Getting a "NoSuchMethodError" when trying to create an array of arrays - arrays

Question

I'm using Ruby 2.4. I'm having trouble creating an array of arrays. I have this
data_cols = [[], []]
lines.each do |line|
parts = [0, *shared_space_indexes, line.size].each_cons(2).map { |a, b| line[a...b].strip }
parts.each_with_index do |part, index|
data_cols[index][data_cols[index] ? data_cols[index].size : 0] = part
end
end
But I get a "NoMethodError: undefined method `[]=' for nil:NilClass" error on the "data_cols[index][data_cols[index] ? data_cols[index].size : 0] = part" line. What I'm trying to do is for each parts array, push each item in "parts" onto its own array correspoding to the index of that element in parts. So, for instance, if the first iteration of the loop has parts equal to
[1, 5, 8, 12]
I would have a data_cols array that looks like
[[1], [5], [8], [12]]
and if the next iteration of the loop had a parts array that looked like
[19, 20, 21, 22]
the data_cols array would then be changed to
[[1, 19], [5, 20], [8, 21], [12, 22]]

Answer 1

It's hard to test without knowing what lines and shared_space_indexes look like.
2 columns → a matrix
zip
For the second part of your question you can use :
[1, 5, 8, 12].zip([19, 20, 21, 22])
#=> [[1, 19], [5, 20], [8, 21], [12, 22]]
transpose
[[1, 5, 8, 12],[19, 20, 21, 22]].transpose
#=> [[1, 19], [5, 20], [8, 21], [12, 22]]
With transpose, both arrays need to have the same size.
3 columns → matrix
array1 = [1, 5, 8, 12]
array2 = [19, 20, 21, 22]
array3 = [99, 88, 77, 66]
p [array1, array2, array3].transpose
#=> [[1, 19, 99], [5, 20, 88], [8, 21, 77], [12, 22, 66]]
columns → rows
If you have an array of arrays :
arrays = [
[1, 5, 8, 12],
[19, 20, 21, 22],
[99, 88, 77, 66]
]
p arrays.transpose
#=> [[1, 19, 99], [5, 20, 88], [8, 21, 77], [12, 22, 66]]
So in your case, I'd just concentrate on creating an array of arrays with the same length, and use transpose at the end.
matrix + column → matrix
As you noticed, if you have
data_cols = [[1, 19], [5, 20], [8, 21], [12, 22]]
arr = [99, 88, 77, 66]
data_cols.zip(arr) would produce a weird half-nested array :
[[[1, 19], 99], [[5, 20], 88], [[8, 21], 77], [[12, 22], 66]]
You can use :
p data_cols.zip(arr).map(&:flatten)
#=> [[1, 19, 99], [5, 20, 88], [8, 21, 77], [12, 22, 66]]