fscanf not scanning input correctly?

Question

fscanf doesn't seem to be reading in values. Any idea why? The input file is poly.dat, and it has several rows corresponding to (x, y) pairs; see below. I check the output value of fscanf, but I don't remember if there's a better way. Any help would be appreciated. Thanks!

poly.dat:

2
0.0 0.0
1.0 0.0
0.75 0.4330127018922193
0.25 0.4330127018922193
0.0 0.0
0.25 0.4330127018922193
0.75 0.4330127018922193
0.5 0.8660254037844386
0.25 0.4330127018922193  

invocation:

% ./program poly.dat poly-converted.dat

source code:

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

int main(int argc, char *argv[]) {

    if (argc != 3 ) {
        printf("usage: %s <input> <output>\n", 
                argv[0]);
        return 1;
    }

    FILE *fp = fopen(argv[1], "r");
    FILE *fo = fopen(argv[2], "w");

    int N = 0; /* How many tiers? */
    double x=0.0, y=0.0; 
    const double MARGIN = 0.10; 

    fscanf(fp, "%d", &N);
    fprintf(fo, "%d\n", N);

    while ( fscanf(fp, "%f %f", &x, &y) == 2) {
        double xprime = x + MARGIN; 
        double yprime = MARGIN + 1.0 - y; 

        fprintf(fo, "%f %f\n", xprime, yprime);

    }

    fclose(fp);
    fclose(fo);

    return 0;

}

Answer 1

When you call fscanf with a pointer to double , you need to use %lf instead of %f . Otherwise, you are going to get wrong results in your x and y variables.

Demo on ideone.

Answer 2

Don't ever use fscanf; when it fails it is very hard to know how much input has been consumed.

fgets and sscanf make for a much more debuggable separation of concerns.