Whenever Im coding I usually come across having to create a new hash
from the key/value pairs of another hash.
For example,
hash = {:a => 1, :b => 2, :c => 3}
I want to do something like this
hash.from_keys(:a,:b) => {:a=> 1, :b=> 2}
Is there something like this already? Perhaps in the facets library? I
looked and couldn’t find anything.
If anybody thinks that I shouldnt even be getting into this situation in
the first place, please let me know that as well.
On Nov 17, 3:50 pm, Aryk G. [email protected] wrote:
Is there something like this already? Perhaps in the facets library? I
looked and couldn’t find anything.
If anybody thinks that I shouldnt even be getting into this situation in
the first place, please let me know that as well.
Posted viahttp://www.ruby-forum.com/.
Hmm… I thought there was a built in method for that. Here’s one
way:
hash = {:a => 1, :b => 2, :c => 3}
Hash[*hash.select {|k,v| [:a,:b].include?(k)}.flatten] #=>
{:b=>2, :a=>1}
HTH,
Chris
Chris S. wrote:
On Nov 17, 4:46�pm, Chris S. [email protected] wrote:
Posted viahttp://www.ruby-forum.com/.
Hmm… I thought there was a built in method for that. �Here’s one
way:
hash = {:a => 1, :b => 2, :c => 3}
Hash[*hash.select {|k,v| [:a,:b].include?(k)}.flatten] �#=>
{:b=>2, :a=>1}
HTH,
Chris
Yeah, or the simpler: hash.reject {|k,v| ![:a,:b].include?(k)}
And indeed, it’s in facets:
require ‘facets’
p hash.slice(:a,:b)
However, this breaks on
hash.slice(:a,:d)
and the hash.reject does not
hth,
Siep
Siep
On Nov 17, 4:46 pm, Chris S. [email protected] wrote:
Posted viahttp://www.ruby-forum.com/.
Hmm… I thought there was a built in method for that. Here’s one
way:
hash = {:a => 1, :b => 2, :c => 3}
Hash[*hash.select {|k,v| [:a,:b].include?(k)}.flatten] #=>
{:b=>2, :a=>1}
HTH,
Chris
Yeah, or the simpler: hash.reject {|k,v| ![:a,:b].include?(k)}
On 18.11.2008, at 01:34 , Einar Magnús Boson wrote:
the first place, please let me know that as well.
HTH,
p selected
and now I see that I was stupd.
heh. nevermind <.<
Einar Magnús Boson
+354-661 1649
[email protected]
[email protected]
On 17.11.2008, at 23:46 , Chris S. wrote:
hash.from_keys(:a,:b) => {:a=> 1, :b=> 2}
Hmm… I thought there was a built in method for that. Here’s one
way:
hash = {:a => 1, :b => 2, :c => 3}
Hash[*hash.select {|k,v| [:a,:b].include?(k)}.flatten] #=>
{:b=>2, :a=>1}
HTH,
Chris
That is ridiculously inefficient and weird 
hash = {:a => 1, :b => 2, :c => 3}
selected = [:a, :b].map{|key|{key, hash[key]}}
p selected
>> [{:a=>1}, {:b=>2}]
Einar Magnús Boson
+354-661 1649
[email protected]
[email protected]
>> [{:a=>1}, {:b=>2}]
and now I see that I was stupd.
heh. nevermind <.<
This is what I meant to do, a lot more efficient to look up the values
you’re lookin for than looping through all keys for every element.
hash = {:a => 1,
:b => 2,
:c => 3,
:d => 4,
:str => “test”}
selected = [:a, :d, :str].inject({}){|result, key| result[key]=
hash[key];result}
p selected
>> {:str=>“test”, :a=>1, :d=>4}
.
On Mon, Nov 17, 2008 at 7:44 PM, Einar Magnús Boson
[email protected] wrote:
hash[key];result}
p selected
>> {:str=>“test”, :a=>1, :d=>4}
You will get nils for none existing keys that way.
selected = [:whatever].inject({}){|result, key| = hash[key]; result}
#=> {:whatever => nil}
If that’s what you want, great.
Todd
On 18.11.2008, at 05:00 , Todd B. wrote:
:str => "test"}
selected = [:whatever].inject({}){|result, key| = hash[key]; result}
#=> {:whatever => nil}
If that’s what you want, great.
Todd
if that is a problem it’s easy to fix
hash = {:a => 1,
:b => 2,
:c => 3,
:d => 4,
:str => “test”}
find = [:a, :d, :str, :extra]
selected = find.inject({}) {
|result, key|
val=hash[key]
result[key]=val if val
result }
p selected
>> {:str=>“test”, :a=>1, :d=>4}
given: f, h = find.size, hash.size
This method is O(f) because hash lookup is O(1)
The other way it’s O(f*h).
so if the hash is big it should make a difference.
If the elements always are just a few it doesn’t really matter.
einarmagnus
On Mon, Nov 17, 2008 at 11:23 PM, Einar Magnús Boson
[email protected] wrote:
:b => 2,
>> {:str=>“test”, :a=>1, :d=>4}
This method is O(f) because hash lookup is O(1)
The other way it’s O(f*h).
Good point. The use of #inject, though, may cloud that performance
analysis.
so if the hash is big it should make a difference.
If the elements always are just a few it doesn’t really matter.
I’m an #inject sort of guy so I like the way you approach this. But,
there must be a reason why you don’t prefer the negative #reject that
seems to work for most people.
I might benchmark this, but I think object creation and destruction
might outweigh the O(f*h).
Todd
Aryk G. wrote:
I want to do something like this
hash.from_keys(:a,:b) => {:a=> 1, :b=> 2}
In ruby1.9, Hash#select returns another Hash. But you’d still be
iterating the ‘wrong way’ (that is, iterating through the hash and doing
a linear search through the keys)
Personally I’d go with:
class Hash
def from_keys(*keys)
keys.inject({}) { |h,k| h[k] = self[k] if has_key?(k); h }
end
end
hash = {:a => 1, :b => 2, :c => 3}
p hash.from_keys(:a, :b)
With ruby19 you can do:
keys.each_with_object({}) { |k,h| h[k] = self[k] if has_key?(k) }
which is more keystrokes but maybe the teeniest bit more efficient. But
I hate each_with_object on the principle that its arguments are the
opposite way round to inject 
On 17 nov. 08, at 23:50, Aryk G. wrote:
hash.from_keys(:a,:b) => {:a=> 1, :b=> 2}
I tried to resist making this a one-liner and went for the clean and
explicit way 
class Hash
def from_keys(*keys)
selected_values = self.values_at(*keys)
selected_key_values = keys.zip(selected_values)
Hash[*selected_key_values.flatten]
end
end
hash = {:a => 1, :b => 2, :c => 3}
p hash.from_keys(:a, :b)
=> {:a=>1, :b=>2}
On Mon, Nov 17, 2008 at 11:04 PM, Todd B. [email protected]
wrote:
selected = [:whatever].inject({}){|result, key| = hash[key]; result}
#=> {:whatever => nil}
I’m not apologizing for the missing result[key] on the lhs, though I
know it’s pretty important to many to have working code in a message.
Anyone with a half a brain knows what I meant, but just to be clean…
h = Hash[:a, 1, :b, 2]
p [:missing_key].inject({}) {|result, key| result[key] = h[key]; result}
=> {:missing_key => nil}
In any case, reducing a hash (being selective) I think is best done
using a reject with a negative in the block (as per Chris S.'s
second example).
Todd
On Nov 18, 3:31 am, Luc H. [email protected] wrote:
selected_key_values = keys.zip(selected_values)
Hash[*selected_key_values.flatten]
end
end
hash = {:a => 1, :b => 2, :c => 3}
p hash.from_keys(:a, :b)
=> {:a=>1, :b=>2}
require ‘facets/hash/slice’
T.