Is there a select and map for hashes instead of arrays?

Ruby
1

Hash#select and Hash#map return arrays. In some cases I want to get
hashes back instead.

I can take the returned arrays and transform them back into hashes,
but it would be nice to avoid the extra step.

Plus, it just surprised me that #select and #map aren’t overridden for
Hash to return new Hashes instead of arrays.

Am I missing something?

Jeff

2

use two vars in an map/select block (or use the index if one var)

array = {:a => 1, :b =>2}.select {|k,v| k == :b}

=> [[:b, 2]]

use inject to make a hash from a list, this is a great technique

hash = array.inject({}) {|h,e| h.merge({e[0] => e[1]}) }

=> {:b=>2}

-franco

3

On Sep 17, 8:35 pm, franco [email protected] wrote:

use two vars in an map/select block (or use the index if one var)

array = {:a => 1, :b =>2}.select {|k,v| k == :b}

=> [[:b, 2]]

use inject to make a hash from a list, this is a great technique

hash = array.inject({}) {|h,e| h.merge({e[0] => e[1]}) }

=> {:b=>2}

-franco

That was exactly my point - why doesn’t Hash#select and Hash#map
return hashes instead of arrays?

Jeff

4

Hi Jeff.
My guess is because you need to enumerate over a Hash. Recall that
entries
in a Hash are indexed by some Hashing algorithm, so it’s kind of had to
enumerate over it’s entries.

It’s probably cheaper to toss all the entries in a Hash into an array
and
iterate over it’s index than to try and divine some means of iterating
over
the non-sequential Hash table containing index values.

James

5

From: Jeff [mailto:[email protected]]

I can take the returned arrays and transform them back into hashes,

but it would be nice to avoid the extra step.

Plus, it just surprised me that #select and #map aren’t overridden for

Hash to return new Hashes instead of arrays.

Am I missing something?

none. just wait for 1.9
http://eigenclass.org/hiki.rb?Changes+in+Ruby+1.9#l81

i think this is an faq. in this regard, i’d like to request that the
ruby faq be on the frontpage of ruby-lang.org and ruby-doc.org and that
it be mirrored.

kind regards -botp

6

With some changes to undo top-posting:
On 9/18/07, James H. [email protected] wrote:

On 9/17/07, Jeff [email protected] wrote:

why doesn’t Hash#select and Hash#map
return hashes instead of arrays?

My guess is because you need to enumerate over a Hash. Recall that entries
in a Hash are indexed by some Hashing algorithm, so it’s kind of had to
enumerate over it’s entries.

There’s no problem enumerating over a Hash. Hash#each is implemented
after all. Enumerable only requires that each yield all of the
elements in SOME order, the fact that Hashes don’t have a natural
ordering doesn’t hamper this. Hash#each yields key,value pairs.

Ruby 1.9 is on track to change Hash#select to return a Hash.

Hash#map is problematic though, map returns a new enumeration which
contains the elements resulting in applying the block to each element
yielded by each. In general this could be anything for example:

{:a => "fred", :b => "barney"}.map { |k,v| "#{k} is associated

with #{v}" } # => [“a is associated with fred”, “b is associated with
barney”]

–
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

7

On Sep 17, 11:45 pm, Peña, Botp [email protected] wrote:

From: Jeff [mailto:[email protected]]
just wait for 1.9http://eigenclass.org/hiki.rb?Changes+in+Ruby+1.9#l81

Thanks for the info.

Jeff

8

Hey Jeff.
You’re in luck! This morning an article was posted to “Softies on
Rails” on
how to override select to get back a Hash:
http://www.softiesonrails.com/2007/9/18/ruby-201-weird-hash-syntax

James