C Programming: Write/Create some code that calculates the perimeter of a polygon

Question

C Programming:

Write/Create some code that calculates the perimeter of a polygon.

The points of the polygon will be stored in a file and this file will be passed on the command line arguments

The file itself will be a binary file containing integers

   The first integer in the file is the number of points contained in the file

   The remaining integers are the points, with the first integer being the x coordinate and the    second integer being the y coordinate

   There is an edge between each adjacent point and between the first point and the last point

   Each file contains at least 3 points

The perimeter of a polygon is the sum of the lengths of all of its edges

Use a double to store your perimeter and report the perimeter to the nearest 2 decimal points.

You MUST store your points in a struct.

The example tests do not form actual polygons but assume that they do.

Example. Assume that there is a file called polygon.txt. It will store the following information but in binary form. This example is just to give you a visualization of the data.

3

287 422

283 -981

781 647

./perimeter polygon.txt

  OUTPUT

The perimeter is 3648.30

Explanation / Answer

//Program to calculate Area and Perimeter of a Polygon

/* Given the coordinates of the vertices of a convex polygon,
calculate its area and perimeter. Subdivide it into triangles
and calculate the area of each triangle with Heron's formula.
Requires data file pvert.txt containing coordinates of each vertex.
Example of data for a polygon with 5 vertices:

3 7 6 4 3 -2 -6 1 -6 7
*/
#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);
}

/* 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);
}

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