Is there a way to group_by with_index in Crystal? - crystal-lang

Question

So I have this (nicely sorted) array.
And sometimes I need all of the elements from the array. But other times I need all of the even-indexed members together and all of the odd-indexed members together. And then again, sometimes I need it split into three groups with indices 0,3,6 etc. in one group, then 1,4,7 in the next and finally 2,5,8 in the last.
This can be done with group_by and taking the modulus of the index. See for yourself:
https://play.crystal-lang.org/#/r/4kzj
arr = ['a', 'b', 'c', 'd', 'e']
puts arr.group_by { |x| arr.index(x).not_nil! % 1 } # {0 => ['a', 'b', 'c', 'd', 'e']}
puts arr.group_by { |x| arr.index(x).not_nil! % 2 } # {0 => ['a', 'c', 'e'], 1 => ['b', 'd']}
puts arr.group_by { |x| arr.index(x).not_nil! % 3 } # {0 => ['a', 'd'], 1 => ['b', 'e'], 2 => ['c']}
But that not_nil! in there feels like a code-smell / warning that there's a better way.
Can I get the index of the elements without needing to look it up and handle the Nil type?

Answer 1

You can also just do:
arr = ['a', 'b', 'c', 'd', 'e']
i = 0
puts arr.group_by { |x| i += 1; i % 1 }
i = 0
puts arr.group_by { |x| i += 1; i % 2 }
i = 0
puts arr.group_by { |x| i += 1; i % 3 }

Answer 2

Besides the nilable return type, it's also very inefficient to call Array#index for each element. This means a runtime of O(N²).
#group_by is used for grouping by value, but you don't need the value for grouping as you just want to group by index. That can be done a lot easier than wrapping around #group_by and #index
A more efficient solution is to loop over the indices and group the values based on the index:
groups = [[] of Char, [] of Char]
arr.each_index do |i|
groups[i % 2] << arr[i]
end
There is no special method for this, but it's fairly simple to implement yourself.
If you don't need all groups, but only one of them, you can also use Int32#step to iterate every other index:
group = [] of Char
2.step(to: arr.size - 1, by: 3) do |i|
group << arr[i]
end