[ruby-core:45688] [ruby-trunk - Bug #6596] New method for Arrays : Array#index

Issue #6596 has been updated by trans (Thomas Sawyer).


Yea, I would have written is completely different myself (even from yours). All I did was search GitHub for #each_with_index for code in which one could use #indexes. This was the first one I found. Unfortunately, as you can imagine, such a search is time consuming. Worse still GitHub's search turned out to have a bug and I could not go beyond page 100 of search results. There in lies one of the problem with "real" use case though --there's always dozens, if not more, way to do something. When you know you don't have a method at your disposal obviously you are going to think of ways to do it that won't require it.

So it can be very difficult to find a case until you are the one that realizes you could use it. And even when you do, lacking the method at hand, before any traction is made in any discussion like this you've already refactored in some fashion or anther to get things to work and have moved on. I know I've used a form of #indexes before but on a String, not an Array. And I remember being frustrated/surprised there was no method for it. But that was years ago now and I don't recall where it was. 

The fact is, sometimes a method makes sense not b/c you have a shining example at hand, but b/c it fills an obvious hole in functionality. We have #index to find the first index. We have #rindex to find the last index. So it becomes obvious to ask about all the other indexes in between. Why do we have to fallback to writing our own loops to get at those? While I agree, it won't find a great deal of use, there are many methods like that. And yet it's nice that they are around when you do need them.

In considering the nature of this method further, I think the most quintessential use would be when you have two arrays corresponding by index and you want to locate the elements of one array based on the positions of a select elements of another. e.g.

    a1 = [:a, :b, :c, :b, :c]
    a2 = [:x, :z, :z, :y, :y]

    g[:b] = a2.indicies(*a1.select_indexes(:b))
    g[:b] #=> [:z, :y]

Yea, yea. Not a "real" use case. We all know code like that exists (and it pretty common back in the days of procedural coding). But I am not going to spend days searching through source code for it.

BTW, I just notice Enumerable#find_index was added to the language (in 1.9.2?). So by analogy this could be Enumerable#select_index, or #select_indexes, right? So that why I used that in the last example.

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Bug #6596: New method for Arrays : Array#index
https://bugs.ruby-lang.org/issues/6596#change-27281

Author: robin850 (Robin Dupret)
Status: Feedback
Priority: Normal
Assignee: 
Category: core
Target version: 2.0.0
ruby -v: 2.0.0


Hello

5 days ago, I submitted a pull request on Github which provides a new method for the array objects which is Array#indexes. I have fist edit the Array#index method in order it to return an array of indexes and not a single index (which is the first occurrence it finds). I found it more logical but a user (trans) tells us that it could break the contract of Array#index so I decided to move it into Array#indexes. Eric (drbrain) tells me I should reasonning why I want to add this method ; it's just a point of view : I don't really understand why Array#index return a single index if the parameter is in the array several times. 

Examples

a = [1, 2, 3, 1]
a.indexes(1)
Return : [0, 3]
a.index(1)
Return : 0
In my opinion, it's not really logical, 1 is in the array twice

Moreover, this pull request doesn't beak anything because we don't edit the Array#index method so programms which were created with previous version of Ruby will work. 

I hope my post is complete. Have a nice day.




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