Asp Forum - Hash#map - comp.lang.ruby

Fredrik

4/29/2009 6:00:00 AM

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

8 Answers

Andrew Timberlake

4/29/2009 6:14:00 AM

On Wed, Apr 29, 2009 at 8:00 AM, Fredrik <fredjoha@gmail.com> 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 =3D oldhash.inject({}) { |h,(k,v)| h[k] =3D some_operation(v); h =
}
>
> but that doesn't look pretty at all in my opinion. I want to just
> write like this:
>
> newhash =3D 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
> =A0def hashmap
> =A0 =A0self.inject({}) do |newhash, (k,v)|
> =A0 =A0 =A0newhash[k] =3D yield(k, v)
> =A0 =A0 =A0newhash
> =A0 =A0end
> =A0end
> end
>
>
> Regards,
> Fredrik
>

Becuase #map comes from form Enumerable (and is an alias for collect)

enum.collect {| obj | block } =3D> array
enum.map {| obj | block } =3D> array

Returns a new array with the results of running block once for every
element in enum.

(1..4).collect {|i| i*i } #=3D> [1, 4, 9, 16]
(1..4).collect { "cat" } #=3D> ["cat", "cat", "cat", "cat"]

Andrew Timberlake
http://ramblingso...
http://www.linkedin.com/in/andrew...

"I have never let my schooling interfere with my education" - Mark Twain

KDr2

4/29/2009 6:20:00 AM

[Note: parts of this message were removed to make it a legal post.]

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}

On Wed, Apr 29, 2009 at 2:00 PM, Fredrik <fredjoha@gmail.com> 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) }
>
> 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
>
>

--
Best Regards,
-- KDr2, at x-macro.com.

Srijayanth Sridhar

4/29/2009 6:30:00 AM

[Note: parts of this message were removed to make it a legal post.]

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

On Wed, Apr 29, 2009 at 11:30 AM, Fredrik <fredjoha@gmail.com> 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) }
>
> 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
>
>

7stud --

4/29/2009 6:37:00 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)? 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.

--
Posted via http://www.ruby-....

Fredrik

4/29/2009 9:43:00 AM

> 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.

Fredrik

4/29/2009 9:49:00 AM

>
> 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 ?

James Gray

4/29/2009 12:40:00 PM

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 Gray II

Fredrik

4/30/2009 3:25:00 AM

On 29 Apr, 21:39, James Gray <ja...@grayproductions.net> wrote:
> 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 Gray 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.)

comp.lang.ruby

Hash#map

Fredrik

Andrew Timberlake

KDr2

Srijayanth Sridhar

7stud --

Fredrik

Fredrik

James Gray

Fredrik

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