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## Feature #8422

closed

Status:
Feedback
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:55046]

Description

they are better when you want descending order,
enum.sort.reverse
does work too, but it is to slow because it needs to build that result array twice

they both can be defined just like #sort and #sort_by but negates the result of the compare block

#### Updated by matz (Yukihiro Matsumoto)about 10 years ago

• Status changed from Open to Feedback

It can be done by `a.sort{|a,b| b<=>a}`. Do we really need to define new methods?

Matz.

#### Updated by marcandre (Marc-Andre Lafortune)about 10 years ago

matz (Yukihiro Matsumoto) wrote:

It can be done by `a.sort{|a,b| b<=>a}`.

That will typically be much slower though.

Currently, the best way to do a reverse sort, performance-wise, is `a.sort.reverse!`

For a random 100 number array, for example:

`sort.reverse!` is faster than `sort.reverse` by 10% ± 1.0%
`sort.reverse` is faster than `sort_by` by 6x ± 0.1
`sort_by` is faster than `sort` by 60% ± 1.0%

#### Updated by Anonymous about 10 years ago

-1, feature creep
But I'm for defining `reverse!` as O(1), not really reversing anything, just treating the last element as first and first as last. (I do not know how collections are exactly implemented, so I am not sure whether this is possible.)

#### Updated by drbrain (Eric Hodel)about 10 years ago

How do you sort an infinite Enumerable?

``````ruby -rprime -e 'p Prime.each.reverse_sort.take 10'
``````
Actions #5

#### Updated by marcandre (Marc-Andre Lafortune)almost 10 years ago

drbrain (Eric Hodel) wrote:

How do you sort an infinite `Enumerable`?

``````ruby -rprime -e 'p Prime.each.reverse_sort.take 10
``````

I'm not sure what your point is. How is that different from `p Prime.each.sort.take 10`? Moreover, an alternative request could be to add `Array#reverse_sort{_by}`.

Maybe we should consider a variation on `sort` that allows multiple criteria and independent sort directions?

#### Updated by drbrain (Eric Hodel)almost 10 years ago

Neither `Prime.each.reverse_sort.take 10` nor `Prime.each.sort.take 10` will return as there is no last value for infinite streams. You can neither reverse nor sort an infinite stream.

I believe adding the feature to `Enumerable` doesn't fit with its existing functionality (all beginning-of-stream oriented).

We need feedback from the submitter. Without it this issue should be closed.

#### Updated by marcandre (Marc-Andre Lafortune)almost 10 years ago

drbrain (Eric Hodel) wrote:

Neither `Prime.each.reverse_sort.take 10` nor `Prime.each.sort.take 10` will return as there is no last value for infinite streams.

Of course. But you are aware that `Enumerable` has a `sort` method that acts as a shortcut to `to_a.sort`?

We need feedback from the submitter. Without it this issue should be closed.

The relevant question here is "should there be a dedicated way to do a reverse sort". The question of where that method should be (`Array` or `Enumerable`) is accessory.

#### Updated by Hanmac (Hans Mackowiak)almost 10 years ago

my idea would be this, for ruby it may be cood if its converted into C++ code

``````module Enumerable
def reverse_sort(&block)
block ||= proc {|x,y| x <=> y }
sort {|x,y| - block.call(x,y)}
end

def reverse_sort_by