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

Hi Guys,

I have one more school task to finish an I need some help understanding where should I start from.

The task is as follows:

"02 - You have been provided with a text file Vals.txt containing some integer numbers. Create a class named A02.c that opens the text file, reads the integer values and dynamically creates a pointerbased array of integers. You should not assume a fixed length but rather dynamically allocate memory. Make sure to appropriately dispose of the variable prior to ending the application."

I already did the code to open the file:

Code:

---

#include <stdio.h>

int main()
{
  FILE *fp;


  fp = fopen("vals.txt","r");


  while( fscanf(fp,"%d",&temp) != EOF )
  {




  } 
}

---

But i need to do this "dynamically creates a pointerbased array of integers. You should not assume a fixed length but rather dynamically allocate memory."

Could you please give me a code example on how to do it?
I already searched google but can't find anything close to what I need.

Any help is greatly appreciated.

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

Quote:

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Originally Posted by GReaper

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

---

It's C. I think that is a typo in the assignment.

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;
}

---

Quote:

---

Originally Posted by laserlight

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;
}

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Thank you very much laserlight.