Realloc an array of double

Question

The exercise, that I have to complete says:

That array_remove function must remove from the array arr the value, that is in the position pos, and scale of a position successive values of pos, and eventually change the array size for no gaps. If this value is not included in the array (if pos is greater than pn (array size)), then you should not do anything.

My problem is:

Probably very wrong to use the malloc function, because when it is performed, it shows the following error:

MAIN.C:

#include "array.h"

int main(void)
{
    double arr[] = { 1.0,2.0,3.0,4.0,5.0 };
    size_t pn = 5;/*array length*/
    size_t pos = 2;/*position of the number to be deleted*/

    array_remove(arr, &pn, pos);
}

ARRAY.C:

#include "array.h"

void array_remove(double *arr, size_t *pn, size_t pos)
{
    int x = *pn;
    int y = pos;
    if (x > y)
    {
        for (int i = y; i < x; i++)
        {
            arr[i] = arr[i + 1];
        }
        realloc(&arr, sizeof(double) * 4);
    }
}

Answer 1

According to the C docs:

realloc Reallocates the given area of memory that must be previously allocated by malloc(), calloc() or realloc() and not yet freed with free, otherwise, the results are undefined.

You have an out of bound problem as well at the following lines when i=x-1 you try to access at arr[i+1] = arr[x=pn]:

for (int i = y; i < ; i++) {
    arr[i] = arr[i + 1]; 

Check the following code out *(live: https://ideone.com/mbSzjL

  #include<stdlib.h>

void array_remove(double **arr, int *pn, int pos) {
    int x = *pn;
    int y = pos;
    if (x > y) {
        //check if after deletion size is zero!
        if (x > y) {
            for (int i = y; i < x-1; i++) {
                (*arr)[i] = (*arr)[i + 1];
            }

            *arr=realloc(*arr, sizeof(double) * x-1);
            *pn=*pn-1;
        }
    }
}

int main(void) {
    int pn = 20;/*array length*/
    int pos = 5;/*position of the number to be deleted*/
    double *arr = malloc(sizeof(double)*pn);
    printf("%p\n",arr);
    for(int i=0;i<pn;i++){
        arr[i] = i;
    }

    for(int i=0;i<pn;i++){
        printf("%.f ",arr[i]);
    }
    printf("\n");

    printf("%i\n",pn);
    array_remove(&arr, &pn, pos);
    printf("%p\n",arr);
    for(int i=0;i<pn;i++){
        printf("%.f ",arr[i]);
    }
    printf("\n");
    printf("%i",pn);


    free(arr);


}

Don't forget to realloc using the right size (not using an hardcoded 4) and check for the edge case in which size is zero after deletion!

In addition, free the memory at the end and to update the size variable.

http://en.cppreference.com/w/c/memory/realloc

Answer 2

arr array is stack allocated. You cannot realloc something that wasn't mallocated.

You probably want something like this:

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

bool array_remove(double **arr, size_t *pn, size_t pos)
{
    int x = *pn - 1;
    int y = pos;
    int i;
    double *temp;

    if (x > y) {
        for (i = y; i < x; i++) {
            (*arr)[i] = (*arr)[i + 1];
        }

        temp = realloc(*arr, sizeof(double) * x);
    }

    if (arr != NULL)
    {
        *arr = temp;

        *pn -=1;

        return true;
    }
    else
    {
        return false;
    }
}

int main(void)
{
    size_t pn = 5;  // array length
    size_t pos = 2; // position of the number to be deleted
    int i;

    double *arr = malloc(pn*sizeof(double));

    if (arr != NULL)
    {
        for (i=0; i<pn; i++)
        {
            arr[i] = (double)(i+1);
        }


        if (array_remove(&arr, &pn, pos) == false)
        {
            printf("Failed to remove element %zu\n", pos);
        }

        for (i=0; i<pn; i++)
         printf ("arr[%d]: %f\n", i, arr[i]);

        free(arr);
    }
    else
    {
        printf("Failed to alloc array\n");
    }

    return 0;
}

As you can see I changed the loop of array_remove. In your code you are addressing the array out of bound on the last loop, because of i=4 and then: arr[i] = arr[i + 1]; is arr[4] = arr[5]

Indexes of a 5 elements array start from 0 to 4.

Answer 3

actually you have a different problem here:

int x = *pn; //x=5
int y = pos; //y=2
if (x > y) {
        for (int i = y; i < x; i++) {
            arr[i] = arr[i + 1];
        }

On the last iteration, you do

 arr[4] = arr[5]

This is out of range addressig and that's probably your problem, or at least your first one.

Also, even though it's not technically wrong it's conceptually wrong:

array_remove(arr, &pn, pos);

Never pass a value by pointer unless you plan on modifying it. Not the case here, so you can pass it by value.