How to use more than one dynamic allocation on C programming?

Question

I'm making a program that reads two sets of data (float) from two different .txt files, and then it transfers these data to two different arrays, which will be used in further calculations. However, when I try to use dynamic allocation more than once, something goes wrong and the data seem not to be stored in the array.

The following simplified program seems to be working fine:

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

int main()
{
    float *VarA;
    int n = 0;
    int *counter;
    int i;
    FILE *input1;

    input1 = fopen("C:\\Users\\...test.txt","r");

    VarA = (float*)calloc(20001, sizeof(float));

    for(i = 0; i < 20001; i++)
    {
        fscanf(input1,"%f",&VarA[i]);
        printf("%f\n",VarA[i]);
    }

    free(VarA);

    fclose(input1);

    return 0;
}

it successfully shows the data stored in the array VarA. However, if I introduce a new array to count the number of lines in the file (which is necessary for my further calculations), I just get the value 0.000000 from every array element:

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

int main()
{
    float *VarA;
    int n = 0;
    int *counter;
    int i;
    FILE *input1;

    input1 = fopen("C:\\Users\\...test.txt","r");

    counter = (int*)calloc(100000, sizeof(int));

    while(fscanf(input1,"%f",&counter[n]) != EOF)
    {
        n++;
    }

    free(counter);

    printf("n = %i\n", n);

    VarA = (float*)calloc(n, sizeof(float));

    for(i = 0; i < n; i++)
    {
        fscanf(input1,"%f",&VarA[i]);
        printf("%f\n",VarA[i]);
    }

    free(VarA);

    fclose(input1);

    return 0;
}

I know that I can avoid using another array to count the number of lines. The point is that every time I use another array, for any purpose, I get the same result. For instance, if I don't use an array to count the number of lines, but I make another one to store my other set of data, one of these arrays just won't present the data after the reading. I tried to modify my program several times in order to find the source of such behavior, but without success.

Answer 1

(At least) two major problems: first,

counter = (int*)calloc(100000, sizeof(int));
while(fscanf(input1,"%f",&counter[n]) != EOF) {
    n++;
}
free(counter);

is basically saying "Grab me a chunk of memory, fill it with data as I read the file, then throw it away without ever using it." Probably not what you intended. Then,

VarA = (float*)calloc(n, sizeof(float));
for (i = 0; i < n; i++) {
    fscanf(input1,"%f",&VarA[n]);
    printf("%f\n",VarA[n]);
}
free(VarA);

which says, "Grab a big chunk of memory, then read data from after the end of the file I just read everything from, put it there, then throw it away."

If you want to read the data from the same file again, you'll have to close it an reopen it (or "seek" to the start). And if you want to do anything with it, you'll have to do it before free()ing the memory you loaded it into.

Answer 2

counter = (int*)calloc(100000, sizeof(int));
         // ^--- `int*`                ^--- `int`
                          // v--- `int` pointer
while(fscanf(input1,"%f",&counter[n]) != EOF)
                   // ^--- `float` designator

Do you see any discrepancies here? Your code allocates ints, then passes a pointer to those ints to fscanf telling it they're floats (using the %f designator). According to the C standard draft n1570, section 7.21.6.2p10 this constitutes undefined behaviour:

If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

My suggestion would be to use the * assignment suppression modifier here, for example:

while (fscanf(input1, "%*f") != EOF) n++;

or, alternatively

while (fscanf(input1, "%f", &(float){0}) != 1) n++;

Note also how I've changed the check from EOF to 1. You can find more information about the return values of fscanf here (which you really should read before using any scanf-related function... and stop guessing, because guessing in C can be harmful).

Additionally, you need to rewind your file once it reaches EOF, otherwise every call to fscanf following this loop will return EOF:

rewind(input1);

P.S. Don't cast malloc in C. This goes for calloc and realloc, too. There's a lot of this quoted stuff that has opengroup manuals of its own; I'll leave it as an exercise to you to find (and read) the opengroup manuals.