On 2009-12-14 03:16:27 -0800, Don <[email protected]> said:
Walter Bright wrote:
Don wrote:
Consider this notorious piece of code:
assert(x>1);
double y = 1 / x;
This calculates y as the reciprocal of x, if x is a floating-point
number. But if x is an integer, an integer division is performed
instead of a floating-point one, and y will be 0.
It's a very common newbie trap, but I find it still catches me
occasionally, especially when dividing two variables or compile-time
constants.
In the opPow thread there were a couple of mentions of inadvertent
integer division, and how Python is removing this error by making /
always mean floating-point division, and introducing a new operator for
integer division.
We could largely eliminate this type of bug without doing anything so
drastic. Most of the problem just comes from C's cavalier attitude to
implicit casting. All we'd need to do is tighten the implicit
conversion rules for int->float, in the same way that the int->uint
rules have been tightened:
"If an integer expression has an inexact result (ie, involves an
inexact integer divison), that expression cannot be implicitly cast to
a floating-point type."
But the compiler cannot reliably tell if it will produce an inexact result.
(This means that double y = int_val / 1; is OK, and also:
double z = 90/3; would be OK. An alternative rule would be:
"If an integer expression involves integer divison, that expression
cannot be implicitly cast to a floating-point type").
This is kinda complicated if one has, say:
double z = x/y + 3;
Integer expressions remain inexact until there's a cast.
(It's very simple to implement, you just use the integer range code,
adding an 'inexact' flag. Division sets the flag, casts clear the flag,
everything else just propagates it if a unary operation, or ORs the two
flags if a binary operation).
Seems like when the product of integer devision is cast to a double is
when it should produce a warning.
-SC
