Mail Archives: djgpp/1998/05/10/11:20:31
Elias Pschernig wrote:
>
> I've a problem:
> I'm working on a game, which uses "float"-type for
> x- and y-coordinates of an object. Sometimes the
> object starts moving very, er, bulky, and i found out,
> this is the case, when either y or x coordinate go
> over 100000; the larger, the more bulky the movement.
> I'm now sure this has to do with the float-type, but
> it's very strange, because there occurs no other
> error, i.e. the position always is correct, even with
> large numbers, just it moves not smoothly any more.
it is in general not a good idea to use float or even double variables
for such purposes. floating point representations are usually not
necessarily exact, and addition is subject to large errors if you are
addin a very small number to a very large number. in essense, the
smaller number becomes indistinguishable from zero. note that this has
nothing to do with djgpp, but is a basic fact of life when you are
dealing with floating point arithmetic. as an example, you can try:
#include <stdio.h>
int main(void)
{
int i;
float sum1 = 0.0, sum2 = 0.0;
float a[] = { 10e6, -10e6, 10e-2};
for(i = 0; i < 3; i++)
sum1 += a[i];
for( ; i; i--)
sum2 += a[i-1];
printf("%g\t%g", sum1, sum2);
return 0;
}
using doubles only delays the onset of the problem. you should keep
track of the coordinates in some integral type, and do conversions as
needed. for more on this, you can search for the document "what every
computer scientist needs to know about floating point arithmetic" on
altavista.
>
> I would like it very much, if somebody could
> explain to me how DJGPP's float-type works.
> (also if it's the i86's float-type)
for the record, this is _NOT_ peculiar to djgpp.
-- Sinan
----------------------------------------------------------------------
A. Sinan Unur
Department of Policy Analysis and Management, College of Human Ecology,
Cornell University, Ithaca, NY 14853, USA
mailto:sinan DOT unur AT cornell DOT edu
http://www.people.cornell.edu/pages/asu1/
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