nil==0
=> false
class NilClass
include Comparable
def <=>(other)
0
end
end
=> nil
[1,nil,2].sort
ArgumentError: comparison of Fixnum with nil failed
from (irb):11:in `sort'
from (irb):11
p nil > 2
false
=> nil
p nil > -1
false
=> nil
p nil == 0
true
=> nil
What I am missing here?
RubyTalk@gmail.com wrote:
nil==0
=> false
class NilClass
include Comparable
def <=>(other)
0
end
end=> nil
[1,nil,2].sort
ArgumentError: comparison of Fixnum with nil failed
from (irb):11:in `sort'
from (irb):11p nil > 2
false
=> nilp nil > -1
false
=> nilp nil == 0
true
=> nilWhat I am missing here?
Probably this:
irb(main):003:0> class NilClass
irb(main):004:1> def <=>(other)
irb(main):005:2> 0
irb(main):006:2> end
irb(main):007:1> end
=> nil
irb(main):008:0> nil <=> 1
=> 0
irb(main):009:0> 1 <=> nil
=> nil
irb(main):010:0>
--
Posted via http://www.ruby-forum.com/\.
Yes! Thats exactly what I was missing. Why would someone write <=> for nil?
On Fri, May 8, 2009 at 4:23 PM, Adam Gardner <adam.oddfellow@gmail.com> wrote:
RubyTalk@gmail.com wrote:
nil==0
=> false
class NilClass
include Comparable
def <=>(other)
0
end
end=> nil
[1,nil,2].sort
ArgumentError: comparison of Fixnum with nil failed
from (irb):11:in `sort'
from (irb):11p nil > 2
false
=> nilp nil > -1
false
=> nilp nil == 0
true
=> nilWhat I am missing here?
Probably this:
irb(main):003:0> class NilClass
irb(main):004:1> def <=>(other)
irb(main):005:2> 0
irb(main):006:2> end
irb(main):007:1> end
=> nil
irb(main):008:0> nil <=> 1
=> 0
irb(main):009:0> 1 <=> nil
=> nil
irb(main):010:0>
--
Posted via http://www.ruby-forum.com/\.
There is probably no good reason because nothing doesn't compare well to anything - and so the method is not defined. 
Kind regards
robert
On 09.05.2009 01:35, RubyTalk@gmail.com wrote:
Yes! Thats exactly what I was missing. Why would someone write <=> for nil?
Robert Klemme wrote:
Yes! Thats exactly what I was missing. Why would someone write <=> for nil?
There is probably no good reason because nothing doesn't compare well to
anything - and so the method is not defined.
It can be useful to have an ordering defined for mixed collections. For
example, CouchDB defines a full ordering across null, numbers, strings,
arrays and objects (hashes):
http://wiki.apache.org/couchdb/View_collation
In Ruby, the solution depends on what exactly you're trying to do. Maybe
sort_by with a default value is good enough for what you need.
arr.sort_by { |e| e || -1 }
You can also use arr.sort { |a,b| ... } and then the block defines the
'<=>' logic for comparing two objects.
Note that it would be dangerous to define nil <=> any and any <=> nil to
be 0, because that would mean (any number) == nil == (any other number).
For example,
[2, nil, 1]
could be treated as an already-sorted array!
Instead, you might want to define nil <=> x as -1 and x <=> nil as +1
(where x is any non-nil value). You can do that in your sort block.
Regards,
Brian.
P.S. Also be aware that ruby's sort is implemented using quicksort
(IIRC), which is not stable for equal keys.