Ruby Array to Histogram: How to group numbers by range?

Question

I'm trying to group an array of integers into an hash based on where the individual values fall in a range. Basically I want to convert an array to a fixed-width histogram.

Example:

values = [1,3,4,4,4,4,4,10,12,15,18]
bin_width = 3

I need to group the array values into a range-based historgram by where they fall into a 3-unit wide bucket like so:

{'0..2'=>[1,3],'3..5'=>[4,4,4,4,4],'6..8'=>[],'9..11'=>[10]....

Is there a simple one line solution ( maybe something like values.group_by{|x| #range calc}) that would work here?

Answer 1

values = [1, 7, 2, 8, 2]
values.group_by { |x| x / 3 }.map { |k, vs| [(3*k..3*k+2), vs] }.to_h
#=> {0..2=>[1, 2, 2], 6..8=>[7, 8]}

If you really need the empty ranges, I don't think a clean one-liner is possible. But this should do:

grouped = values.group_by { |x| x / 3 }
min, max = grouped.keys.minmax
(min..max).map { |n| [(3*n..3*n+2), grouped.fetch(n, [])] }.to_h
#=> {0..2=>[1, 2, 2], 3..5=>[], 6..8=>[7, 8]}

Answer 2

I came up with a rather inefficient but quite clear solution:

ranges = 0.step(values.max, bin_width).each_cons(2).map { |s, e| Range.new(s, e, true) }
values.group_by { |v| ranges.find { |r| r.cover? v } }