Append the results of a Ruby .map into an array?

Question

I'm trying to build a items array. I'm looping through a CSV file and when i find matches expanding the list of items. See code below:

items = Array.new
csv.each_with_index do |row,i|
   items << ["a","b","c"].map {|x| row.to_s.gsub(/XXXXXXXXXX/, x.downcase)}
end
puts items.length

This is not returning to desired items array. Am I appended the results of map incorrectly to the array?

You don't use i in your example. Is it because it's just a simpler example? — Eric Duminil
– Eric Duminil, Commented Jan 14, 2017 at 21:40
Show your current result and what you would like to have as a result. — user2012677
– user2012677, Commented Jan 14, 2017 at 22:17

max pleaner · Accepted Answer · 2017-01-14 20:39:36Z

6

Yes, you are doing it incorrectly here:

items << ["a","b","c"].map

Items would end up as a nested array. Here's an example of what's happening here:

arr = []
arr << [1].map { |x| x }
arr
# => [[1]]

Instead, you can use +=.

You can also use push if you use the splat operator:

arr = []
arr.push *["a","b","c"].map { |x| x }
arr
# => ["a", "b", "c"]

answered Jan 14, 2017 at 20:39

max pleaner

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3 Comments

AnApprentice Over a year ago

perfect! Than you. I didn't know about the * what does that do exactly? I'll accept as soon as the time passes and stackoverflow lets me

max pleaner Over a year ago

it has a few uses, see here. In this case push accepts sequential arguments, so push(:a, :b) == push(*[:a, :b]) == push(*%i{a b}). The last one uses a shorthand to construct an array of symbols.

mu is too short Over a year ago

There's also Array#concat: arr.concat %w[a b c].map { |x| x }

Eric Duminil · Accepted Answer · 2017-01-14 21:47:53Z

1

As a rule of thumb, if you're :

initializing an empty Array
iterating over elements, modifiying the array
returning the Array at the end

There's a Ruby Enumerable method to help you!

In this case, you could use flat_map and with_index :

csv = %w(1;2;3 4;5;6 7;8;9)

items = csv.flat_map.with_index do |row,i|
   ["a","b","c"].map {|x| row.to_s.gsub(/[159]/, x.downcase)}
end

p items
#=> ["a;2;3", "b;2;3", "c;2;3", "4;a;6", "4;b;6", "4;c;6", "7;8;a", "7;8;b", "7;8;c"]

answered Jan 14, 2017 at 21:47

Eric Duminil

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