Iterating through a hash

ruby-talk
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

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

Farrel

···

2008/8/20 Brian Ross <p.brian.ross@gmail.com>:

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

--
Aimred - Ruby Development and Consulting

4

#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:

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

5

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

···

On 20.08.2008 19:34, Brian Ross wrote:

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?

6

# 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

···

From: Brian Ross [mailto:p.brian.ross@gmail.com]

7

For fun, a solution with zip:

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

Clay

8

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

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

martin

···

On Wed, Aug 20, 2008 at 11:26 AM, Robert Klemme <shortcutter@googlemail.com> 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

9

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

···

2008/8/21 Peña, Botp <botp@delmonte-phil.com>:

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

--
use.inject do |as, often| as.you_can - without end

10

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? :slight_smile:
--
Posted via http://www.ruby-forum.com/\.

11

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

···

On Wed, Aug 20, 2008 at 2:32 PM, Martin DeMello <martindemello@gmail.com>wrote:

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

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

12

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

···

2008/8/20 Martin DeMello <martindemello@gmail.com>:

On Wed, Aug 20, 2008 at 11:26 AM, Robert Klemme > <shortcutter@googlemail.com> 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})}

--
use.inject do |as, often| as.you_can - without end

13

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

···

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.