How to return a `realloc` array in C function

Question

I want to append numbers to an empty array and the amount of these numbers is unknown at the start. For example, generating numbers from 1 to 10 and appending one after another.

generateFromOneToTen will save my result in output and count should be 10 after execution. Everything's alright if I print the result in this function.

int generateFromOneToTen(int *output, int count)
{
    for (int i = 0; i < 10; i++) {
        output = arrayAppendInt(output, i + 1, count);
        count++;
    }

    // Print result of `output` is 1,2,3...10 here

    return count;
}

And I implemented arrayAppendInt to dynamic increase the length of an array and append new value after the old ones.

int *arrayAppendInt(int *array, int value, int size) 
{
    int newSize = size + 1;
    int *newArray = (int*) realloc(array, newSize * sizeof(int));

    if (newArray == NULL) {
        printf("ERROR: unable to realloc memory \n");
        return NULL;
    }

    newArray[size] = value;

    return newArray;
}

Here comes the question. When invoking the generation function, numbers will always be NULL. How can I return the generated numbers to the numbers variable?

int *numbers = NULL;
int count = 0;
count = generateFromOneToTen(numbers, 0);
                             ^^^^^^^

man realloc():: `... if ptr is NULL, then the call is equivalent to malloc(size), for all values of size;...` — wildplasser, May 10 '18 at 09:11
So you want to modify the argument `numbers` of the function? [Possible duplicate](https://stackoverflow.com/questions/9459691/how-to-change-value-of-variable-passed-as-argument)? — user202729, May 10 '18 at 09:11
The cleanest solution is IMHO to pack the array+the bookkeeping (size,used) into a structure, and use (a pointer to) this structure as an argument. — wildplasser, May 10 '18 at 09:17

Fabrice Derepas · Accepted Answer · 2018-05-10T09:55:57.440

3

You could use a pointer to a pointer of integer (int **):

int generateFromOneToTen(int **output, int count)
{
    for (int i = 0; i < 10; i++) {
        *output = arrayAppendInt(*output, i + 1, count);
        count++;
    }
    // `*output` is 1,2,3...10 here
    return count;
}

You could re-write the arrayAppendInt function like that:

int *arrayAppendInt(int *array, int value, int size) 
{
    int newSize = size + 1;
    int *newArray;
    if (array==NULL)
      newArray = (int*) malloc ((1+size) * sizeof(int));
    else
      newArray = (int*) realloc(array, newSize * sizeof(int));

    if (newArray == NULL) {
        printf("ERROR: unable to realloc memory \n");
        return NULL;
    }

    newArray[size] = value;

    return newArray;
}

And call it like that *output = arrayAppendInt(*output, i + 1, i);.

edited May 10 '18 at 09:55

answered May 10 '18 at 09:16

Fabrice Derepas

87
5

How to modify `arrayAppendInt` then? – WangYudong May 10 '18 at 09:26
I updated my answer with modifications to `arrayAppendInt`. Here is the full synthesis of my answers : https://taas.trust-in-soft.com/tsnippet/t/856bfc98 – Fabrice Derepas May 10 '18 at 09:58
I needn't change `arrayAppendInt` I think. – WangYudong May 10 '18 at 09:58
I agree, no need to change actually! – Fabrice Derepas May 10 '18 at 10:04
1

`if (array==NULL) newArray = (int*) malloc ((1+size) * sizeof(int));` You dont need the special case. (and you dont need the cast,either) – wildplasser May 10 '18 at 10:27

score 0 · Answer 2 · answered May 10 '18 at 09:48

The cleanest solution is (in my opinion) to pack the array+the bookkeeping (size,used) into a structure, and use (a pointer to) this structure as an argument.

#include <stdlib.h>

struct dopedarray {
        unsigned size;
        unsigned used;
        int *array;
        };

Now you can put all your allocation and bookkkeeping stuff into a single function (which can be inlined) :

int array_resize(struct dopedarray *ap, unsigned newsize)
{
int *newp;

if(!ap) return -1;

newp = realloc (ap->array, newsize*sizeof*ap->array);
  // check return value here...
if (!newp) return -1;

free(ap->array);
ap->array = newp;
ap->size = newsize;

  // bookkeeping sanity
if(ap->size > ap->used ) { ap->used > ap->size; }

return 0;
}

The add_element function needs to be changed a bit, too:

int array_add_element(struct dopedarray *ap, int value)
{
if(ap->used >= ap->size){
        unsigned newsz;
        newsz= ap->used ? 2*ap->used: 4;
        array_resize(ap, newsz);
        // check return value here...
        }

ap->array[ap->used++] = val;
return 0;
}

score 0 · Answer 3 · answered May 10 '18 at 10:07

The complete code to my question:

int generateFromOneToTen(int **output, int count) // +
{
    for (int i = 0; i < 10; i++) {
        *output = arrayAppendInt(*output, i + 1, count); // ++
        count++;
    }

    return count;
}

int *arrayAppendInt(int *array, int value, int size) 
{
    int newSize = size + 1;
    int *newArray = (int*) realloc(array, newSize * sizeof(int));

    if (newArray == NULL) {
        printf("ERROR: unable to realloc memory \n");
        return NULL;
    }

    newArray[size] = value;

    return newArray;
}

int *numbers = NULL;
int count = 0;
count = generateFromOneToTen(&numbers, 0); // +

This answer also worths reading: https://stackoverflow.com/a/9459803/1951254

How to return a `realloc` array in C function

3 Answers3