And now for my implementation:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

class Looper
  def initialize(*args)
    @items = args
  end

  def detect
    @items.inject(nil) {|found, i| found ||= i if yield(i); found }
  end

  def select
    @items.inject([]) {|matches, i| matches << i if yield(i); matches }
  end

  def reject
    @items.inject([]) {|matches, i| matches << i unless yield(i); matches }
  end

  def collect
    @items.inject([]) {|matches, i| matches << yield(i) }
  end
end

How did you do it?

blog comments powered by Disqus