Transposing rows to columns (with limitations on what methods I can use) - ruby

Question

I'm doing a practice problem and the question is to transpose rows to columns in Ruby.
I understand while loops are "rookie-esque" in Ruby, but I think they prefer I use basic methods and control flow statements: while, each, map. Obviously, I can't use Array#transpose. They want me to basically write Array#transpose from scratch.
It keeps telling me:
undefined method `[]=' for nil:NilClass (NoMethodError)
Here is my code:
def transpose(rows)
idx1 = 0
cols = []
while idx1 < rows.count
idx2 = 0
while idx2 < rows[idx1].count
cols[idx1][idx2] = rows[idx2][idx1]
idx2 += 1
end
idx1 += 1
end
return cols
end
puts transpose(rows = [
[0, 1, 2],
[3, 4, 5],
[6, 7, 8]
])

Answer 1

Perhaps this will help you:
def transpose(rows)
idx1 = 0 # => 0
cols = [] # => []
while idx1 < rows.count # => true
idx2 = 0 # => 0
while idx2 < rows[idx1].count # => true
cols[idx1][idx2] = rows[idx2][idx1] # ~> NoMethodError: undefined method `[]=' for nil:NilClass
idx2 += 1
end
idx1 += 1
end
return cols
end
puts transpose(rows = [
[0, 1, 2], # => [0, 1, 2]
[3, 4, 5], # => [3, 4, 5]
[6, 7, 8] # => [6, 7, 8]
])
# ~> NoMethodError
# ~> undefined method `[]=' for nil:NilClass
That was created using "Seeing Is Believing" in Sublime Text 2.
Breaking it down, here's where you're going wrong:
cols = [] # => []
cols[0] # => nil
cols[0][0] # ~> NoMethodError: undefined method `[]' for nil:NilClass
You can't use a sub-index on a nil. The first index doesn't exist in the empty array.

Answer 2

In your outer while loop you need to make cols[idx1] an empty array, else it is nil in your inner while loop:
while idx1 < rows.count
cols[idx1] = []
idx2 = 0
while idx2 < rows[idx1].count
# ...
end
end

Answer 3

You could do that using Array#new with a block:
Array.new(rows.size) { |i| Array.new(rows.size) { |j| rows[j][i] } }
#=> [[0, 3, 6], [1, 4, 7], [2, 5, 8]]
Alternatively, you could use methods from the class Matrix, for which you need:
require 'matrix'
Here's an easy way that does not use Array#transpose, though it may not satisfy the spirit of the question:
Matrix[*rows].transpose.to_a
#=> [[0, 3, 6], [1, 4, 7], [2, 5, 8]]
However, you could do this:
Matrix.build(rows.size) { |i,j| rows[j][i] }.to_a
#=> [[0, 3, 6], [1, 4, 7], [2, 5, 8]]

Answer 4

Another way to do this for comparison using nested Enumerable#maps:
rows = [[0, 1, 2],
[3, 4, 5],
[6, 7, 8]]
def my_transpose(matrix)
matrix.first.size.times.map { |column| matrix.map { |row| row[column] } }
end
my_transpose(rows)
# => [[0, 3, 6], [1, 4, 7], [2, 5, 8]]
my_transpose(my_transpose(rows))
# => [[0, 1, 2], [3, 4, 5], [6, 7, 8]]