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digitalmars.D.bugs - [Issue 12452] New: To mitigate unwanted integer division precision loss

reply d-bugmail puremagic.com writes:

           Summary: To mitigate unwanted integer division precision loss
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc

--- Comment #0 from bearophile_hugs eml.cc 2014-03-24 07:14:56 PDT ---
This kind of code sometimes is wrong, because you forget to cast x to double
before the division and you lose precision (note that here the compiler knows
that the result of the division will go in a floating point variable):

void main() {
    int x = 15;
    double y = x / 10;

(The cause is that unfortunately in D the integer division uses the same
operator as the FP division. In Python there is the / and // operators. In
OcaML there are the / and /., in Delphi there are the / and div operators, in
Ada the two operands need to be of the same type).

Seasoned C/C++/D programmers watch for the types every time they perform a
division, to avoid that trap, and they often use explicit casts in that kind of
code (this uses a recently introduce syntax that avoids the use of a cast()):

double y = double(x) / 10;

But less experienced programmers introduce bugs with divisions. Can D help the
programmers reduce the frequency of similar bugs?

- - - - - - - - - - - -

A comment by Daniel Murphy:

 Newbies have to realize that integers aren't real numbers eventually.

I have seen that even programmers with some experience once in a while create this bug. Because you have to keep attention to the types of each division usage, and once in a while your attention slips (or perhaps some bugs of this kind are created by successive type changes). An example of this possible code found in real code: Search for "GetTransmission()" in this page: http://www.viva64.com/en/b/0242/ The possible bug: int SpectralLMM5Interface::GetTransmission(...., double& transmission) { .... int16_t tr = 0; memcpy(&tr, answer + 1, 2); tr = ntohs(tr); transmission = tr/10; .... } - - - - - - - - - - - - Don suggests:
 It is indeed a common floating-point bug.

 I came up with a solution for this a couple of years ago, never
 got around to doing a pull request, but it's on the newsgroup
 somewhere. It's a little extension to the range propagation
 implementation. You add a boolean flag to the range, which
 indicates 'a fractional part has been discarded'. This flag gets
 set whenever you perform an integer division (or integer
 exponentiation with a negative power), and is cleared whenever
 there is a cast or a bitwise operation.

 Then, disallow implicit casting from integer to floating point
 whenever the fractional bit is set. Catches all these kinds of
 bugs, doesn't require any changes to the language.

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parent d-bugmail puremagic.com writes:

Andrei Alexandrescu <andrei erdani.com> changed:

           What    |Removed                     |Added
             Status|NEW                         |ASSIGNED

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