Joel VanderWerf
5/31/2007 5:21:00 PM
Eric Torreborre wrote:
> Hi,
>
> I have been trying to intersect 2 sets of objects that where I want the "intersected" objects to share a common property.
>
> This is what I came up with:
>
> Here, I want to get all the Errors having the same file name:
>
> def intersection_set
> Set.new(event_errors).&(Set.new(socket_channel_errors)) {|x| file_name(x)}
> end
>
> To allow that I extended the Set class with:
>
> def &(enum, &property)
> n = self.class.new
> if block_given?
> mapped = self.map{|e| property.call(e)}
> enum.each { |o| n.add(o) if mapped.include?(property.call(o)) }
> else
> enum.each { |o| n.add(o) if include?(o) }
> 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__
Output:
#<Set: {#<Test:0xb79f73e4 @prop=2>, #<Test:0xb79f73d0 @prop=3>}>
#<Set: {2, 3}>
--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407