# Question about the Set class, XOR, and arrays

Hi all,

Ruby 1.8.5

irb(main):002:0> Set[1,2,3] ^ Set[3,4,5,5]
=> #<Set: {5, 1, 2, 4}>

Ok, looks good.

irb(main):003:0> Set[1,2,3] ^ [3,4,5,5]
=> #<Set: {1, 2, 4}>

What?! I’m confused. Do I need a refresher in Set theory or something?

Thanks,

Dan

On Nov 1, 2006, at 12:25 PM, Daniel B. wrote:

irb(main):002:0> Set[1,2,3] ^ Set[3,4,5,5]
=> #<Set: {5, 1, 2, 4}>

Ok, looks good.

irb(main):003:0> Set[1,2,3] ^ [3,4,5,5]
=> #<Set: {1, 2, 4}>

What?! I’m confused. Do I need a refresher in Set theory or something?

It is the way it is implemented… subsequent 5’s will toggle it on
and off. You should file a bug.

Daniel B. wrote:

=> #<Set: {1, 2, 4}>

What?! I’m confused. Do I need a refresher in Set theory or something?

I think I can guess why it is happening:

irb(main):009:0> (0…10).each {|n| p Set[1,2,3] ^ ([3,4]+[5]*n)}
#<Set: {1, 2, 4}>
#<Set: {5, 1, 2, 4}>
#<Set: {1, 2, 4}>
#<Set: {5, 1, 2, 4}>
#<Set: {1, 2, 4}>
#<Set: {5, 1, 2, 4}>
#<Set: {1, 2, 4}>
#<Set: {5, 1, 2, 4}>
#<Set: {1, 2, 4}>
#<Set: {5, 1, 2, 4}>
#<Set: {1, 2, 4}>
=> 0…10

Looks like the array argument is used to flip-flop the elements of the
set.

Daniel B. wrote:

=> #<Set: {1, 2, 4}>

What?! I’m confused. Do I need a refresher in Set theory or something?

Are your irb entries above copied directly from the display? Or is this
a
1.8.5 problem?

\$ ruby -v
\$ ruby 1.8.4 (2005-12-24) [i386-linux]

\$irb
irb(main):002:0> require ‘set’
=> true
irb(main):003:0> Set[1,2,3] ^ Set[3,4,5,5]
=> #<Set: {5, 1, 2, 4}>
irb(main):004:0> Set[1,2,3] ^ Set[3,4,5,5]
=> #<Set: {5, 1, 2, 4}>
irb(main):005:0> Set[1,2,3] ^ Set[3,4,5,5]
=> #<Set: {5, 1, 2, 4}>

Ryan D. wrote:

What?! I’m confused. Do I need a refresher in Set theory or something?

It is the way it is implemented… subsequent 5’s will toggle it on
and off. You should file a bug.

Done.

Thanks for confirming my sanity.

• Dan

Jeffrey S. wrote:

=> #<Set: {1, 2, 4}>
about the intersection of a set with an array.
Arrays already behave like sets in some ways. They have set-like
operators:

irb(main):009:0* [1,2,3] & [2,3,4]
=> [2, 3]
irb(main):010:0> [1,2,3] | [2,3,4]
=> [1, 2, 3, 4]
irb(main):011:0> [1,2,3] - [2,3,4]
=> [1]

But this argument falls down because arrays don’t have symmetric
difference:

irb(main):012:0> [1,2,3] ^ [2,3,4]
NoMethodError: undefined method `^’ for [1, 2, 3]:Array
from (irb):11

Daniel B. wrote:

What?! I’m confused. Do I need a refresher in Set theory or something?
It is the way it is implemented… subsequent 5’s will toggle it on
and off. You should file a bug.

Done.

Thanks for confirming my sanity.

In what way is this a bug? I’m not aware of set theory saying anything
about the intersection of a set with an array.