Hash#map

Ruby
1

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

2

On Wed, Apr 29, 2009 at 8:00 AM, Fredrik [email protected] 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://www.linkedin.com/in/andrewtimberlake

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

3

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}

4

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

5

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

6

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.

7

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 ?

8

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! :slight_smile: Though o.merge
(o) doesn’t look as pedagogically clear as o.map, it’ll have to do.

9

On 29 Apr, 21:39, James G. [email protected] 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.)

10

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!