Help with finding the area of a polygon.

[Angel45]

This program takes in coordinates and then outputs the area, but there seems to be a logic error somewhere. The file it is reading from has this in it.
4
1.0 0.0
13.2 1.25
20.5 18.4
16.37 24.54

#include<stdio.h>
#include<stdlib.h>
int main(int argc, char **argv[]) 
{
    int size = 0;
    char fileName[] = "pointData.txt";
    double sum=0;
    int i;
    FILE *instream=NULL;
    instream = fopen(fileName, "r");
    fscanf(instream,"%d",&size);
    double x[size];
    double y[size];
    for(i=0; i<size; i++){
        fscanf(instream,"%lf",&x[i]);
        fscanf(instream,"%lf",&y[i]);}
    fclose(instream);
    for(i=0; i<size-1; i++){
        sum+=(((x[i])*(y[i+1]))-((x[i+1])*(y[i])));}
    sum/=2.0;
    printf("Area of the polygon is: %.4f\n", sum);
  return 0;
}

It outputs 210.1835, it should be 197.9135

[smokeyangel]

You're missing the last term in the sum. For a 4 sided polygon you should be summing:

x[0]y[1] - y[0]x[1]
x[1]y[2] - y[1]x[2]
x[2]y[3] - y[2]x[3]
x[3]y[0] - y[3]x[0]

Your for loop takes care of the first 3, but not the last one. You just need another addition after the loop.

[mgracecar]

Ok this is weird.

I changed it to how I have it in here now and it works on gcc which is what it's graded on. No errors or anything. But on Dev-C It's not compiling. Hmmm

[Angel45]

You're missing the last term in the sum.

I tried adding one more and it gives me the same result.

Also I am compiling with gcc too. I am not sure what is the problem.

[quzah]

But on Dev-C It's not compiling. Hmmm

I hope someday they come out with compilers that tell us why things don't compile.


Quzah.

[Angel45]

Ok I understand why it was not working now, thanks . Haha compilers tell you why they don't compile, but sometimes it is so hard to understand why.

[smokeyangel]

Great!

I changed it to how I have it in here now and it works on gcc which is what it's graded on. No errors or anything. But on Dev-C It's not compiling. Hmmm

Haha, yah, the error would be helpful.

It's probably the variable length arrays:

    double x[size];
    double y[size];

Variable length arrays are part of C99, and most C compilers don't default to C99. gcc supports variable length arrays as an extension of both C90 and C++, so they're always allowed.