Nicer syntax for looping through array of arrays in Ruby

Question

I have A, an array of arrays of length 3. For example A = [[a1, b1, c3], [a2, b2, c2], [a3, b3, c3], [a4, b4, c4]]. Right now I am looping over it like this:

A.each do |elem|
  puts(elem[0].foo)
  puts(elem[1].bar)
  puts(elem[2].baz)
end

Since I am using a lot of different properties in the loop, the code gets pretty messy and unreadable. Plus, the local name elem[0] isn't very descriptive. Is there a way to use something like this?

A.each do |[a,b,c]|
  puts(a.foo)
  puts(b.bar)
  puts(c.baz)
end

I'm pretty new to ruby I don't really know where to look for something like this.

Thanks, works perfectly. Out of curiosity, what is this kind of thing called? — Christian Doucette
– Christian Doucette, Commented Oct 7, 2020 at 15:44
Multiple assignment, several languages have this feature including ruby — Nick M
– Nick M, Commented Oct 7, 2020 at 16:21
The Ruby docs refer to it as array decomposition. Note that the size of the array must be known to contain three elements to express the block variables as |a,b,c|. — Cary Swoveland
– Cary Swoveland, Commented Oct 7, 2020 at 17:30
What @CarySwoveland said; however if the array contains more than 3 elements but a and b are the imperative, one can use array decomposition with the splat (*) operator as well such that |a,b,*c| (it is customary to use *rest in place of *c). In this case a and b will be the first and second elements of the Array and *c will contain 0 or more elements that identify the rest of the Array. — engineersmnky
– engineersmnky, Commented Oct 7, 2020 at 19:00

max · Accepted Answer · 2020-10-08 14:30:28Z

4

Its known as de-structuring (or decomposition in the Ruby docs):

A.each do |(a,b,c)|
  puts(a.foo)
  puts(b.bar)
  puts(c.baz)
end

You can also use a splat (*) if the number of elements is unknown which will gather the remaining elements:

[[1, 2, 3, 4], [5, 6, 7, 8]].each do |(first, *rest)|
  puts "first: #{first}"
  puts "rest: #{rest}"
end


[[1, 2, 3, 4], [5, 6, 7, 8]].each do |(first, *middle, last)|
  puts "first: #{first}"
  puts "middle: #{middle}"
  puts "last: #{last}"
end

edited Oct 8, 2020 at 14:30

answered Oct 7, 2020 at 20:51

max

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2 Comments

3limin4t0r Over a year ago

Note that the parentheses are optional if the block is yielded with one parameter that is an array. Meaning that A.each do |a,b,c| would also work here.

max Over a year ago

@3limin4t0r true. I still do like them as they give a heads up that there is de-structuring going on.

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