Feature #15143: Extend `Enumerable#to_h`

• Status: Closed
• Assignee: nobu (Nobuyoshi Nakada)
• Target version: -

[sawa (Tsuyoshi Sawada)]

Often, we call Array#to_h to the result of Enumerable#map:

(1..5).map{|x| [x, x ** 2]}.to_h
#=> {1=>1, 2=>4, 3=>9, 4=>16, 5=>25}

I am thinking of a feature to do this in a single method call.

Currently, Enumerable#to_h does not accept a block. I propose that, when Enumerable#to_h is called with a block (that has a subarray representing a key-value pair), return a hash that would be returned by applying the block to map, and to_h to the result:

(1..5).to_h{|x| [x, x ** 2]}
#=> {1=>1, 2=>4, 3=>9, 4=>16, 5=>25}

Ideally, I request this to be done internally to Ruby without creating an intermediate parent array.

[sawa (Tsuyoshi Sawada)]

I forgot to mention that people alternatively do:

(1..5).each_with_object({}){|x, h| h[x] = x ** 2}
#=> {1=>1, 2=>4, 3=>9, 4=>16, 5=>25}

which may be more optimal than calling map. The proposed feature also has the effect of making this technique unnecessary (although the proposal is not as effective as this in that the former creates intermediate subarrays).

[matz (Yukihiro Matsumoto)]

• Assignee set to nobu (Nobuyoshi Nakada)

Sounds reasonable. Accepted.

Matz.

[nobu (Nobuyoshi Nakada)]

• Is duplicate of Feature #10208: Passing block to Enumerable#to_h added

[nobu (Nobuyoshi Nakada)]

• Subject changed from Extend `Enumerrable#to_h` to Extend `Enumerable#to_h`

Have you changed the mind since #10208?

[mame (Yusuke Endoh)]

If the block returns an array whose length is > 2, how does it behave?

%w(aa1 ab2 ba3 bb4).to_h {|s| s.chars }
 #=> { "a" => { "a" => "1",
                "b" => "2" },
       "b" => { "a" => "3",
                "b" => "4" } }

I expect very useful trie generator :-)

[marcandre (Marc-Andre Lafortune)]

nobu (Nobuyoshi Nakada) wrote:

Have you changed the mind since #10208?

Or since... #666!

[sawa (Tsuyoshi Sawada)]

Sorry that I missed the previous duplicate proposals (I found that I had even commented on one of them, which I had forgotten about). But it looks like four years have passed since then, and the situation has changed perhaps. Or, it may have worth proposing again to indicate that many people want such feature.

I am grateful to Matz for accepting this.

[sawa (Tsuyoshi Sawada)]

mame (Yusuke Endoh) wrote:

If the block returns an array whose length is > 2, how does it behave?

%w(aa1 ab2 ba3 bb4).to_h {|s| s.chars }
 #=> { "a" => { "a" => "1",
                "b" => "2" },
       "b" => { "a" => "3",
                "b" => "4" } }

I expect very useful trie generator :-)

My original proposal is to follow the behaviour of the current map.{...}.to_h, which means to raise ArgumentError: wrong array length at 0 (expected 2, was 3).

However, mame's proposal to extend the feature also sounds interesting and good. Though, after having Matz' acceptance of this proposal, I am not sure whether we can still get another approval on top of it from him assuming mame's extension, or whether the extension should be another issue.

[matz (Yukihiro Matsumoto)]

I changed my mind when I approved to_h.

Matz.

[matz (Yukihiro Matsumoto)]

I don't agree with @mame's idea. The return value from the block should be strictly 2 elements array.

Matz.

[sawa (Tsuyoshi Sawada)]

Matz, thanks for confirmation. I am happy with my original proposal.

[nobu (Nobuyoshi Nakada)]

OK, not only Enumerable, but Array, Hash, ENV, NilClass and Struct too?

[nobu (Nobuyoshi Nakada)]

nil isn't an Enumerable, so it's irrelevant.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset trunk|r64794.

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Enumerable#to_h with block and so on

[Feature #15143]

[sawa (Tsuyoshi Sawada)]

nobu (Nobuyoshi Nakada) wrote:

OK, not only Enumerable, but Array, Hash, ENV, NilClass and Struct too?

Thanks for reminding me about that and implementing it.