Program to calculate Area of a Polygon

#include stdio.h
#include stdlib.h
#include math.h
#define MAX_VERT 50
enum {x, y};
typedef struct triangle
{
double v1[2];
double v2[2];
double v3[2];
} triangle;
double area(triangle a);
double perimeter(double *vertices, int size);
double side(double *p1, double *p2);

int main(void)
{
int n, idx;
int triangles;
int index;
int xycount;
double xy;
double triangle_area;
double polygon_area;
double perim;
double polygon_vertices[MAX_VERT] = {0.0};
triangle a;
FILE *data;

xycount = 0;
polygon_area = 0;
if((data = fopen("pvert.txt", "r")) == NULL)
{
fprintf(stderr, "can't open data file");
exit(EXIT_FAILURE);
}

/* Read x-y coordinates of the vertices
of the polygon from a file. */
while(fscanf(data, "%lf", &xy) == 1)
polygon_vertices[xycount++] = xy;
fclose(data);
idx = 0;
/* triangles in polygon = vertices - 2 */
triangles = (xycount / 2) - 2;
putchar('');

for(index = 2, idx = 0; idx < triangles; index += 2, ++idx)
{
/* Load vertices of a triangle into struct.
1st vertex of the polygon will be the 1st
vertex of each triangle. index holds the
starting index of each consecutive set of
triangle vertices after the 1st. */
a.v1[x] = polygon_vertices[0];
a.v1[y] = polygon_vertices[1];
a.v2[x] = polygon_vertices[index+0];
a.v2[y] = polygon_vertices[index+1];
a.v3[x] = polygon_vertices[index+2];
a.v3[y] = polygon_vertices[index+3];

/* calculate the area of the triangle */
triangle_area = area(a);
printf("area of triangle = %.2f
", triangle_area);

/* add triangle area to polygon area */
polygon_area += triangle_area;
}
printf("area of polygon = %.2f", polygon_area);

/* calculate the perimeter of the polygon */
perim = perimeter(polygon_vertices, xycount);
printf("perimeter of polygon = %.2f", perim);

return 0;
}

/* calculate triangle area with Heron's formula */
double area(triangle a)
{
double s1, s2, s3, S, area;

s1 = side(a.v1, a.v2);
s2 = side(a.v2, a.v3);
s3 = side(a.v3, a.v1);
S = (s1 + s2 + s3) / 2;
area = sqrt(S*(S - s1)*(S - s2)*(S - s3));

return area;
}

/* calculate polygon perimeter */
double perimeter(double *vertices, int size)
{
int idx, jdx;
double p1[2], p2[2], pfirst[2], plast[2];
double perimeter;

perimeter = 0.0;
/* 1st vertex of the polygon */
pfirst[x] = vertices[0];
pfirst[y] = vertices[1];
/* last vertex of polygon */
plast[x] = vertices[size-2];
plast[y] = vertices[size-1];
/* calculate perimeter minus last side */
for(idx = 0; idx <= size-3; idx += 2)
{
for(jdx = 0; jdx < 4; ++jdx)
{
p1[x] = vertices[idx];
p1[y] = vertices[idx+1];
p2[x] = vertices[idx+2];
p2[y] = vertices[idx+3];
}
perimeter += side(p1, p2);
}
/* add last side */
perimeter += side(plast, pfirst);

return perimeter;
}

/* calculate length of side */
double side(double *p1, double *p2)
{
double s1, s2, s3;

s1 = (p1[x] - p2[x]);
s2 = (p1[y] - p2[y]);
s3 = (s1 * s1) + (s2 * s2);

return sqrt(s3);
}