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comp.lang.ruby

Help me with some nested Hashes.

Daniel Finnie

12/9/2006 11:33:00 PM

What I want to do is have 2 nested hashes, the outer hash returning a
new hash on an unknown key and the inner hash returning a space on an
unknown key.

For example:
values = Hash.new # as described above
values[4][4] = "H" # The 4 is an unknown key, so a new Hash is
formed # and the value of that inner hash's 4 key is "H"
values[5][6] # => " "

I got this to work correctly with the following (only ints are keys):
@value = []
0.upto(300) do |x|
@value[x] = Hash.new(" ")
end
But that is horrible.

So I've tried this:
value = Hash.new(Hash.new(" "))
But if you pass an object as the default value for a hash, it is not
cloned for each unknown key.

So then this:
value = Hash.new { Hash.new(" ") }
But that has some problems as well:
irb(main):013:0> value = Hash.new {Hash.new(" ") }
=> {}
irb(main):014:0> value[5][6] = "H"
=> "H"
irb(main):015:0> value[5][6]
=> " "
irb(main):016:0> value[5][7] = "I"
=> "I"
irb(main):017:0> value[5][6]
=> " " # It just changed from what I set it to above.


Is there anyway to get this to work?
4 Answers

Joel VanderWerf

12/9/2006 11:38:00 PM

0

Daniel Finnie wrote:
> What I want to do is have 2 nested hashes, the outer hash returning a
> new hash on an unknown key and the inner hash returning a space on an
> unknown key.

value = Hash.new {|h,k| h[k] = Hash.new {|h1,k1| h1[k1] = " "}}

p value[5][6] # ==> " "
value[5][6] = "H"
p value # ==> {5=>{6=>"H"}}

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Joel VanderWerf

12/9/2006 11:46:00 PM

0

Joel VanderWerf wrote:
> Daniel Finnie wrote:
>> What I want to do is have 2 nested hashes, the outer hash returning a
>> new hash on an unknown key and the inner hash returning a space on an
>> unknown key.
>
> value = Hash.new {|h,k| h[k] = Hash.new {|h1,k1| h1[k1] = " "}}
>
> p value[5][6] # ==> " "
> value[5][6] = "H"
> p value # ==> {5=>{6=>"H"}}
>

Another option, in case you don't want all those empty strings
cluttering up your hash:

value = Hash.new {|h,k| h[k] = Hash.new {" "}}

p value[5][6] # ==> " "
p value # ==> {5=>{}}

value[5][6] = "H"
p value # ==> {5=>{6=>"H"}}

# But, with this variation, beware:

value[5][7] << "add some characters after the space"
p value[5][7] # ==> " "

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Daniel Finnie

12/9/2006 11:52:00 PM

0

Thanks, that works perfectly! (I'm only storing 1 char strings)

Joel VanderWerf wrote:
> Joel VanderWerf wrote:
>> Daniel Finnie wrote:
>>> What I want to do is have 2 nested hashes, the outer hash returning a
>>> new hash on an unknown key and the inner hash returning a space on an
>>> unknown key.
>>
>> value = Hash.new {|h,k| h[k] = Hash.new {|h1,k1| h1[k1] = " "}}
>>
>> p value[5][6] # ==> " "
>> value[5][6] = "H"
>> p value # ==> {5=>{6=>"H"}}
>>
>
> Another option, in case you don't want all those empty strings
> cluttering up your hash:
>
> value = Hash.new {|h,k| h[k] = Hash.new {" "}}
>
> p value[5][6] # ==> " "
> p value # ==> {5=>{}}
>
> value[5][6] = "H"
> p value # ==> {5=>{6=>"H"}}
>
> # But, with this variation, beware:
>
> value[5][7] << "add some characters after the space"
> p value[5][7] # ==> " "
>

Ashley Moran

12/9/2006 11:57:00 PM

0


On Dec 09, 2006, at 11:32 pm, Daniel Finnie wrote:

> What I want to do is have 2 nested hashes, the outer hash returning
> a new hash on an unknown key and the inner hash returning a space
> on an unknown key.


Daniel

I was beat to it, but since I spent ten minutes figuring it out, here
is my solution anyway :)

require 'rubygems'
require 'spec'

# a subclass, instead of a re-opened base class... how un-rubyish ;)
class SpaceHash < Hash
def [](key)
value = super(key)
value = self[key] = Hash.new(" ") unless value
value
end
end

context "SpaceHash" do
setup do
@space_hash = SpaceHash.new
@space_hash[5][6] = "H"
end

specify "unassigned keys return a space" do
@space_hash[5][5].should == " "
end

specify "assigned keys should return their value" do
@space_hash[5][6].should == "H"
end

specify "assigned another key to the second layer should not
reassign othe keys" do
@space_hash[5][7] = "I"
@space_hash[5][6].should == "H"
end
end

Ashley