# Reverse-range alternatives?

**URL:** <https://www.ruby-forum.com/t/reverse-range-alternatives/105955>\
**Category:** Ruby\
**Created:** [June 4, 2007, 10:52pm UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955 "2007-06-04T22:52:40Z")\
**Posts on this page:** 11\
**Page:** 1

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 4, 2007, 10:52pm UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/1 "2007-06-04T22:52:40Z")

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Since a reverse range (eg. 4…1) is functionally almost the same as an  
empty range, is there an alternative in the standard library, where  
.each would actually iterate over the elements from first to last, in  
this case 4, 3, 2?

Thanks,  
Ken

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 4, 2007, 11:24pm UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/2 "2007-06-04T23:24:44Z")

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On 6/4/07, Kenneth McDonald [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Since a reverse range (eg. 4…1) is functionally almost the same as an  
> empty range, is there an alternative in the standard library, where  
> .each would actually iterate over the elements from first to last, in  
> this case 4, 3, 2?
> 
> Thanks,  
> Ken

You could do 4.downto(2) { |i| … }

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 12:08am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/3 "2007-06-05T00:08:12Z")

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fREW wrote:

> You could do 4.downto(2) { |i| … }  
> Oh, of course. I’m still not entirely used to thinking of numbers as  
> having a bunch of their own methods. Thanks!

Ken

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 2:03am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/4 "2007-06-05T02:03:21Z")

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On 04.06.2007 22:51, Kenneth McDonald wrote:

> Since a reverse range (eg. 4…1) is functionally almost the same as an  
> empty range, is there an alternative in the standard library, where each  
> would actually iterate over the elements from first to last, in this  
> case 4, 3, 2?

$ ruby -e ‘4.downto 1 do |i| p i end’  
4  
3  
2  
1

robert@fussel ~  
$ ruby -e ‘4.step 1, -1 do |i| p i end’  
4  
3  
2  
1

Kind regards

robert

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 3:40am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/5 "2007-06-05T03:40:32Z")

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From: Kenneth McDonald [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] :

# Since a reverse range (eg. 4…1) is functionally almost the

# same as an

# empty range, is there an alternative in the standard library, where

# .each would actually iterate over the elements from first to last, in

# this case 4, 3, 2?

assumming we’re all talking about range.

irb(main):014:0\> x  
=\> 1…4  
irb(main):015:0\> x.class  
=\> Range  
irb(main):016:0\> x.last  
=\> 4  
irb(main):017:0\> x.first  
=\> 1  
irb(main):018:0\> x.last.downto x.first do |e|  
irb(main):019:1\* p e  
irb(main):020:1\> end  
4  
3  
2  
1  
=\> 4

but i really hope something of a bidirectional range, ie, (4…1).to\_a ==  
[4,3,2,1] and (4…1)==(1…4).reverse, and then (4…1).each{|x| p x  
=\>4,3,2,1

right now (4…1) is useless, but it does not \_err…

kind regards -botp

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 6:48am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/6 "2007-06-05T06:48:17Z")

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Kenneth McDonald wrote:

> Since a reverse range (eg. 4…1) is functionally almost the same as an  
> empty range, is there an alternative in the standard library, where  
> .each would actually iterate over the elements from first to last, in  
> this case 4, 3, 2?

I can’t resist this one…

class Range  
def reverse  
r = dup  
def r.each(&block)  
last.downto(first, &block)  
end  
r  
end  
end

> > (1…9).to\_a  
> > =\> [1, 2, 3, 4, 5, 6, 7, 8, 9]
> 
> > (1…9).reverse.to\_a  
> > =\> [9, 8, 7, 6, 5, 4, 3, 2, 1]

😃

Daniel

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 7:07am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/7 "2007-06-05T07:07:19Z")

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I’d thought of that, but it’s simply too risky. Changing the behavior of  
something as fundamental as Range could really screw up if another  
required module counted on that behavior.

Generally, I’ll reopen a class to add methods to it, but not to change  
its behavior.

Too bad, though, that the original Range type didn’t have different  
semantics, if only to throw an exception when given an inverted range.

Cheers,  
Ken

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 7:31am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/8 "2007-06-05T07:31:20Z")

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On Jun 4, 4:51 pm, Kenneth McDonald [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Since a reverse range (eg. 4…1) is functionally almost the same as an  
> empty range, is there an alternative in the standard library, where  
> .each would actually iterate over the elements from first to last, in  
> this case 4, 3, 2?

Would be nice if Range supported this. It would mean working off a  
#pred, not just #succ. It’s been a while since I’ve messed with it,  
but I’m pretty sure Facets’ Interval class does this.

For a lite solution however you might consider:

(-4…-1).each { |i| i.abs }

T.

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 6, 2007, 12:41am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/9 "2007-06-06T00:41:26Z")

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Oops, my bad, I saw the class being reopened and jumped to a conclusion  
without even looking at the code. Thanks for pointing this out.

But I still wish it were possible to write (3…1) and have it do  
something that (IMHO) would be a bit more useful than the current  
behavior. 🙂 Oh well, too late now.

K

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 5, 2007, 11:03pm UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/10 "2007-06-05T23:03:15Z")

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First let me take the liberty of reversing the top posting:  
On 6/5/07, Kenneth McDonald [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > ```
> > r
> > 
> > ```
> > 
> > end  
> > end
> 
> I’d thought of that, but it’s simply too risky. Changing the behavior of  
> something as fundamental as Range could really screw up if another  
> required module counted on that behavior.
> 
> Generally, I’ll reopen a class to add methods to it, but not to change  
> its behavior.

Actually if you look carefully that’s what his code does. He added a  
method to range which returns a new instance of range with a singleton  
method which overrides each. Normal instances of range won’t be  
affected.

It’s a nice usage of the nested method definitions we were discussing  
recently.

Bravo Daniel.

Of course the reversed range should probably also invariants like:

```
(1..3).reverse.last == (1..3).reverse.to_a.last

```

And methods like to\_s and step should also do the right thing too.

If you want to go that far it’s probably better to have a ReverseRange  
class and have the Range#reverse return an instance of that.

–  
Rick DeNatale

My blog on Ruby  
[http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)

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**Author:** ![Kenneth\_McDonald](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/dc4da7/32.png) [@Kenneth\_McDonald](https://www.ruby-forum.com/u/Kenneth_McDonald)\
**Post date:** [June 6, 2007, 10:51am UTC](https://www.ruby-forum.com/t/reverse-range-alternatives/105955/11 "2007-06-06T10:51:28Z")

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On 06.06.2007 00:40, Kenneth McDonald wrote:

> Oops, my bad, I saw the class being reopened and jumped to a conclusion  
> without even looking at the code. Thanks for pointing this out.
> 
> But I still wish it were possible to write (3…1) and have it do  
> something that (IMHO) would be a bit more useful than the current  
> behavior. 🙂 Oh well, too late now.

The real issue here is that there are at least two useful ways to deal  
with ranges where the second element lies before the first one:

1. no iteration

This is useful for scenarios where you somehow determine the end point  
and you want to iterate only if it is to the right of the starting  
point. For example

def show\_silly\_example(s, start, char)  
(start … s.index(char)).each do |i|  
puts s[i]  
end  
end

1. backwards iteration

This is useful when you want to be able to do backward iteration.

Given the fact that not foo all possible range endpoints there is a  
meaningful #pred method, I guess option 1 is actually a better choice:

irb(main):012:0\> “ab”.succ  
=\> “ac”  
irb(main):013:0\> “ab”.pred  
NoMethodError: undefined method `pred’ for “ab”:String  
from (irb):13  
from :0  
irb(main):014:0\>

IMHO a ReverseRange class would be good, but at the moment I cannot  
think of a compelling syntax that would make creation as straightforward  
as for Range.

Kind regards

robert