My conversion identifies my polygon as ccw. I know it is cw.
Did I make a mistake converting from C to C#?
I noted the assumption that the last point is not repeated and have tried my code with n-1, but still returns ccw.
Also, my polygon is in the Western Hemisphere, meaning that Longitude is expressed as a negative number.
I have tried all the solutions to deal with the negative number I can think of.
Thanks for any help,
Michael

C function by Paul Bourke
/*
Return the clockwise status of a curve, clockwise or counterclockwise
n vertices making up curve p
return 0 for incomputables eg: colinear points
CLOCKWISE == 1
COUNTERCLOCKWISE == -1
It is assumed that
- the polygon is closed
- the last point is not repeated.
- the polygon is simple (does not intersect itself or have holes)
*/
int ClockWise(XY *p,int n)
{
int i,j,k;
int count = 0;
double z;
if (n < 3)
return(0);
for (i=0;i<n;i++) {
j = (i + 1) % n;
k = (i + 2) % n;
z = (p[j].x - p[i].x) * (p[k].y - p[j].y);
z -= (p[j].y - p[i].y) * (p[k].x - p[j].x);
if (z < 0)
count--;
else if (z > 0)
count++;
}
if (count > 0)
return(COUNTERCLOCKWISE);
else if (count < 0)
return(CLOCKWISE);
else
return(0);
}
But I don't remember much C
I have tried to rewrite to C#:
private static int ClockWise(double[] y, double[]x, int n)
{
int i,j,k;
int count = 0;
double z;
if (n < 3)//not a polygon
return(0);
for (i=0;i<n-1;i++)
{
j = (i + 1) % n;
k = (i + 2) % n;
z = (x[j] - x[i]) * (y[k] - y[j]);
z -= (y[j] - y[i]) * (x[k] - x[j]);
if (z < 0)
count--;
else if (z > 0)
count++;
}
if (count > 0)//counterclockwise
return(1);
else if (count < 0)//clockwise
return(-1);
else
return(0);
}

I have solved this problem, and would like to close this question. I did not actually get an answer to my question, but than-you ragi0017: for you input. If someone still wants to answer the basic question,

try this one out
what i have tried here is creating a point class and using a generic list
i am assuming that you C program i taking the input of points array which is similar to the List<Point>

class Point
{
public double x;
public double y;
}
private int ClockWise (List<Point> p, int n)
{
int i, j, k;
int count = 0;
double z;
if (n < 3)
return (0);
for (i = 0; i < n; i++)
{
j = (i + 1) % n;
k = (i + 2) % n;
z = (p[j].x - p[i].x) * (p[k].y - p[j].y);
z -= (p[j].y - p[i].y) * (p[k].x - p[j].x);
if (z < 0)
count--;
else if (z > 0)
count++;
}
if (count > 0)
return (-1);
else if (count < 0)
return (1);
else
return (0);
}

Thank-you,
No, I do not use C. The question is: did I properly translate the C code to C#? My C# translation works for a simple 4-sided polygon but not for a 2500-sided polygon. Either the original C code is incorrect or my translation is incorrect. The simplest solution would be that my translation is incorrect. If you are able to assure me that my translation is either correct or incorrect, that would be the solution to this question.

I have solved this problem, and would like to close this question. I did not actually get an answer to my question, but than-you ragi0017: for you input. If someone still wants to answer the basic question, that is: Is my translation of the C code to C# correct or not, I am still interested. If the translation is correct, the C code algorithm is incorrect (or not robust enough for my needs) I did find another alorithm which I was able to adapt to C# and it works.

0

Manage projects of all sizes how you want. Great for personal to-do lists, project milestones, team priorities and launch plans.
- Combine task lists, docs, spreadsheets, and chat in one
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Even though I have solved this problem, I would still like to know if my translation from C to C# is correct or not. I will award the points for a yes or no answer with explanation if incorrect.

Although it is irrelevant, but for those who too want to check if conversion is good or not, then with my knowledge it is good.

0

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