Ruby divide an array of arrays by variable condition

April 3, 2022

Suppose I have an array like this:

arr=[["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"], 
     ["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"], ["1"], ["2"], 
     ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]

ie, a series of sublist (1 or more) of length 1 followed by a series of sublists (1 or more) of length greater than 1.

I want to divide that list into:

[[["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"]], 
 [["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"]], 
 [["1"], ["2"], ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]]

ie, the sub lists of length 1 combined with the following sublists of length greater than 1 into a single sublist of those sublists.

This works:

data = []

while arr.length > 0  
    tmp = []

    while arr.length > 0 && arr[0].length() == 1
        tmp << arr.shift 
    end

    while arr.length > 0 && arr[0].length() > 1
        tmp << arr.shift 
    end

    data << tmp 
end

p data
# [[["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"]], [["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"]], [["1"], ["2"], ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]]

But that seems super clumsy. Is there a .groupby or flip/flop or some form of Ruby enumerator I am missing to do this more easily?

>Solution :

As an improvement on @dawg’s answer, if the block we pass to :slice_when checks for the length of b being greater than the length of a:

data = arr.slice_when { |a, b| b.length < a.length }.to_a

Result is:

[[["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"]], 
 [["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"]], 
 [["1"], ["2"], ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]]

To make this more robust, we can check that we’re only doing this when b has length 1.

data = arr.slice_when { |a, b| b.length == 1 && b.length < a.length }.to_a

Now, if:

arr = [["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"], ["1", "3", "4"], ["1", "1"], 
       ["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"], ["1"], ["2"], 
       ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]

The result is:

[[["1"], ["1"], ["2", "2"], ["2.1", "0.8"], ["2.2", "0.2"], ["1", "3", "4"], ["1", "1"]], 
 [["1"], ["2", "3"], ["2", "0.8"], ["2", "0.4"]], 
 [["1"], ["2"], ["1", "0.8"], ["2", "0.4"], ["3", "0.3"]]]