Real life use of each_cons?


#1

Hello,

I’ve been going through the enumerable/enumerator methods that I’m
unfamiliar with and came across each_cons. Apart from a post by a guy
who wasn’t sure where it was even defined, I have only come across one
use of it in the wild:

(From Prawn)

def polygon(*points)
move_to points[0]
(points << points[0]).each_cons(2) do |p1,p2|
line_to(*p2)
end
end

Does anyone have any ideas on other cases where each_cons would be
useful? Or perhaps insight into why it’s in Ruby?

Best,

M


#2

On Thu, Jan 29, 2009 at 12:19 AM, Mischa F. removed_email_address@domain.invalid
wrote:

 move_to points[0]
 (points << points[0]).each_cons(2) do |p1,p2|
   line_to(*p2)
 end

end

Does anyone have any ideas on other cases where each_cons would be
useful? Or perhaps insight into why it’s in Ruby?

I think you may have been confused by my ugly code there. I have
replaced it with:

def polygon(*points)
  move_to points[0]
  (points[1..-1] << points[0]).each do |point|
    line_to(*point)
  end
end

The reason why it’s not needed is because we draw the lines from point
to point.

But if we were drawing them segment by segment, it’d make sense.

[[1,2],[3,4],[5,6],[1,2]].each_cons(2) { |a| p a }
[[1, 2], [3, 4]]
[[3, 4], [5, 6]]
[[5, 6], [1, 2]]

I imagine I had refactored a line p1, p2 call down to just line_to(p2)
without fixing the each_cons()… sorry about that.

In general each_cons is useful when you need a sliding window of size
n across a dataset.

-greg


#3

On Thu, Jan 29, 2009 at 12:53 AM, Gregory B.
removed_email_address@domain.invalid wrote:

def polygon(*points)
move_to points[0]
(points << points[0]).each_cons(2) do |p1,p2|
line_to(*p2)
end
end

Does anyone have any ideas on other cases where each_cons would be
useful? Or perhaps insight into why it’s in Ruby?

Here’s a more reasonable usage, for solving a simple tree-traversal
problem:
http://blog.majesticseacreature.com/archives/2008.10/euler_67.html


#4

On Thu, Jan 29, 2009 at 1:15 AM, Mischa F. removed_email_address@domain.invalid
wrote:

Also, it seems that if you refactor prawn, there will be no actual usage
of each_cons on github, apart from the ruby projects of course!

Already done.

http://github.com/sandal/prawn/commit/7c68dadeae3c89822a6ab0713af435b912441600


#5

2009/1/29 Mischa F. removed_email_address@domain.invalid:

 move_to points[0]
 (points << points[0]).each_cons(2) do |p1,p2|
   line_to(*p2)
 end

end

Does anyone have any ideas on other cases where each_cons would be
useful? Or perhaps insight into why it’s in Ruby?

You can find some uses in this mailing list’s archive:

http://blade.nagaokaut.ac.jp/cgi-bin/vframe.rb?key=each_cons&cginame=namazu.rb&submit=Search&dbname=ruby-talk&max=50&whence=0

Useful applications are those where you need a moving window over a
collection of data, for example when calculating moving averages
(smoothing a plot) or distances between adjacent values.

Kind regards

robert


#6

Gregory B. wrote:

On Thu, Jan 29, 2009 at 12:53 AM, Gregory B.
removed_email_address@domain.invalid wrote:

def polygon(*points)
move_to points[0]
(points << points[0]).each_cons(2) do |p1,p2|
line_to(*p2)
end
end

Does anyone have any ideas on other cases where each_cons would be
useful? Or perhaps insight into why it’s in Ruby?

Here’s a more reasonable usage, for solving a simple tree-traversal
problem:
http://blog.majesticseacreature.com/archives/2008.10/euler_67.html

Interesting. Thanks a bunch Gregory. Traversal had occurred to me as a
possible use, but I wasn’t sure.

Also, it seems that if you refactor prawn, there will be no actual usage
of each_cons on github, apart from the ruby projects of course!

M


#7

each_cons just saved me an unpleasant headache. I use it together with
each_line to implement file parsing with lookahead:

f.each_line.each_cons(2) do |line, next_line|

end