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#define _CRT_SECURE_NO_WARNINGS

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

int main()
{

   // 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("polygon4.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 i=1;
   int n=0;
   double xarray[50];
   double yarray[50];


   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);
        }

        xarray[i]=x;
        yarray[i]=y;

      printf("Lese Eckpunkt: %6.2f/%6.2f\n", x, y);
      i++;
      n++;
   }

    double sum = 0;
   double temp;

   i= 0;
//if n= 4
if(n == 4){
   for(i=1;i<n;i++){

       temp =( yarray[i]+yarray[i+1])*( xarray[i]-xarray[i+1])/2;

printf("\nx = %.1f |x+1 = %.1f | y = %.1f |y+1 = %.1f | i = %d | temp = %.1f",xarray[i],xarray[i+1],yarray[i],yarray[i+1],i,temp);
        sum = sum + temp;
   }

}

   //if n = 3 (xarray[1]-xarray[2])*(yarray[3]-yarray[2]
   if(n == 3){

       sum = 0.5*(xarray[0]-xarray[1])*(yarray[2]-yarray[1]);


   }
   // output results
   printf("\nErgebnisse:\n");
   printf("-----------\n\n");
printf("\nFlaecheninhalt = %.5f",sum);
   // finally close the input file and clean up memory
   fclose(fp);

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