Question

I am trying to read a file which contains the coordinate values for my code.

My question is, how do I make the array size large enough to contain future addition to the file?

Below is my code; when I set the array to size 905, my loop continues until the space is filled. Why is that?

FILE.txt:

S (60,70)(200,200)
S (30,40)(100,200)
S (10,20)(80,10)
S (60,400)(700,200)
S (160,70)(240,20)
S (160,70)(240,20)
S (160,70)(240,20)

My code:

#include <stdio.h>
int a;
int b;
int c;
int d;
int data[905][4];
int main ( int argc, char *argv[] )
{
    if ( argc != 2 ) /* argc should be 2 for correct execution */
    {
        /* We print argv[0] assuming it is the program name */
        printf( "usage: %s filename", argv[0] );
    }
    else 
    {
        // We assume argv[1] is a filename to open
        FILE *file = fopen( argv[1], "r" );

        /* fopen returns 0, the NULL pointer, on failure */
        if ( file == 0 )
        {
            printf( "Could not open file\n" );
        }
        else 
        {
            int j=0;int count=1
            for (j=0; j < count; j++) 
            {    
                fscanf(file, "S (%d,%d)(%d,%d)", &a, &b, &c, &d);

                printf("%d,%d,%d,%d\n",a, b, c, d);
                count++
            }             

            fclose( file );
        }
    }
}

Answer 1

You will need to use the malloc and realloc functions from <stdlib.h> for this. The basic idea is to allocate a certain amount of space up front, and then enlarge the array when you discover that it is not big enough.

It will make life easier if you use an array of structures rather than an array of arrays:

struct segment { int x0, y0, x1, y1; };

and then you do something like this:

size_t nsegs = 0, segs_allocated = 10;
struct segment *segs = malloc(segs_allocated * sizeof(struct segment));
if (!segs) abort();

while (getline(&line, &linesz, stdin) >= 0)
{
    if (!parse_line(&segs[nsegs], line)) continue;
    nsegs++;
    if (nsegs == segs_allocated)
    {
        segs_allocated *= 2;
        segs = realloc(segs, segs_allocated * sizeof(struct segment));
        if (!segs) abort();
     }
}

Obligatory tangential comment: Forget you ever heard of fscanf. It is far more trouble than it is worth. Read entire lines with getline (if you don’t have it, it’s not hard to implement), extract individual number-strings from the line with a hand-coded parser, and convert them to machine integers with strtol, strtoul, or strtod as appropriate.