Thread: School task Read from file and create pointer based array of integers

Is this C or C++? I'm asking because the assignment refers to A02.c as a "class".

Ok then, about your question: You can resize a dynamically allocated array with the realloc function. Read up on it, it's quite useful.

Don't forget to call fclose at the end.

realloc is what you want to use - If you are new to malloc/realloc you might want to read up on it.
C Programming/stdlib.h/malloc - Wikibooks, open books for an open world
Casting malloc - Cprogramming.com

In it's simplest form "malloc()" is the allocation of memory at run time - Note that that memory must also be deallocated "free()" at run time.


"realloc()" is the changing of size for the allocation of memory at run time. i.e. I originally wanted 5 bytes, but now I need 10... It also needs to be deallocated at run time.

Give it a go and see if you run into any problems

Code:

#include <stdio.h>#include <stdlib.h>
int main(int argc, char *argv[])
{
  FILE *fp;
  int temp;
  int *arr;
  int counter = 0;

  fp = fopen("vals.txt", "r");

  while (fscanf(fp, "%d", &temp) != EOF) {
    if (arr == NULL) {
      arr = (int *) malloc(sizeof(int));
      arr[0] = &temp;
      counter++;
    } else {
      arr = (int *) realloc(arr, (counter + 1) * sizeof(int));
      arr[counter++] = &temp;
    }
  }

  fclose(fp);
  free(arr);

  return 0;
}

I tried but it fails with segmentation error on the line after realloc. Any idea why this would happen?

Maybe

Code:

    if (arr == NULL) {
      arr = malloc(sizeof(int));
      arr[0] = temp;
      counter++;
    } else {
      arr = realloc(arr, (counter + 1) * sizeof(int));
      arr[counter++] = temp;
    }

Remove the casts, you don't need them in C.
If you're getting "cannot convert void*" errors, then stop using the C++ compiler to compile your C code.

Your attempt to check if arr == NULL doesn't make sense because you have not initialised arr. Reallocating the dynamic array on each iteration is also rather slow. We might reallocate by some large constant, or by some small factor greater than 1, e.g.,

Code:

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

#define INITIAL_CAPACITY 2
#define REALLOCATION_FACTOR 2

int main(void)
{
    FILE *fp = fopen("vals.txt", "r");
    if (!fp)
    {
        // Handle file opening error
        // ...
        // exit by returning EXIT_FAILURE or calling exit
        // ...
    }

    size_t capacity = INITIAL_CAPACITY;
    int *arr = malloc(sizeof(arr[0]) * capacity);
    if (!arr)
    {
        // Handle memory allocation failure
        // ...
        // free file resource and exit
        // ...
    }

    size_t size = 0;
    int temp;
    while (fscanf(fp, "%d", &temp) == 1)
    {
        if (size >= capacity)
        {
            size_t resized_capacity = capacity * REALLOCATION_FACTOR;
            int *resized_arr = realloc(arr, sizeof(arr[0]) * resized_capacity);
            if (!resized_arr)
            {
                // Handle memory reallocation failure
                // ...
                // either free both file and memory resources and exit,
                // or don't free memory and break instead of exit
                // ...
            }
            capacity = resized_capacity;
            arr = resized_arr;
        }

        arr[size++] = temp;
    }

    fclose(fp);

    // Do stuff with arr
    // ...

    free(arr);

    return 0;
}