Feature #6240
closed
Enumerable#drop with negative argument
Added by marcandre (Marc-Andre Lafortune) almost 12 years ago.
Updated about 5 years ago.
Description
Currently, Enumerable#drop works only for non-negative arguments.
It could be extended so that negative arguments means dropping from the end:
[:hello, :world].drop(-1) # => [:hello]
This could especially be interesting for Lazy#drop
, which would keep a circular buffer of elements before yielding them.
(1..6).lazy.drop(-3).each{|x| puts x} # -> prints 1, 2 and 3
Thoughts?
marcandre (Marc-Andre Lafortune) wrote:
Currently, Enumerable#drop works only for non-negative arguments.
It could be extended so that negative arguments means dropping from the end:
[:hello, :world].drop(-1) # => [:hello]
This could especially be interesting for Lazy#drop
, which would keep a circular buffer of elements before yielding them.
(1..6).lazy.drop(-3).each{|x| puts x} # -> prints 1, 2 and 3
Thoughts?
How Enumerable#drop can know the total number of elements?
The source of elements might be IO. Besides that, the total number of elements might be infinite.
Hi,
shugo (Shugo Maeda) wrote:
How Enumerable#drop can know the total number of elements?
The source of elements might be IO. Besides that, the total number of elements might be infinite.
drop
would have to consume the whole iteration, indeed, which is why I was talking about a buffer. The buffer holds elements until we know they are not in the last (-n) elements. Here's a Ruby implementation:
class Lazy
def drop(n)
return to_enum :drop, n unless block_given?
if n < 0
buffer = []
each do |e|
buffer << e
yield buffer.shift if buffer.size > -n
end
else
# ...
end
end
end
For infinite sequences, drop
with negative argument would not be very useful, but it would still yield all elements.
marcandre (Marc-Andre Lafortune) wrote:
How Enumerable#drop can know the total number of elements?
The source of elements might be IO. Besides that, the total number of elements might be infinite.
drop
would have to consume the whole iteration, indeed, which is why I was talking about a buffer. The buffer holds elements until we know they are not in the last (-n) elements. Here's a Ruby implementation:
Ah, I see. It's interesting.
Do you have any use case in mind?
- Status changed from Open to Assigned
- Assignee set to matz (Yukihiro Matsumoto)
- Target version changed from 2.0.0 to 2.6
- Target version deleted (
2.6)
- Status changed from Assigned to Feedback
Using an existing feature, [1,2,3,4,5].lazy.drop(-n)
could be written as [1,2,3,4,5].each_cons(n+1).lazy.map(&:first)
.
Enumerable#each_cons does not use a circular buffer, though. (It currently uses push & shift)
- Status changed from Feedback to Closed
knu (Akinori MUSHA) wrote:
Using an existing feature, [1,2,3,4,5].lazy.drop(-n)
could be written as [1,2,3,4,5].each_cons(n+1).lazy.map(&:first)
.
Enumerable#each_cons does not use a circular buffer, though. (It currently uses push & shift)
That is clever.
I'll close this request, since it's not clear how frequent this could be needed and there's already an easy way of doing it (even though it might not be obvious).
Also available in: Atom
PDF
Like0
Like0Like0Like0Like0Like0Like0Like0Like0Like0