Adam is a friend of mine learning to program. Every once in a while, he’ll send me a bit of code to look over to learn how he might be able to do it better. The latest one was an exercise involving splitting an array based on the parity of the number, that is to say whether the number is even or odd.

Can you check this over for me?

Method that takes an array as argument, counts and displays the odds and evens from the array. It works as expected—I’m just looking for best practices.

Here is Adam’s original code:

def numbereven(numarr)
  neven = 0
  nodd = 0
  narray = []
  narrayo = []

  numarr.each do |n|
    if n%2 == 0
      neven += 1
      nodd += 1

  puts "There are #{neven} even numbers and #{nodd} odd numbers in this array."
  print "Evens: ", narray, "\n"
  print "Odds: ", narrayo, "\n"

my_array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 1001]


I may have scared him off with the sheer volume of my reply. I wrote a bit how I would write it, another way to write it, and why I wouldn’t write it that way.

How I Would Write It

def parity_counts(numbers)
  evens = { |number| number.even? }
  odds  = numbers - evens

  puts "There are #{evens.size} even numbers and #{odds.size} " +
       "odd numbers in this array."
  puts "Evens: #{evens}"
  puts "Odds:  #{odds}"
  • An array knows its size, so there is no need to keep track of it separately.
  • Ruby integers have handy even? and odd? methods that return true or false. Under the hood, that method is doing exactly what you did: n % 2 == 0.1
  • Reducing if/else branches and loops helps the reader understand what is happening.
  • Yours might have better performance, especially as input size approaches infinity.
  • Your print statements ending in \n are fine, but you’ll more likely see people using puts with string interpolation (#{}) when they want to print a string ending in a new line.
  • In a real-world application, I would probably not put the screen output as part of the computation logic. You’d probably want to return a hash or other data structure and put the presentation logic somewhere else.

An Alternate Implementation

There is more than one way to skin a cat. Here’s an alternate implementation:

def parity_counts(ns)
  ns.reduce([[],[]]){|(e,o),n|n.odd? ? o<<n : e<<n;[e,o]}

This will return an array where the first element is an array of even numbers, and the second is an array of odd numbers. But there is very little about this solution that is best practice. Clarity should be preferred to brevity:

  • Variable names should reflect what is stored in them. Single letter names and even some abbreviations don’t make it immediately clear as to what it is to be used for.
  • reduce is a concept that is not easy to grasp. I use it sparingly when it really makes the most sense, rather than just because it makes me feel smart.
  • When you have more than one statement inside an iterator, it’s good style to use do/end instead of curly braces, and break up the statement into multiple lines. There are varying schools of thought on this:
    • Some people use curly braces when they are using the value returned from the iterator (e.g. give me the parent contact email for each student), and do/end when they are mutating state (e.g. take this list of students and calculate each one’s grade)
    • Other people say to always use curly braces for single line and use do/end for multiple lines. I follow this pattern; I think it looks nicer.

A Better Version of the Alternate Implementation

So, given the concepts above, using reduce, I would write it this way:

def parity_counts(numbers)
  numbers.reduce([[], []]) do |(evens, odds), number|
    number.odd? ? odds << number : evens << number
    [evens, odds]

The [[], []] sets up the data structure that we are going to pass to the first iteration to store the values in. Reduce takes the return value of the block and passes as the first argument to the next iteration. By putting parens around (even, odd), we can “unwrap” the array into separate arguments. It’s kind of a confusing concept to wrap your mind around, but once you get it, it’s incredibly handy for writing iterators.


In conclusion, a couple takeaways: First, Ruby has a huge standard library, along with helper methods to do common things. It’s usually a good idea to check out the documentation for a given class to see if there is already a method to do what you’re trying to do (even? and odd? from above, for example).

Second, with Ruby, it’s possible to write code that can almost be read as a sentence. Variable names and function names following this pattern make your code easier to understand and, in my opinion, much more fun to write.

UPDATE: How about Enumerable#partition?

There’s almost always a better way. Turns out, Ruby already has a method for partitioning an enumerable. In this implementation, we ask each element in the array if it is even. If so, it goes into the first partition. Otherwise, it goes into the second.

#=> [[2, 4, 6], [1, 3, 5]]

This is the shorthand syntax (using Symbol#to_proc—a post for another day, perhaps) that would be equivalent to writing

numbers.partition {|number| number.even? }

So to put this back in to solve the original problem:

def parity_counts(numbers)
  evens, odds = numbers.partition(&:even?)

  puts "There are #{evens.size} even numbers and #{odds.size} " +
       "odd numbers in this array."
  puts "Evens: #{evens}"
  puts "Odds:  #{odds}"
  1. The source for this function is in C, but here it is if you want to take a look at it.