Re : Set intersection with a specific criteria: a better idi

Thanks Joel,

Do you think this could/should be added to the standard library or to
the Facets library?

Eric.


Eric TORREBORRE
tel: +81 (0)90 5580 3280
e-mail: [email protected] / [email protected]
blog: http://etorreborre.blogspot.com

----- Message d’origine ----
De : Joel VanderWerf [email protected]
À : ruby-talk ML [email protected]
Envoyé le : Vendredi, 1 Juin 2007, 2h21mn 21s
Objet : Re: Set intersection with a specific criteria: a better idiom?

Eric T. wrote:

end
end
n
end

I have met this requirement quite some times while scripting and I may miss something here.
Do you know a more idiomatic, Ruby-way of doing that?

Hm, it’s like Schwartzian transform: intersect_by instead of sort_by.

Your implementation looks good, but might be more efficient with:

   mapped = self.class.new(map{|e| property.call(e)})

which would make mapped a set, for faster lookups.

Also, using yield will probably be more efficient than instantiating a
proc and calling it:

def &(enum)
n = self.class.new
if block_given?
mapped = self.class.new(map{|e| yield e})
enum.each { |o| n.add(o) if mapped.include?(yield o) }
else
enum.each { |o| n.add(o) if include?(o) }
end
n
end

It would be more elegant to use select than to iterate through enum
using #each and #add to the set, but Set#select returns an Array. At the
cost of using two iterations instead of once, you could do this:

require ‘set’

class Set
def intersect_by(other)
props = self.class.new(other.map {|t| yield t})
self.class.new(select {|t| props.include? t.prop})
end

Actually the following is closer to the origial:

def intersect_by(other)

props = self.class.new(map {|t| yield t})

self.class.new(other.select {|t| props.include? t.prop})

end

alias old_intersect &

def &(other)
if block_given?
intersect_by(other) {|t| yield t}
else
old_intersect(other) {|t| yield t}
end
end
end

class Test
attr_accessor :prop
def initialize prop
@prop = prop
end
end

s1 = Set.new([
Test.new(1),
Test.new(2),
Test.new(3)
])

s2 = Set.new([
Test.new(2),
Test.new(3),
Test.new(4)
])

p(s1.intersect_by(s2) {|t| t.prop})
p(Set[1,2,3]&Set[2,3,4])

END

#<Set: {#<Test:0xb79f73e4 @prop=2>, #<Test:0xb79f73d0 @prop=3>}>
#<Set: {2, 3}>


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


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