From: marcandre-ruby-core@...
Date: 2020-12-16T15:47:24+00:00
Subject: [ruby-core:101467] [Ruby master Feature#17277] Make Enumerator#with_index yield row and col indices for Matrix

Issue #17277 has been updated by marcandre (Marc-Andre Lafortune).

Status changed from Assigned to Closed

Closing for lack of a viable solution

----------------------------------------
Feature #17277: Make Enumerator#with_index yield row and col indices for Matrix
https://bugs.ruby-lang.org/issues/17277#change-89242

* Author: greggzst (Grzegorz Jakubiak)
* Status: Closed
* Priority: Normal
* Assignee: marcandre (Marc-Andre Lafortune)
----------------------------------------
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, 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 indices.



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