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 <[email protected]>:
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 <[email protected]>
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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