Iterating through a hash

Ruby
1

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

2

How about something like:

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

–wpd

3

2008/8/20 Brian R. [email protected]:

end

p map

End of code

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

Farrel

4

-----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 G.

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

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

iEYEARECAAYFAkisX3AACgkQJA/zY0IIRZYC0ACfVfK1H80uupfXn/g0jhp69uzB
O4YAoMIao4OCp1WfSUkpTWKW3LQA3tKP
=WHKL
-----END PGP SIGNATURE-----

5

On 20.08.2008 19:34, Brian R. 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. :slight_smile:

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

Cheers

robert

6

On Wed, Aug 20, 2008 at 11:26 AM, Robert K.
[email protected] wrote:

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

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

without the ugly “h” at the end :slight_smile: :

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

martin

7

On Wed, Aug 20, 2008 at 2:32 PM, Martin DeMello
[email protected]wrote:

without the ugly “h” at the end :slight_smile: :

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

8

On Wed, Aug 20, 2008 at 12:48 PM, Brian R. [email protected]
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

9

From: Brian R. [mailto:[email protected]]

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

10

2008/8/20 Martin DeMello [email protected]:

without the ugly “h” at the end :slight_smile: :

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

11

2008/8/21 Peña, Botp [email protected]:

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

12

For fun, a solution with zip:

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

Clay

13

Farrel L. wrote:

2008/8/20 Brian R. [email protected]:

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? :slight_smile: