[phobos] [Fwd: "Unsigned-related bugs never occur in real code."]

[Andrei Alexandrescu]

On a 64-bit system you have a portability problem. Generally casts are 
bad because they specify the target type when you actually want a 
different manipulation (in this case, changing signedness).

std.traits has a function called "unsigned" that takes an integral of 
any length and returns the corresponding unsigned integral. I'll define 
also "signed". With that and with the change to abs(), abs(j - i) is an 
error (static assert), and will actually suggest that you use 
abs(signed(j - i)) in its error message.

At any rate, all these fixes don't make me feel much better. Bugs are 
easy to come by and difficult to find (my initial code involved an 
associative array, and AAs have their _own_ bugs that made me suspicious).

In brief, D still has big problems with handling signed and unsigned 
integrals.


Andrei

Sean Kelly wrote:
> I got a reasonable answer by casting to int before calling abs():
> 
> import std.math, std.stdio;
> 
> void main() {
>    auto a = [ 4, 4, 2, 3, 2 ];
>    float avgdist = 0;
>    uint count;
> 
>    foreach (i, e1; a) {
>        foreach (j, e2; a) {
>            if (i == j) continue;
>            if (e1 != e2) continue;
>            ++count;
>            writeln("adding ", cast(int)(i - j));
>            avgdist += abs(cast(int)(i - j));
>        }
>    }
> 
>    writeln(count, " ", avgdist / count);
> }
> 
> On Jan 21, 2010, at 12:41 AM, Andrei Alexandrescu wrote:
> 
>> The problem persists even if I remove the call to abs(). At least its removal makes things look more suspicious.
>>
>> Andrei
>>
>> Don Clugston wrote:
>>> 2010/1/21 Sean Kelly <sean at invisibleduck.org>:
>>>> My first inclination would be for abs to only accept signed values.  Unsigned values don't really seem appropriate for unchecked math operations.
>>> I agree, it doesn't make sense to take the absolute value of something
>>> which doesn't have a sign. With range checking, it ought to be
>>> possible to implicitly cast from uint to int, which would allow all of
>>> the sensible uses. Unfortunately, implicit casting and templates don't
>>> interact well. So I think abs() might need to go back to being a
>>> non-template function.
>>>> On Jan 20, 2010, at 8:50 PM, Andrei Alexandrescu wrote:
>>>>
>>>>> Is there anything we can do about this?
>>>>>
>>>>> Andrei
>>>>>
>>>>> -------- Original Message --------
>>>>> Subject: "Unsigned-related bugs never occur in real code."
>>>>> Date: Wed, 20 Jan 2010 20:42:50 -0800
>>>>> From: Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org>
>>>>> Organization: Digital Mars
>>>>> Newsgroups: digitalmars.D
>>>>>
>>>>> "It's an academic problem. Don't worry about it and move on."
>>>>>
>>>>> That's what Walter kept on telling me. Yet I've spent the better part of
>>>>> an hour reducing a bug down to the following:
>>>>>
>>>>> import std.math, std.stdio;
>>>>>
>>>>> void main() {
>>>>>   auto a = [ 4, 4, 2, 3, 2 ];
>>>>>   float avgdist = 0;
>>>>>   uint count;
>>>>>
>>>>>   foreach (i, e1; a) {
>>>>>       foreach (j, e2; a) {
>>>>>           if (i == j) continue;
>>>>>           if (e1 != e2) continue;
>>>>>           ++count;
>>>>>           avgdist += abs(i - j);
>>>>>       }
>>>>>   }
>>>>>
>>>>>   writeln(count, " ", avgdist / count);
>>>>> }
>>>>>
>>>>> May this post be an innocent victim of the war against unsigned-related
>>>>> bugs.
>>>>>
>>>>>
>>>>> Andrei
>>>>> _______________________________________________
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