# Hash#map **URL:** \ **Category:** Ruby\ **Created:** [April 29, 2009, 8:04am UTC](https://www.ruby-forum.com/t/hash-map/166752 "2009-04-29T08:04:32Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 8:04am UTC](https://www.ruby-forum.com/t/hash-map/166752/1 "2009-04-29T08:04:32Z")
Is there any good reason why Hash#map does not give back a Hash (Ruby 1.8 but same in 1.9 as far as I know)? I often find myself writing these kind of things: newhash = oldhash.inject({}) { |h,(k,v)| h[k] = some\_operation(v); h } but that doesn’t look pretty at all in my opinion. I want to just write like this: newhash = oldhash.map { |k,v| some\_operation(v) } I finally got around to change this behaviour for my own code, but are all Ruby users supposed to invent this wheel on their own? Wouldn’t it be better if Hash#map behaved like this? Or is there something I am missing? class Hash def hashmap self.inject({}) do |newhash, (k,v)| newhash[k] = yield(k, v) newhash end end end Regards, Fredrik ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 8:14am UTC](https://www.ruby-forum.com/t/hash-map/166752/2 "2009-04-29T08:14:31Z")
On Wed, Apr 29, 2009 at 8:00 AM, Fredrik [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > end > end > end > > Regards, > Fredrik Becuase #map comes from form Enumerable (and is an alias for collect) enum.collect {| obj | block } =\> array enum.map {| obj | block } =\> array Returns a new array with the results of running block once for every element in enum. (1..4).collect {|i| i\*i } #=\> [1, 4, 9, 16] (1..4).collect { “cat” } #=\> [“cat”, “cat”, “cat”, “cat”] Andrew T. [http://ramblingsonrails.com](http://ramblingsonrails.com) [http://www.linkedin.com/in/andrewtimberlake](http://www.linkedin.com/in/andrewtimberlake) “I have never let my schooling interfere with my education” - Mark Twain ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 8:21am UTC](https://www.ruby-forum.com/t/hash-map/166752/3 "2009-04-29T08:21:01Z")
irb(main):041:0\> o={“a”,1,“b”,2} =\> {“a”=\>1, “b”=\>2} irb(main):042:0\> o.merge(o){|k,ov|ov\*2} =\> {“a”=\>2, “b”=\>4} ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 8:30am UTC](https://www.ruby-forum.com/t/hash-map/166752/4 "2009-04-29T08:30:28Z")
class Hash def new\_map result = self.map do |k,v| r = yield v [k,r] end Hash[\*result.flatten] end end a = {} a[2] = 4 a[3] = 6 b=a.new\_map do |v| v+2 end puts b.class puts b.inspect yields: Hash {2=\>6, 3=\>8} Does this help? Jayanth ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 2:40pm UTC](https://www.ruby-forum.com/t/hash-map/166752/5 "2009-04-29T14:40:17Z")
On Apr 29, 2009, at 1:00 AM, Fredrik wrote: > Is there any good reason why Hash#map does not give back a Hash (Ruby > 1.8 but same in 1.9 as far as I know)? It wouldn’t make sense for Hash#map to return a Hash since you are not required to transform it into key-value pairs: > > {1 =\> “one”, 2 =\> “two”}.map { |k, v| “#{k} is #{v}” } > > =\> [“1 is one”, “2 is two”] > I often find myself writing > these kind of things: > > newhash = oldhash.inject({}) { |h,(k,v)| h[k] = some\_operation(v); h } 1.9 adds each\_with\_object() for this very usage. You don’t want the return value of the block to carry forward, but instead the object you started with. That’s what each\_with\_object() does. So your code becomes the following in Ruby 1.9: newhash = oldhash.each\_with\_object({}) { |(k, v), h| h[k] = some\_operation(v) } Hope that helps. James Edward G. II ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 29, 2009, 8:35am UTC](https://www.ruby-forum.com/t/hash-map/166752/6 "2009-04-29T08:35:24Z")
Fredrik wrote: > Is there any good reason why Hash#map does not give back a Hash (Ruby > 1.8 but same in 1.9 as far as I know)? I often find myself writing > these kind of things: > > newhash = oldhash.inject({}) { |h,(k,v)| h[k] = some\_operation(v); h } > > but that doesn’t look pretty at all in my opinion. I want to just > write like this: > > newhash = oldhash.map { |k,v| some\_operation(v) } class Hash def hashbackmap result = {} ``` self.each do |key, val| result[key] = yield val end result ``` end end h = { “red” =\> 10, “blue” =\> 20, “green” =\> 30 } p h.hashbackmap {|x| x\*2} –output:– {“green”=\>60, “blue”=\>40, “red”=\>20} I HAVE A PATENT!! My licensing fees are cheap: $10 mth. ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 30, 2009, 1:10am UTC](https://www.ruby-forum.com/t/hash-map/166752/7 "2009-04-30T01:10:34Z")
> Becuase #map comes from form Enumerable (and is an alias for collect) Well ok, that is a reason but it’s not a good motivation why it HAS to be like that. What use does one really have of getting an Array (with elements in an undefined order) from a Hash#map ? ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 30, 2009, 1:21am UTC](https://www.ruby-forum.com/t/hash-map/166752/8 "2009-04-30T01:21:06Z")
> irb(main):041:0\> o={“a”,1,“b”,2} > =\> {“a”=\>1, “b”=\>2} > irb(main):042:0\> o.merge(o){|k,ov|ov\*2} > =\> {“a”=\>2, “b”=\>4} Thanks! That’s the piece missing from my Ruby skills! 🙂 Though o.merge (o) doesn’t look as pedagogically clear as o.map, it’ll have to do. ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 30, 2009, 5:28am UTC](https://www.ruby-forum.com/t/hash-map/166752/9 "2009-04-30T05:28:06Z")
On 29 Apr, 21:39, James G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > newhash = oldhash.each\_with\_object({}) { |(k, v), h| > h[k] = some\_operation(v) > > } > > Hope that helps. > > James Edward G. II Thanks! It’s not a very big improvement over inject though. I don’t see the use of Hash#map → Array so if it was up to me, Hash#map would give a Hash. If one really wants an Array it seems more reasonable to use something like hash.to\_a{|k,v| “#{k} is #{v}” } Maybe it’s just me, but when I read “map” I am thinking “map this block to each element and give back the same structure” (i.e. Array#map gives an Array and Hash#map gives a Hash). (I know map is just an alias for collect, but I never understood why “collect” is a good name for this method.) ---
**Author:** ![Fredrik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/cab0a1/32.png) [@Fredrik](https://www.ruby-forum.com/u/Fredrik)\ **Post date:** [April 12, 2014, 12:28am UTC](https://www.ruby-forum.com/t/hash-map/166752/10 "2014-04-12T00:28:54Z")
KDr2 wrote in post #811055: > irb(main):041:0\> o={“a”,1,“b”,2} > =\> {“a”=\>1, “b”=\>2} > irb(main):042:0\> o.merge(o){|k,ov|ov\*2} > =\> {“a”=\>2, “b”=\>4} Awesome, thanks!