Array comparison.. why does this work.

[Jean Nibee]

How does Ruby do Array comparision?

old_inventory = [ 'a', 'b', 'd' ]
new_inventory = [ 'a','e', 'c' ]

puts "The following files have been added: "
puts new_inventory - old_inventory
puts ""
puts "The following files have been deleted: "
puts old_inventory - new_inventory

Notice the output is correct so it's not a case of element 0's being
compared in each array.

So...

Does Ruby sort arrays in memory first? or is a scan done 1 element at a
time?

Thanks.

 

[dblack]

Hi --

How does Ruby do Array comparision?

One element at a time.

old_inventory = [ 'a', 'b', 'd' ]
new_inventory = [ 'a','e', 'c' ]

puts "The following files have been added: "
puts new_inventory - old_inventory
puts ""
puts "The following files have been deleted: "
puts old_inventory - new_inventory

Notice the output is correct so it's not a case of element 0's being
compared in each array.

So...

Does Ruby sort arrays in memory first? or is a scan done 1 element at a
time?

You're not doing comparison, though; you're doing subtraction.
Subtraction doesn't depend on order. You can think of it as
"without":

[1,2,3] - [1] => [2,3] # [1,2,3] without 1
[1,2,3,1] - [1] => [2,3] # [1,2,3,1] without 1
[1,2,3] - [2] => [1,3] # [1,2,3] without 2

etc.


David

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[Jean Nibee]

You're not doing comparison, though; you're doing subtraction.
Subtraction doesn't depend on order. You can think of it as
"without":

[1,2,3] - [1] => [2,3] # [1,2,3] without 1
[1,2,3,1] - [1] => [2,3] # [1,2,3,1] without 1
[1,2,3] - [2] => [1,3] # [1,2,3] without 2

etc.


David

Sorry, when I mentioned comparison I meant "Under the hood". (I realise
'-' is subtractions ), but the interpreter needs to perform comparisoin
to see if an element exists somewhere.

I was curious if it just interates through one array looking for that
element in the second, or does it (internally) sort the array, iterate
each array performing comparision and if a > b skipping the equality
determination to save CPU cycles and doing a 'next'.

I guess what I"m saying (Makes more sense when you see pseudo code) is
how does it work in the interpreter.

 

[Jean Nibee]

Don't forget, you can always just look at the source[1].
It looks to me like it converts the second array to a hash, then
iterates through the first array, checking if each element is in the
newly created hash.
If it is, it skips it. If not, the value goes in the new array.

-Justin

[1] http://ruby-doc.org/core/classes/Array.src/M002261.html

Awesome I didn't now where to find the sources to read them.

Thanks!