Descending ranges in Ruby

Ruby
1

Dear list members,

(5…1).to_a #returns []
(1…5).to_a #returns [1,2,3,4,5]
(5…1).each{|x| puts x} #prints (-5…-1)
(1…5).each{|x| puts x} #prints
1
2
3
4
5

I don’t get an error message when I declare a descending Range or when I
apply an iterator method to it, but I’m not able to iterate over a
descending Range the same way I can an ascending Range. Can someone
explain what the rationale is behind that language restriction?

Thanks,
Harry

2

5.downto(1) { |i| puts i }
5
4
3
2
1

3

Thanks for this work-around but my question was not so much how to get
around this restriction but rather “why I’m restricted from iterating
over a descending range if descending Ranges are legal?”. For example a
Range returned by a function in my program that I may not know whether
it is ascending or descending?

Harry

4

AFAIK, ruby simply considers values like (5…1) as empty range.
Iterating over it has no sense. You can iterate over ascending range in
reverse order using Enumerable#reverse_each method, for example.

5

On Fri, Nov 19, 2010 at 5:49 AM, Harry S. [email protected]
wrote:

Thanks for this work-around but my question was not so much how to get
around this restriction but rather “why I’m restricted from iterating
over a descending range if descending Ranges are legal?”. For example a
Range returned by a function in my program that I may not know whether
it is ascending or descending?

IIRC a Range finds the next value via #succ. And Fixnum#succ always
yields the next higher number. If you want to use a Range you must
use a type which reverses #succ and #<=>, example:

irb(main):001:0> RFN = Struct.new :value do
irb(main):002:1* def <=>(o) o.value <=> value end
irb(main):003:1> def succ; self.class.new(value - 1) end
irb(main):004:1> end
=> RFN
irb(main):005:0> (RFN.new(5) … RFN.new(1)).to_a
=> [#, #, #, #, #]

I’d rather stick with the other approaches presented or use
Array#reverse.

Kind regards

robert

6

On Nov 19, 5:01am, Harry S. [email protected] wrote:

Dear list members,

(5…1).to_a #returns []
(1…5).to_a #returns [1,2,3,4,5]

(1…5).to_a.reverse
=> [5, 4, 3, 2, 1]

– Maurice