Collections/Enumerables Deep dive - Part 1 of n

Introduction

In this series I am going to do a deep dive into the Ruby Enumerables module. I am going to talk about problem you will face in everyday coding and how you can solve them using Ruby enumerables. I believe Ruby Enumerables module is extremely powerful. Enumerables along with ActiveRecord querying according to me are the 2 most important tools for any Ruby/Rails developer. I am going to do another series for ActiveRecord deep dive.

Fasten your seat belts and take the dive.

Problem 1

Given an array of 1000 random numbers (rand_arr). Split the array into 2 arrays, odd_arr and even_arr. odd_arr will contain odd numbers while even_arr will contain even numbers. The challenge is to do it in a single ruby statement

Attempt 1

odd_arr = []
even_arr = []
rand_arr.each do |elem|
  odd_arr << elem if elem % 2 == 0
  even_arr << elem if elem % 2 == 1
end

Attempt 2

odd_arr = rand_arr.select{|elem| elem%2 ==0 }
even_arr = rand_arr.select{|elem| elem%2 ==0 }

Attempt 3

odd_arr = rand_arr.select{|elem| elem.odd?}
even_arr = rand_arr.select{|elem| elem.even?}

Attempt 4

odd_arr = rand_arr.select(&:odd?)
even_arr = rand_arr.select(&:even?)

Attempt 5

odd_arr,even_arr = rand_arr.partition(&:odd?)

Problem 2

You would want to create an array of random numbers, for trying out few of your ideas. So here is the easiest way to do it. Believe me whenever you want to create an array, you wouldn’t want to start typing an array of numbers, and even if you want to how many numbers can you type?

Array.new(100){rand(1..1000)}

If you want to create a series of natural numbers.

(1..30).to_a
#[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]

If you want to create numbers with a step

(0..30).step(2).to_a
#[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
(0..100).step(5).to_a
#[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100]

Alternatively,

3.step(30,3).to_a
#[3, 6, 9, 12, 15, 18, 21, 24, 27, 30]

And if you want to do more

(1..10).to_a.map{|n| 2**n}
#[2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]
(1..10).to_a.collect{|n| 2**n}
#[2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]

Problem 3

You have been given an array of strings(or numbers) and you want to find repetitions in the array. Also which string repeated itself how many times.

Lets generate some random numbers

arr = Array.new(20){rand(1..5)}
#[1, 1, 3, 3, 4, 2, 5, 3, 1, 2, 1, 5, 1, 1, 4, 5, 5, 5, 4, 1]

lets attempt this problem.

repetitions = {}
arr.each { |a|
    repetitions[a].nil? ? repetitions[a]=0 : repetitions[a]+=1
}
repetitions
# {5=>2, 2=>2, 4=>2, 3=>3, 1=>1}

A more elegant solution is

repetitions = Hash.new(0)
arr.each{|n| repetitions.store(n, repetitions[n]+1)}
repetitions
# {5=>2, 2=>2, 4=>2, 3=>3, 1=>1}

This is a good solutions, but lets attempt it in a different way.

arr.group_by{|n| n}.map{|k,v| {k=>v.count}}
#[{5=>2}, {2=>2}, {4=>2}, {3=>3}, {1=>1}]

What we have is an array of hashes, we can also get Array of arrays.

repetitions = arr.group_by{|n| n}.map{|k,v| [k,v.count]}
#[[5, 2], [2, 2], [4, 2], [3, 3], [1, 1]]

Now if we want to convert an array of arrays to a Hash

Hash[*repetitions.flatten]
#{5=>2, 2=>2, 4=>2, 3=>3, 1=>1}