[swift-users] Why can I not filter or map a dictionary to another dictionary?

Dave Abrahams dabrahams at apple.com
Thu Oct 27 12:29:54 CDT 2016

on Thu Oct 27 2016, Rick Mann <rmann-AT-latencyzero.com> wrote:

>> On Oct 26, 2016, at 22:23 , Dave Abrahams via swift-users <swift-users at swift.org> wrote:
>> on Wed Oct 26 2016, Rick Mann <swift-users-AT-swift.org> wrote:
>>> It seems fairly natural to want to do this:
>>> let bigDictionary = ...
>>> let smallerDictionary = bigDictionary.filter { key, value in <some test returning Bool> }
>>> Similarly, it seems natural to want to map this way.
>> It's reasonable for filter, but maybe not for map.  Map follows certain
>> laws, and one of them is that you get the same number of elements out as
>> you put in.  But you can easily map all keys to the same key, and the
>> law would be violated.
> Sure map would produce a dictionary of the same number of entries. I
> didn't mean to imply map would also filter. 

I think you're missing my point.  What result do you expect from

  {1:2, 3:4}.map { _ in (1, 1) }

? How many elements does it have?  Note that you can't have the same key
twice in a Dictionary.

> But I'm basically transforming a JSON dictionary into a new version of
> that dictionary with fewer elements. I'd like to first filter it, then
> map it, and have the result be a dictionary.

What we need are Dictionary initializers that operate on Sequences, per

    Dictionary(merging: d.lazy.filter {... }.map {...})

That would get you the result you're looking for in a principled way,
without creating an intermediate Array.


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