On Tuesday, 12 November 2013 at 04:41:56 UTC, Vladimir Panteleev
wrote:
double[] arr = [1, 2, 3, double.nan, 1, 2];
assert(arr.isSorted);
arr.sort();
This code will fail with an assert error, but not the one on
the second line. Rather, it will fail inside std.range, when
sort calls assumeSorted, which checks elements in an order
different than sort and isSorted.
Here's a case where the odd way NaN interacts with generic code
messes things up in an ugly way.
This is concerning. It's very easy to overlook this while
writing an application, and it can become a hidden
vulnerability.
We can't fix the operators... but, perhaps, we could define a
safe default comparison predicate (e.g. std.algorithm.less) for
use with sorting and related tasks? Aside from bitwise
comparison, is it even possible to efficiently compare
floating-point values in a way suitable for sorting?
You cannot sort NaNs. A comparison with NaN must always evaluate
to false.
One could change isSorted to check for false instead of true, so
it would accept NaN. That would probably break too much code,
though.
arr[n] < arr[n+1] => !(arr[n+1] < arr[n])
Apparently, isSorted contains an antisymmetry check, which is not
triggered, because it relies on true results.
