Feature #17277
Updated by sawa (Tsuyoshi Sawada) about 4 years ago
Given a matrix:
```ruby
matrix = Matrix[[0,2,3,4], [6,7,8,9], [1,4,5,8]]
```
You could get the row and col indices of a matrix using `Matrix#each_with_index`:
```ruby
matrix
.each_with_index { |e, row, col| p [row, col] }
[0, 0]
[0, 1]
[0, 2]
[0, 3]
[1, 0]
[1, 1]
[1, 2]
[1, 3]
[2, 0]
[2, 1]
[2, 2]
[2, 3]
```
You can chain it with other enumerators and access indices within them:
```ruby
matrix
.each_with_index
.filter_map { |e, row, col| [row, col] if e % 4 == 0}
# => [[0, 0], [0, 3], [1, 2], [2, 1], [2, 3]]
```
Meanwhile, `with_index` after `Matrix#each` returns flattened indices, not row or column indices, seems to iterate over the elements of the internal array, which does not look right:
```ruby
matrix
.each.with_index { |e, index| p index }
0
1
2
3
4
5
6
7
8
9
10
11
```
I feel we should override `with_index` for `Matrix` so it returns row and column col indices.