C - Polygonberechnung (ohne Aufgabe 4)

#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
#include <stdlib.h>
#include <math.h>


int main()
{
    double flaecheninhalt;

    // variables for reading the measurements from file
    FILE * fp;
    size_t len = 0;
    char resp;

    double x, y;

    // open the input file "messwerte.txt" for reading
    fp = fopen("polygon.txt", "r");
    if (fp == NULL)
    {
        // if file could not be opened (wrong path, not readable, ...)
        // output a short message and immediately exit
        printf("Eingabedatei kann nicht geoeffnet werden.\n");
        scanf("%c", &resp);
        exit(EXIT_FAILURE);
    }

    // print program header
    printf("\n\nProgramm zur Berechnung eines Polygons\n");
    printf("--------------------------------------------------------\n");

    // the following loop reads a new value in every iteration
    // until the end of the file or some invalid token is found
    int n=0;
    int a[999];
    int b[999];

    while (1)
    {
        len = fscanf(fp, "%lf %lf", &x, &y);

        if (len == EOF)
            break;
        else if (len == 0) {
            printf("Unerwartetes Zeichen in der Eingabedatei.");
            scanf("%c", &resp);
            exit(EXIT_FAILURE);
        }

        printf("Lese Eckpunkt: %6.2f/%6.2f\n", x, y);
       a[n] = x;
       b[n] = y;
        n++;
    }

    flaecheninhalt=0;

    for(int i=0; i<n;i++){
        if(i==(n-1)){
            flaecheninhalt +=((b[i]+b[0])*(a[i]-a[0]));

        }
        else{
            flaecheninhalt += ((b[i]+b[i+1])*(a[i]-a[i+1]));
        }
    }
    flaecheninhalt = flaecheninhalt/2;
    double xschwer, yschwer;
    for(int j=0;j<n;j++){
        if(j==(n-1)){
            xschwer += (a[j] + a[0]) * (a[j] * b[0] - a[0] * b[j]);
            yschwer += (b[j] + b[0]) * (a[j] * b[0] - a[0] * b[j]);

        }
        else {
            xschwer += (a[j] + a[j + 1]) * (a[j] * b[j + 1] - a[j + 1] * b[j]);
            yschwer += (b[j] + b[j + 1]) * (a[j] * b[j + 1] - a[j + 1] * b[j]);
        }
        }
    xschwer = xschwer / (6*flaecheninhalt);
    yschwer = yschwer / (6*flaecheninhalt);


    // output results
    printf("\nErgebnisse:\n");
    printf("-----------\n\n");
    printf("A = %.2f\n", flaecheninhalt);
    printf("Schwerpunkt = (%.2f|%.2f) \n", xschwer, yschwer);

    // finally close the input file and clean up memory
    fclose(fp);

    // wait for user input before closing terminal
    scanf("%c", &resp);
}