appending the outputs of an array method into another array

Question

array3 =['Maths', 'Programming', 'Physics'], ['Maths', 'Intro to comp. science', 'Programming'], ['English', 'Intro to comp. science', 'Physics']
course = 'Programming'    
index = []
array3.find_index do |i|
  if array3.include?(course) == true
  index << i
  end
end

i created an array (array3) that contains the respective elements and i want to add the elements of array3 which hold the condition true but after executing the code i get a blank array like "[[], [], []]" how can i fix this issue?

Tip: In Ruby x == true is almost always better written as x. — tadman
– tadman, Commented Dec 27, 2022 at 14:16
Hint: This is really just index. The find_index name is misleading. Additionally the argument to the index block is not i as in index, but the value. It returns the index. — tadman
– tadman, Commented Dec 27, 2022 at 14:18
Do you want to add the elements, or the index of the elements? — tadman
– tadman, Commented Dec 27, 2022 at 14:19
Defining array3 like that is problematic for two reasons. 1. It should be array3 = [ [ ... ], ... ] where it's clear that's a 3 part array. 2. The name is terrible. Call it courses or something meaningful. — tadman
– tadman, Commented Dec 27, 2022 at 14:20
Or as it stands maybe course_lists is a fitting name. This also helps the reader to understand that they're working with a nested arrays. — 3limin4t0r
– 3limin4t0r, Commented Dec 27, 2022 at 14:23

engineersmnky · Accepted Answer · 2022-12-27 14:55:59Z

2

find_index does not iterate over indices. It iterates over values and returns the first index of the value that matches. It sounds like you want to iterate over every element, making note of all of the indices that match some condition.

To that end, you can use each_with_index.

index = []
array3.each_with_index do |courses, i|
  if courses.include?(course) == true
  index << i
  end
end

or you can use each_index and filter the results.

index = array3.each_index.select { |index| array3[index].include? course }

or, filtering with each_with_index,

index = array3.each_with_index
        .select { |list, _| list.include? course }
        .map(&:last)

on Ruby 2.7 or newer, you can shorten this with filter_map.

index = array3.each_with_index
              .filter_map { |obj, index| index if obj.include? course }

edited Dec 27, 2022 at 14:55

engineersmnky

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answered Dec 27, 2022 at 14:21

Silvio Mayolo

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7 Comments

tadman Over a year ago

The second form is more typical Ruby and it works great.

tadman Over a year ago

Though I think the second form is missing the right arguments. It should be each_with_index and then |list, index| as that'd make it easier to do tests: list.include?(course).

Silvio Mayolo Over a year ago

@tadman I don't either, to be honest. And I'm surprised Ruby doesn't have a find_all_indices method built-in. If it did, this would become incredibly simple: array3.find_all_indices { |list| list.include? course }, but sadly such a method does not appear to exist.

tadman Over a year ago

Great thing about Ruby is if you don't like that, you can just patch it in!

engineersmnky Over a year ago

@SilvioMayolo It does now have filter_map so array3.each_with_index.filter_map {|list,i| i if list.include? course } but still a bit of a mouthful in comparison.

|

thiebo · Accepted Answer · 2022-12-27 14:30:18Z

0

You can achieve this with each

array3 =['Maths', 'Programming', 'Physics'], ['Maths', 'Intro to comp. science', 'Programming'], ['English', 'Intro to comp. science', 'Physics']
course = 'Programming'    
index = []
array3.each do |i|
    if i.include?(course)
        index << i
    end
end

answered Dec 27, 2022 at 14:30

thiebo

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2 Comments

Dave Newton Over a year ago

Did you run this?

thiebo Over a year ago

yes of course. In irb. And in the end index contains the 2 elements of the array.

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