Re: Sort function modifies the table being sorted?

[Paul K <paulclinger@...>]

I suspect so too, but (1) it's not stated anywhere that it's against
the rules and (2) changing to "o[a] and 0 or 9" fixes it, which should
make no difference.

And I'm only reading the table, which should not trigger any
modification in it (even though it may be in some strange/intermediate
state).

Paul.

On Wed, Jun 13, 2012 at 10:26 PM, Dimiter 'malkia' Stanev
<malkia@gmail.com> wrote:
> I think you are introducing non-determinism.
>
> You are getting two values to be compared (a and b), and yet
> instead you use these values as indices to the current state of the array
> (which is not your business, hence the non-determinism).
>
> the sort routine just asked you to compare two things, not to use them as
> indices vs the current (random in a sense) state of the array.
>
>
> On 6/13/2012 8:56 PM, Paul K wrote:
>>
>> I'm utterly puzzled by the behavior of this fragment:
>>
>> local alphanumsort = function(o)
>>     table.sort(o, function(a,b)
>> print('---', a, b) for k,v in ipairs(o) do print(k,v) end
>>       return (o[a] or 9)..a<  (o[b] or 9)..b
>>     end) end
>> alphanumsort({1,2,3,4,5,6,7,'a',1.2,-1})
>>
>> This is a simplified fragment from an application I have been working
>> on. On my machine (Windows with Lua 5.1.4) it dies with:
>>
>> sort.pl:4: attempt to concatenate local 'b' (a nil value)
>> stack traceback:
>>        sort.pl:4: in function<sort.pl:2>
>>        [C]: in function 'sort'
>>        sort.pl:2: in function 'alphanumsort'
>>        sort.pl:6: in main chunk
>>        [C]: ?
>>
>> Where does this "nil" value come from? It is clearly not in the
>> original table. if I replace ..a and ..b with tostring(a) and
>> tostring(b), I get "invalid order function for sorting", which usually
>> indicates that the function returns inconsistent results (1<  5 in one
>> case and 1>  5 in another), but I don't see why it would do it in this
>> case. Basically, the idea is to sort elements that match their
>> positions first and sort everything else after that.
>>
>> Even more puzzling, when I replace "o[a] or 9" with "o[a] and 0 or 9",
>> things start working as expected, but maybe it just hides the issue;
>> not sure.
>>
>> If you review the output, you notice that some elements got swapped
>> even though I would not expect them to be (that may be a red herring
>> though as they get swapped in "normal" cases too). For example, values
>> for 5 and 9 get swapped, even though 5 is "before" 1.2 given the
>> result of the comparator.
>>
>> ---     -1      5
>> 1       1
>> 2       2
>> 3       3
>> 4       4
>> 5       5
>> 6       6
>> 7       7
>> 8       a
>> 9       1.2
>> 10      -1
>> ---     2       5
>> 1       1
>> 2       2
>> 3       3
>> 4       4
>> 5       1.2
>> 6       6
>> 7       7
>> 8       a
>> 9       5
>> 10      -1
>>
>> Paul.
>>
>>
>