# [fpc-pascal] C translation question

Bernd Oppolzer bernd.oppolzer at t-online.de
Sun Oct 23 22:20:27 CEST 2016

it might help if I translate the C for loop into an equivalent
while loop for you, (and eliminating the ++ construct),
simply mechanically, without knowing anything about the application.

This results in:

int pnpoly (int nvert, float *vertx, float *verty, float testx, float testy)
{
int i, j, c = 0;

i = 0, j = nvert-1;

while (i < nvert)
{
if (((verty [i] > testy) != (verty [j] > testy)) &&
(testx < (vertx[j] - vertx[i]) * (testy - verty [i])
/ (verty [j] - verty [i]) + vertx [i]))
c = !c;

j = i;
i = i + 1;
}

return c;
}

HTH,

kind regards

Bernd

Am 22.10.2016 um 11:06 schrieb Ryan Joseph:
> I’m trying to translate a function from C (taken from http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html) and although I think I got it correctly it’s not behaving 100% accurately so I’m not sure if it was my translation or not. The “for” statement is pretty confusing to my eyes despite the author of the function giving a description of his code.
>
> Does anyone spot any errors with my translation?
>
> int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy)
> {
>    int i, j, c = 0;
>    for (i = 0, j = nvert-1; i < nvert; j = i++) {
>      if ( ((verty[i]>testy) != (verty[j]>testy)) &&
>       (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
>         c = !c;
>    }
>    return c;
> }
>
>
> function TPolygonHelper.ContainsPoint (point: TPoint): boolean;
> var
> 	i, j, c: integer;
> begin
> 	i := 0;
> 	j := high(self);
> 	c := 0;
> 	for i := 0 to high(self) do
> 		begin
> 			j := i + 1;
> 			if ((self[i].y > point.y) <> (self[j].y > point.y))
> 					and (point.x < (self[j].x - self[i].x) * (point.y - self[i].y) / (self[j].y - self[i].y) + self[i].x) then
> 				c := not c;
> 		end;
> 	result := c <> 0;
> end;
>
> Regards,
> 	Ryan Joseph
>
> _______________________________________________
> fpc-pascal maillist  -  fpc-pascal at lists.freepascal.org
> http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal