Program to calculate Area of a Polygon in c

#include
#include
#include
#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);
}

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)
{

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];


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

polygon_area += triangle_area;
}
printf("
area of polygon = %.2f
", polygon_area);

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

return 0;
}

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;
}

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

perimeter = 0.0;
pfirst[x] = vertices[0];
pfirst[y] = vertices[1];
plast[x] = vertices[size-2];
plast[y] = vertices[size-1];
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);
}
perimeter += side(plast, pfirst);

return perimeter;
}

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);
}