Finding index of elements in an array of arrays

Not that fancy, but you could use two loops:

ary = [
  [0, 0, 0, 0],
  [0, 1, 0, 0],
  [0, 0, 0, 1],
  [0, 0, 0, 0]
]

result = []
ary.each_with_index do |row, i|
  row.each_with_index do |value, j|
    result << [i, j] if value == 1
  end
end
result
#=> [[1, 1], [2, 3]]

a = [
  [0,0,0,0],
  [0,1,0,0],
  [0,0,0,1],
  [0,0,0,0]
].flatten

a.each.with_index.select{|e, _| e == 1}.map{|_, i| i.divmod(4)}
# => [[1, 1], [2, 3]]

To be interpreted as (row 1, column 1) and (row 2, column 3).

Consider using the Matrix class.

require 'matrix'

arr = [
  [0, 0, 0, 0],
  [0, 1, 0, 0],
  [0, 0, 0, 1],
  [0, 0, 0, 0]
]    

target = 1

Matrix[*arr].each_with_index.with_object([]) { |(e,row,col),a|
  a << [row,col] if e==target }
  #=> [[1, 1], [2, 3]]

I like how this reads.

Ruby has a very flexible, functional style Enumerable module which is included in many of the standard collections (Arrays, Hashes, Sets). You can find the documentation at ruby-doc.org.

A nice one-liner to solve this in max #rows * 2 #columns time would be the following:

matrix
  .each.with_index
  .inject([]) { |acc, (el,idx)| el.include?(1) ? acc.push([idx, el.index(1)]) : acc }
# => [[1,1],[2,3]]

z = [
  [0,0,0,0],
  [0,1,0,0],
  [0,0,0,1],
  [0,0,0,0]
]

First, let's find the matching rows:

rows = z.each_with_index.select { |row, index| row.include? 1}.map(&:last)
# => [1, 2]

And then for each row let's find the index of the matching 1:

cols = rows.map {|row| z[row].each_with_index.select { |item, index| item == 1}.map(&:last) }
# => [[1], [3]]

If we want, we can then combine them using zip:

rows.zip(cols)
# => [[1, [1]], [2, [3]]]

The above works even if a row of z contains multiple occurrences of 1

For example if we had:

z = [
  [1,0,0,1],
  [0,1,0,0],
  [0,0,0,1],
  [1,1,1,0]
]

then

rows = z.each_with_index.select { |row, index| row.include? 1}.map(&:last)
# => [0, 1, 2, 3]
cols = rows.map {|row| z[row].each_with_index.select { |item, index| item == 1}.map(&:last) }
# => [[0, 3], [1], [3], [0, 1, 2]]
rows.zip(cols)
# => [[0, [0, 3]], [1, [1]], [2, [3]], [3, [0, 1, 2]]]