Ruby Forum Ruby > ruby and list comprehension

In Python, I can do this to arrays:

added = [x for x in new_data if x not in old_data]
removed = [x for x in old_data if x not in new_data]
same = [x for x in new_data if x in old_data]

I believe this is known as list comprehension in Python. How is this 
done in
Ruby?

Thanks,
Brad

Brad Tilley wrote:
> Brad
>
>   
This question comes up from time to time. You can search the archives of
comp.lang.ruby on Google Groups for "list comprehension" to find the
previous threads on this topic.

http://groups.google.com/group/comp.lang.ruby/search?group=comp.lang.ruby&q=list+comprehension&qt_g=1&searchnow=Search+this+group

On 11/25/06, Brad Tilley <rtilley@vt.edu> wrote:

> In Python, I can do this to arrays:
>
> added = [x for x in new_data if x not in old_data]
> removed = [x for x in old_data if x not in new_data]
> same = [x for x in new_data if x in old_data]

Short answer:

    added   = new_data.reject {|i| old_data.include? i }
    removed = old_data.reject {|i| new_data.include? i }
    same    = new_data.select {|i| old_data.include? i }

Provided ordering isn't important here, you can do the same thing with
set operations.

    require 'set'

    old_data = old_data.to_set
    new_data = new_data.to_set

    added   = new_data - old_data
    removed = old_data - new_data
    same    = new_data.intersection(old_data)

Note those returns sets, not arrays.

Brad Tilley wrote:
> Brad
Because Ruby's select() returns a value, not just a boolean like in 
Smalltalk,
you can do the following:

[1,2,3].select { |x| ![2,3,4].include? x }
[2,3,4].select { |x| ![1,2,3].include? x }
[1,2,3].select { |x| [2,3,4].include? x }

You can also use Array operators:

[1,2,3] - [2,3,4]
[2,3,4] - [1,2,3]
[1,2,3] & [2,3,4]


Mike

Quoting Mike Austin <noone@nowhere.com>:

> [1,2,3] - [2,3,4]
> [2,3,4] - [1,2,3]
> [1,2,3] & [2,3,4]

Thanks for all the examples guys! That's great stuff.

On 2006-11-25 18:47:26 -0500, Brad Tilley <rtilley@vt.edu> said:

> Brad
I found this while web searching for the same thing recently; I can't
recall where I found it. It's a cute little hack.

class Array
  def comprehend
    return self unless block_given?
    result = []
    self.each { |i| result.push yield(i) }
    result.compact
  end
end

Then:

added = new_data.comprehend { |x| x if not old_data.include? x }
removed = old_data.comprehend { |x| x if not new_data.include? x }
same = new_data.comprehend { |x| x if old_data.include? x }

Best,
James

Hi --

On Mon, 27 Nov 2006, James Cunningham wrote:

>> Ruby?
>   result = []
>   self.each { |i| result.push yield(i) }
>   result.compact
> end
> end
>
> Then:
>
> added = new_data.comprehend { |x| x if not old_data.include? x }
> removed = old_data.comprehend { |x| x if not new_data.include? x }
> same = new_data.comprehend { |x| x if old_data.include? x }

I'm not getting how that's better than:

   added = new_data.select {|x| not old_data.include?(x) }

(or the reject equivalent) and so on.


David

On 2006-11-26 17:52:37 -0500, dblack@wobblini.net said:

>>> same = [x for x in new_data if x in old_data]
>> class Array
>> added = new_data.comprehend { |x| x if not old_data.include? x }
> David
I should have clarified. In your example there's no difference, but the
above gives a general replacement for list comprehensions.

irb(main):018:0> (1..25).to_a.comprehend { |x| x**2 if not x % 2 == 0 }
=> [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

Best,
James

On 2006-11-26 19:01:53 -0500, James Cunningham
<jameshcunningham@gmail.com> said:

>>>> added = [x for x in new_data if x not in old_data]
>>> recall where I found it. It's a cute little hack.
>>> Then:
>> 
> Best,
> James

Er, "your" meaning "Brad Tilley's".

Best,
James

On 11/27/06, James Cunningham <jameshcunningham@gmail.com> wrote:
>
> added = new_data.comprehend { |x| x if not old_data.include? x }
> removed = old_data.comprehend { |x| x if not new_data.include? x }
> same = new_data.comprehend { |x| x if old_data.include? x }

I wrote http://zem.novylen.net/ruby/fproduct.rb a while ago to emulate
list comprehensions in ruby - for example:

for x,y,z in product 1..40,
                       1..40, proc {|x,y| x <= y},
                       1..40, proc {|x,y,z| x**2 + y**2 == z**2}
    p [x,y,z]
end

The benefit is that the filters do get applied in order (sorted on
arity), so that it doesn't generate all the combinations first and
then filter.

martin

On 27.11.2006 01:01, James Cunningham wrote:
> On 2006-11-26 17:52:37 -0500, dblack@wobblini.net said:
>> I'm not getting how that's better than:
>>
>>    added = new_data.select {|x| not old_data.include?(x) }
>>
>> (or the reject equivalent) and so on.

> I should have clarified. In your example there's no difference, but the 
> above gives a general replacement for list comprehensions.
> 
> irb(main):018:0> (1..25).to_a.comprehend { |x| x**2 if not x % 2 == 0 }
> => [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

Frankly, I am not sure I find this better than using the built in 
methods:

irb(main):001:0> (1..25).inject([]) {|a,x| a << x**2 unless x % 2 == 0; 
a}
=> [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

irb(main):002:0> (1..25).inject([]) {|a,x| a << x**2 if x % 2 == 1; a}
=> [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

irb(main):003:0> (1..25).select {|x| x % 2 == 1}.map! {|x| x**2}
=> [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]

Kind regards

	robert

I've always thought list comprehension is just a bunch of
map/filter/... transformation until I saw the following version of
permutation:

in Haskell:
permutation [] = [[]]
permutation xs = [x:ys | x <- xs, ys <- permutation (delete x xs)]

in Erlang:
permutation([]) -> [[]];
permutation(L)  -> [[H|T] || H <- L, T <- permutation(L--[H])].

really neat, isn't it?

On 11/26/06, Mike Austin <noone@nowhere.com> wrote:
>
> Brad Tilley wrote:
> <snip>
> Because Ruby's select() returns a value, not just a boolean like in
> Smalltalk,
>
>
OT of course ;)
... this does not seem true for all Smalltalks, I am using Squeak and
select: returns the filtered array, I am quite surprised that it worked
differently before.
<snip>
Robert



--
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore all progress
depends on the unreasonable man.

- George Bernard Shaw

On 2006-11-27 03:30:58 -0500, Robert Klemme <shortcutter@googlemail.com> 
said:

>> 
> 
> irb(main):003:0> (1..25).select {|x| x % 2 == 1}.map! {|x| x**2}
> => [1, 9, 25, 49, 81, 121, 169, 225, 289, 361, 441, 529, 625]
> 
> Kind regards
> 
> 	robert

That's fair enough, but I think list comprehension is clearer than
method chaining and one-liner array accumulation. I'm afraid

a << x**2 if x % 2 == 1; a

is perhaps just a little less elegant than

x**2 if x % 2 == 1

I think the nicest thing about Ruby, though, is that it's even possible.

Best,
James

piggybox wrote:
> in Haskell:
> permutation [] = [[]]
> permutation xs = [x:ys | x <- xs, ys <- permutation (delete x xs)]
> 
> in Erlang:
> permutation([]) -> [[]];
> permutation(L)  -> [[H|T] || H <- L, T <- permutation(L--[H])].
> 
> really neat, isn't it?
Yes. :)

James Cunningham wrote:
> end
Maybe I don't comprehend comprehending, but why the result/each instead
of map?

class Array
  def comprehend
    if block_given?
      map{ |i| yield( i ) }.compact
    else
      self
    end
  end
end

or perhaps better

class Array
  def comprehend( &block )
    block ? map( &block ).compact : self
  end
end

On 2006-11-27 23:39:57 -0500, "Phrogz" <gavin@refinery.com> said:

> class Array
>   def comprehend( &block )
>     block ? map( &block ).compact : self
>   end
> end

The same.

You're just cleverer than I am. ;)

Best,
James

On 28.11.2006 05:39, Phrogz wrote:
> class Array
>   def comprehend( &block )
>     block ? map( &block ).compact : self
>   end
> end


This could go into Enumerable instead.  There is no special Array
functionality involved.

Kind regards

	robert