On Thursday, 23 October 2014 at 18:26:53 UTC, Steven
Schveighoffer wrote:
On 10/23/14 2:18 PM, deed wrote:
Using equality is not a good idea with floating point.
The compiler will on a whim, or depending on whether it can
inline or
not, use higher precision floats, changing the outcome
slightly.
I cannot say for certain whether this explains all the issues
you
have, the very last one seems troubling to me at least.
Sure, in many cases it's a bad idea. While I understand that
sin(PI) !=
0.0, but approxEqual(sin(PI), 0.0) == true, I would expect the
following
to pass:
assert (0.0 == 0.0);
assert (1.2345 == 1.2345);
F a = 1.2345, b = 9.8765;
assert (a+b == b+a);
assert (a*b == b*a);
None of these fail on my system
Same for me, that's the point; it's perfectly valid to compare
floating points.
F fun (F a) pure;
assert (fun(a) + fun(b) == fun(b) + fun(a));
assert (fun(a) * fun(b) == fun(b) * fun(a));
auto a = fun(100);
auto b = fun(100);
assert (a == b);
assert (fun(100) == fun(100));
Not sure what body of fun is, so I cannot test this.
Could be anything taking a floating point and returning a
floating point.
You would expect to get the same back for the same input,
especially when
it's pure. If so, the asserts above are expected to hold.
Now, if fun's body is { return sin(a); }, the behaviour
changes to:
auto c = fun(100);
auto d = fun(100);
assert (c == d); // Ok
assert (fun(100) != fun(100)) // I have a hard time
understanding
// this is correct behaviour
Tried that out, it does not fail on my machine. Can you be more
specific on your testing? What compiler/platform? Stock
compiler, or did you build it yourself?
Right. It doesn't fail, and that's the problem.
assert (fun(100) == fun(100)); // Should pass, and does with
// body { return a + 1; }
// but not with
// body { return sin(a); }
assert (fun(100) != fun(100)); // Shouldn't pass, but passes with
// body { return sin(a);}
Compiler: DMD32 D Compiler v2.066.0
Also tried dpaste.dzfl.pl with 2.065.0 and DMD 2.x Git
(cfb5842b49),
which had slightly different behaviour; more of the sin tests
which should
pass in my opinion did with those two. Hence, it appears to be
regressions.
-Steve
