Question

Can someone tell me where I’m going wrong here? basically I’ve written some code that should take values from a file and then print the first value from the file as output along with the number of values in the file.

Although I get the correct number of values, my first value printed in the output isn’t the same as the first value in my file. here’s the code:

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

int main()
{
    const char Project_Data[] = "filedata.dat";
    FILE *input = fopen(Project_Data, "r");

    int t = 0;
    int N = 0;
    float *a;

    a = (float*)malloc(N*sizeof(float));

    if(input != (FILE*) NULL)
    {
        while(fscanf(input, "%e", &a[t]) == 1)
        {
            N++;
            if(a[t] == EOF)
                break;
        }

        printf("first value in file: %e\n",a[0]);
        printf("number of values in file: %d\n", N);
    }
    else
        printf("coudlnt read input file.\n");

    return(0);
}

I feel it’s got something to do with how I’m using the malloc function. If so, how do I use it correctly? I want to read my values from the file into an array size N, but how can I do that if I don’t know the value of N beforehand?

---

EDIT:

Here’s some of the values from the file. The values are ordered one after the other with a single space between them. Because the number of values is quite large, they kind of form a several diagonals across the page (if you’ve it before).

   9.0100000e+00   8.9663752e-01
   9.0200000e+00   1.5041077e+00
   9.0300000e+00   2.5992505e+00
   9.0400000e+00   1.5242828e+00
   9.0500000e+00   3.6815660e-01
   9.0600000e+00   5.4889676e-01
   9.0700000e+00   1.2371257e+00
   9.0800000e+00   1.2163317e+00
   9.0900000e+00   5.4318479e-01
   9.1000000e+00   1.5906641e+00
   9.1100000e+00   2.6775285e+00
   9.1200000e+00   1.1608307e+00
   9.1300000e+00   1.2084299e+00
   9.1400000e+00  -7.8752191e-01
   9.1500000e+00   6.4048690e-01
   9.1600000e+00   2.2727416e-02
   9.1700000e+00   1.0307653e+00
   9.1800000e+00   1.9435864e+00
   9.1900000e+00   2.9422693e+00
   9.2000000e+00   3.2184945e+00
   9.2100000e+00   1.3041157e+00
   9.2200000e+00   1.1018038e+00

The values on the left aren’t orders; they are genuine values, it’s just the other data is meant to be the ‘noise’ in the data i think.

Answer 1

You can allocate an array with, say, 10 elements at first. While reading the file, remember to increase the index variable (t in your example). Should t become greater than your allocated size, use realloc to double the size of your array. This is how most dynamic vector implementations work as far as I know.

Something like this:

int capacity = 10;
float* a = (float*)malloc(capacity*sizeof(float));
int t = 0;    

while(fscanf(input, "%e", &a[t]) == 1) {
   t++;
   if (t >= capacity) {
      capacity *= 2;
      void* temp = realloc(a, capacity*sizeof(float));
      if (temp == 0) exit(1); // or handle allocation failure properly, a is still valid
      a = (float*)temp;
   }
}

At this point a should contain your items and t the number of items read.