Thread: Strange issue: Different results of the calculation of a polygn area and perimeter

Im have a program to calculate the area and perimeter of a polygn. Also Im confiming if the result of the area and perimeter calculations are equal to the expected result.

I dont understand what is happening but the verification part, to confirm if the area and perimeter are equal to what it is expected is not working corretly.

For exemple, I test now and I get that "Arrays are the same" in all cases, which is correct because I gave the correct coordinates in the txt file to match each one of the expected results. But I test a little bit later or on a diferente machine, I tested in Linux also, and in some cases it appears "Arrays are diferent". This is really strange to happen without changing the coordinates file or the code.

Do you understand why this strange issue is happening?


This is the texto file, with the correct coordinaes to mach the expected results:

1.0 2.5 5.1 5.8 5.9 0.7
1.2 4.1 5.1 5.8 6.8 1.9 2.9 0.2
1.7 4.9 5.1 5.8 7.0 2.8 4.8 0.1 1.5 1.4
2.1 5.3 5.1 5.8 7.0 3.5 5.9 0.7 2.9 0.2 1.0 2.5
2.5 5.6 5.1 5.8 6.9 3.9 6.5 1.3 4.2 0.0 1.8 1.0 1.0 3.4
2.8 5.7 5.1 5.8 6.7 4.2 6.8 1.9 5.2 0.3 2.9 0.2 1.3 1.8 1.2 4.1

Program:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
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);

char result[256];
char expected[] = "11.7715.65";     //triangle regular
char expected1[] = "18.1017.02";    //quadrilateral regular
char expected2[] = "21.3317.60";    //pentagon regular
char expected3[] = "23.5518.07";    //hexagon regular
char expected4[] = "24.8018.29";    //heptagon regular
char expected5[] = "25.1418.26";     //octagon regular

int main()
{
    int n, idx;
    int triangles;
    int index;
    int xycount;
    double xy;
    double triangle_area;
    double polygon_area;
    double perim;
    double polygon_vertices[50] = {0.0};
    triangle a;
    FILE* data;
    char line[256];
    char* token;
    if ((data = fopen("test.txt", "r")) == NULL) {
        fprintf(stderr, "can't open data file\n");
        exit (EXIT_FAILURE);
    }
    while (fgets(line, sizeof (line), data)){
        xycount = 0;
        polygon_area = 0;
        line[strlen(line) - 1] = 0;
        token = strtok(line, " ");
        while (token != NULL){
            xy = atof(token);
            token = strtok(NULL, " ");
            polygon_vertices[xycount++] = xy;
        }
        idx = 0;
        triangles = (xycount / 2) - 2;
        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);
            polygon_area += triangle_area;
        }
        printf("area=%.2f\t", polygon_area);
        perim = perimeter(polygon_vertices, xycount);
        printf("perimeter=%.2f\n", perim);
        sprintf(result, "%.2f%.2f", polygon_area, perim);
        if(strcmp(result,expected) == 0) {
            printf("Arrays are the same\n");
        }
        else if(strcmp(result,expected1) == 0){
            printf("Arrays are the same\n");
        else if(strcmp(result,expected2) == 0){
            printf("Arrays are the same\n");        
        }
        else if(strcmp(result,expected3) == 0){
            printf("Arrays are the same\n");        
        }
        else if(strcmp(result,expected4) == 0){
            printf("Arrays are the same\n");        
        }
        else if(strcmp(result,expected5) == 0){
            printf("Arrays are the same\n");
        }
        else {
            printf("Arrays are the different\n");        
        }
    }
    fclose(data);
    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);
}