[ruby-core:114343] [Ruby master Feature#19830] Allow `Array#transpose` to take an optional size argument

Issue #19830 has been updated by nobu (Nobuyoshi Nakada).


Why not `ary.transpose[1]&.join`?

----------------------------------------
Feature #19830: Allow `Array#transpose` to take an optional size argument
https://bugs.ruby-lang.org/issues/19830#change-104079

* Author: tomstuart (Tom Stuart)
* Status: Open
* Priority: Normal
----------------------------------------
One benefit of supplying an initial value to `Enumerable#inject` is that it avoids an annoying edge case when the collection is empty:

```
>> [1, 2, 3].inject(:+)
=> 6 # good

>> [].inject(:+)
=> nil # bad

>> [].inject(0, :+)
=> 0 # good
```

A similar edge case exists for `Array#transpose`:

```
>> [[1, :a], [2, :b], [3, :c]].transpose
=> [[1, 2, 3], [:a, :b, :c]] # good

>> [].transpose
=> [] # bad
```

Although no explicit `nil` is produced here, the subtle problem is that the caller may assume that the result array contains arrays, and that assumption leads to `nil`s in the empty case:

```
>> [[1, :a], [2, :b], [3, :c]].transpose.then { _2.join }
=> "abc"

>> [].transpose.then { _2.join }
undefined method `join' for nil:NilClass (NoMethodError)
```

If we allow `Array#transpose` to take an optional argument specifying the size of the result array, we can use this to always return an array of the correct size:

```
>> [[1, :a], [2, :b], [3, :c]].transpose(2)
=> [[1, 2, 3], [:a, :b, :c]] # good

>> [].transpose(2)
=> [[], []] # good
```

By avoiding an unexpectedly empty result array, we also avoid unexpected downstream `nil`s:

```
>> [[1, :a], [2, :b], [3, :c]].transpose(2).then { _2.join }
=> "abc"

>> [].transpose(2).then { _2.join }
=> ""
```

Here is a patch which adds an optional argument to `Array#transpose` to support the above usage: https://github.com/ruby/ruby/pull/8167

Something similar was requested eleven years ago in #6852. I believe this feature addresses the problem expressed in that issue without compromising backward compatibility with existing callers of #transpose.



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