Issue #5663 has been updated by Alexey Muranov.


It seems that in full generality this method needs to accept two blocks: one for selecting and one for mapping, but this would be an unusual syntax.
 
So how about a lazy `#selecting` first, which would store a block for selecting inside `enum`, and to make `#map` check if a block for selecting is defined, and to use it?  Maybe, instead of changing `#map`, a new variant of `#map` can be created that would take lazy operations into account. (For the name, i would propose `#partial_map` or `#map_partially`  ( http://en.wikipedia.org/wiki/Partial_function ), or `#map_selected`.)  Then the code would look like this:

enum.selecting { |i| i.even? }.map_selected { |i| i + 1 }

and only one loop will be needed.

I've read about gems defining lazy methods for Enumerable, but i do not remember if any of them is doing exactly this.

-Alexey.
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Feature #5663: Combined map/select method
http://redmine.ruby-lang.org/issues/5663

Author: Yehuda Katz
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


It is pretty common to want to map over an Enumerable, but only include the elements that match a particular filter. A common idiom is:

enum.map { |i| i + 1 if i.even? }.compact

It is of course also possible to do this with two calls:

enum.select { |i| i.even? }.map { |i| i + 1 }

Both cases are clumsy and require two iterations through the loop. I'd like to propose a combined method:

enum.map_select { |i| i + 1 if i.even? }

The only caveat is that it would be impossible to intentionally return nil here; suggestions welcome. The naming is also a strawman; feel free to propose something better.


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