Forum: Ruby Iterating through a hash

2d985777f09653ef6e8ce9d9f5a0e7c2?d=identicon&s=25 Brian Ross (Guest)
on 2008-08-20 19:38
(Received via mailing list)
How can I iterate through a hash so that each key is modified and saved
into
a new hash?

# Beginning of code

hash = {"name"=>"greg", "job"=>"boring", "hair"=>"plenty"}
p hash

map = hash.each_key do |key|
  key.upcase
end

p map

# End of code

I'd imagine that this would return map as a new hash with the keys
modified.
Is there anything like collect! for hashes?

Brian
Ed437e52d8d6720308720e7e678f3e6d?d=identicon&s=25 Patrick Doyle (Guest)
on 2008-08-20 19:53
(Received via mailing list)
How about something like:

map = {}
hash.each_pair do |k,v|
  map[k.upcase] = v
end

--wpd
05be5d6610e2c3f1780aa0e39e902e93?d=identicon&s=25 Farrel Lifson (Guest)
on 2008-08-20 19:56
(Received via mailing list)
2008/8/20 Brian Ross <p.brian.ross@gmail.com>:
> end
>
> p map
>
> # End of code

Hash[*hash.map{|key,value| [key.upcase, value]}.flatten]

Farrel
D1f1c20467562fc1d8c8aa0d328def62?d=identicon&s=25 Florian Gilcher (skade)
on 2008-08-20 20:17
(Received via mailing list)
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

#upcase doesn't modify the receiver.

#upcase! doesn't work because you can't modify String keys, as they
are frozen.

So, you have to rebuild the hash, the others pointed out some
solutions for this.

Regards,
Florian Gilcher

On Aug 20, 2008, at 7:34 PM, Brian Ross wrote:

>  key.upcase
> Brian
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.8 (Darwin)

iEYEARECAAYFAkisX3AACgkQJA/zY0IIRZYC0ACfVfK1H80uupfXn/g0jhp69uzB
O4YAoMIao4OCp1WfSUkpTWKW3LQA3tKP
=WHKL
-----END PGP SIGNATURE-----
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2008-08-20 20:58
(Received via mailing list)
On 20.08.2008 19:34, Brian Ross wrote:
> end
>
> p map
>
> # End of code
>
> I'd imagine that this would return map as a new hash with the keys modified.
> Is there anything like collect! for hashes?

Folks, thanks for leaving the #inject solution to me. :-)

map = hash.inject({}) do |h,(k,v)|
   h[k.upcase] = v
   h
end

Cheers

  robert
Ae16cb4f6d78e485b04ce1e821592ae5?d=identicon&s=25 Martin DeMello (Guest)
on 2008-08-20 21:06
(Received via mailing list)
On Wed, Aug 20, 2008 at 11:26 AM, Robert Klemme
<shortcutter@googlemail.com> wrote:
>
> Folks, thanks for leaving the #inject solution to me. :-)
>
> map = hash.inject({}) do |h,(k,v)|
>  h[k.upcase] = v
>  h
> end

without the ugly "h" at the end :) :

  map = hash.inject({}) {|h, (k,v)| h.update({k.upcase => v})}

martin
2d985777f09653ef6e8ce9d9f5a0e7c2?d=identicon&s=25 Brian Ross (Guest)
on 2008-08-20 21:52
(Received via mailing list)
On Wed, Aug 20, 2008 at 2:32 PM, Martin DeMello
<martindemello@gmail.com>wrote:

> without the ugly "h" at the end :) :
>
>  map = hash.inject({}) {|h, (k,v)| h.update({k.upcase => v})}
>

Thanks everyone! Using Martin's example, how would I then use upcase for
an
array of hashes?

I'd imagine it would be

this_array.collect do |hash|
  @map = {}
  hash.each_pair do |k,v|
   @map[k.upcase] = v
  end
end

but while the @map gives me the scope to see it outside of the block, it
looks like only the first hash within the array is passed through.

Brian
Ae16cb4f6d78e485b04ce1e821592ae5?d=identicon&s=25 Martin DeMello (Guest)
on 2008-08-20 22:52
(Received via mailing list)
On Wed, Aug 20, 2008 at 12:48 PM, Brian Ross <p.brian.ross@gmail.com>
wrote:
>
> this_array.collect do |hash|
>  @map = {}
>  hash.each_pair do |k,v|
>   @map[k.upcase] = v
>  end
> end
>
> but while the @map gives me the scope to see it outside of the block, it
> looks like only the first hash within the array is passed through.

new_array = this_array.collect do |hash|
  map = {}
  hash.each_pair do |k,v|
     map[k.upcase] = v
  end
  map
end

collect collects the return value of the block

martin
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2008-08-21 09:46
(Received via mailing list)
2008/8/20 Martin DeMello <martindemello@gmail.com>:
> without the ugly "h" at the end :) :
>
>  map = hash.inject({}) {|h, (k,v)| h.update({k.upcase => v})}

But with ugly and unnecessary brackets.  This works as well

map = hash.inject({}) {|h, (k,v)| h.update(k.upcase => v)}

Note though that this unfortunately creates a lot of temporary Hashes
that are thrown away immediately. I personally find that more ugly
than the h at the end.  You can reformat to make it less stand out as
in

map = hash.inject {} do |h,(k,v)|
 h[k.upcase] = v; h
end

Kind regards

robert
6087a044557d6b59ab52e7dd20f94da8?d=identicon&s=25 Peña, Botp (Guest)
on 2008-08-21 10:44
(Received via mailing list)
From: Brian Ross [mailto:p.brian.ross@gmail.com]
# I'd imagine that this would return map as a new hash with the
# keys modified. Is there anything like collect! for hashes?

the required pairings for hashes makes it fragile to implement a
map/collect like feature similar to plain arrays.

you can create one if you like. just be careful.

eg, here is my simple-minded implementation,

irb(main):084:0> class Hash
irb(main):085:1> def map2
irb(main):086:2>   h={}
irb(main):087:2>   self.each do |k,v|
irb(main):088:3*      kk,vv=yield(k,v)
irb(main):089:3>      key = kk || k
irb(main):090:3>      val = vv || v
irb(main):091:3>      h[key] = val
irb(main):092:3>   end
irb(main):093:2>   h
irb(main):094:2> end
irb(main):095:1> end
=> nil
irb(main):096:0> hash.map2{|k,_| k.upcase}
=> {"NAME"=>"greg", "JOB"=>"boring", "HAIR"=>"plenty"}
irb(main):097:0> hash.map2{|k| k.upcase}
=> {"NAME"=>"greg", "JOB"=>"boring", "HAIR"=>"plenty"}
irb(main):098:0> hash.map2{|k,v| k.upcase}
=> {"NAME"=>"greg", "JOB"=>"boring", "HAIR"=>"plenty"}
irb(main):099:0> hash.map2{|k,v| [k.upcase,v.capitalize]}
=> {"NAME"=>"Greg", "JOB"=>"Boring", "HAIR"=>"Plenty"}


kind regards -botp
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2008-08-21 13:29
(Received via mailing list)
2008/8/21 Peña, Botp <botp@delmonte-phil.com>:
> irb(main):084:0> class Hash
> irb(main):095:1> end
> => nil

I'd like to use that bit for a small discussion if you allow. I
believe the implementation would be better off by not messing with
return values from the block because the changes lead to more complex
code and results may be surprising (i.e. if you relied on returning
nil or false and expected that to show up in the new Hash).  By having
the method do less things it becomes more flexible to use.

Another interesting question is which is the proper method for
creating the new Hash?  I can think at least of these as reasonable
solutions

h = {}
h = Hash.new(default)
h = Hash.new(&default_proc)
h = default_proc ? Hash.new(&default_proc) : Hash.new(default)
h = self.class.new
h = self.class.new(default)
h = self.class.new(&default_proc)
h = dup.clear

The solution might be to do

def map2(h = {})
  each do |k,v|
    k, v = yield k, v
    h[k] = v
  end
  h
end

i.e. allow the caller to decide where he wants the mapped data to go
while defaulting to the most straightforward case, a new Hash. This is
similar to what Enumerable#map does, i.e. it creates a new Array
regardless of the type at hand.

Kind regards

robert
0e89a218ff1ebb034b3e90628718dea6?d=identicon&s=25 Angus Kwan (anguskwan)
on 2008-08-22 22:02
Farrel Lifson wrote:
> 2008/8/20 Brian Ross <p.brian.ross@gmail.com>:
>> end
>>
>> p map
>>
>> # End of code
>
> Hash[*hash.map{|key,value| [key.upcase, value]}.flatten]
>
> Farrel

*hash.map{|key,value| [key.upcase, value]}.flatten

what * do to the array pls? :)
9f2cac8aadde1264d56151d3757f238b?d=identicon&s=25 clay (Guest)
on 2008-08-30 02:23
(Received via mailing list)
For fun, a solution with zip:

class Hash
  def map_keys &block
    Hash[ * keys.map(&block).zip(values) ]
  end
end

Clay
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