What I wanted to say:
when translating the for loop to Pascal, you are in danger of introducing
logic errors, because the Pascal for loop is kind of restricted compared to
the C for loop (which is in fact simply a while loop with another
notation).
But if you translate the C for loop to a while loop first, then you can
translate this loop in a straightforward manner to Pascal, and the
probability
of inserting logic errors is much lower.
Kind regards
Bernd
Am 23.10.2016 um 22:20 schrieb Bernd Oppolzer:
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
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