Allocating memory to 2D array using an array of NULL (c)

Question

thanks for taking the time in reading this.
In my question a "vector" is defined as a 1D dimensional array of integers.
Therefore an array of vectors would be a 2D dimensional array in which every vector can be of a different length.
I'm asked to use:
int** vectors- the 2D array
int size -an integer that represents how many vectors exist inside **vectors
int* sizes-a 1D array of integers that represents the length of the vectors

for example,for:
vectors = {{4,3,4,3},{11,22,33,44,55,66},NULL,{5},{3,33,333,33,3}}.
size is 5 (there are 5 vectors inside vectors).
sizes is {4,6,0,1,5} (4 is the length of the first vector and so on).
size is inputted by the user at the beginning of main() and **vectors&*sizes are dynimacilly allocated with size's value.

I'm asked to write the function:
int init(int ***vectors, int **sizes, int size) which initializes **vectors to be an array of NULLs and *sizes to be an array of zeros.
I came up with this code:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
int init(int*** vectors, int** sizes, int size)
{
    int i, k,j;
    printf("check\n");
    *vectors = (int**)malloc(size * sizeof(int*));
    if (*vectors == NULL)
        return 0;
    for (i = 0; i < size; i++)
    {
        *(vectors + i) = NULL;
    }
    printf("check 2\n");
    for (k = 0; k<size; k++)
    {
        if (*(vectors+k) != NULL)
            printf("didn't work\n");
        else
            printf("current is null\n");
    }
    *sizes= (int*)malloc(size * sizeof(int));
    if (*sizes == NULL)
        return 0;
    for (j= 0; j < size; j++)
    {
        *(sizes + j) = 0;
        printf("%d ", *(sizes + j));
    }
    printf("\n");
    return 1;
}
int main()
{
    int size, i;
    int** vectors = NULL;
    int* sizes = NULL;
    printf("\nPlease enter an amount of vectors:\n");
    scanf("%d", &size);
    printf("%d\n", init(&vectors, &sizes, size));
    printf("size is %d now\n", size);
//  for (i = 0; i < size; i++)
    //  printf("%d ", *(sizes+i));
    printf("check 3\n");
    free(sizes);
    free(vectors);
    printf("check 4\n");
    printf("check 5\n");
    return 0;
}

forgot to mention that init returns 0 if it fails to allocate memory and 1 otherwise.
printing the "checks" was so I could see where the program fails.
the problem is that no matter what,after printing the last check (check 5) the program fails.(Run-Time Check Failure #2)
if anyone could help me understand what I'm doing wrong I would HIGHLY appreciate it.
thanks alot for reading and have an amazing day. edit:
i also printed the array sizes/vectors inside init just to see if it prints zeros/nulls,i don't actually need to do it.

Don't be a Three Star Programmer. Oh, and a pointer to a pointer is not a 2d-Array ... — user2371524
– user2371524, Commented May 2, 2018 at 7:12
I was just introduced in class to pointers and my homework spesficially demands us to use three stars...sorry if it's dumb — Ultraviolence
– Ultraviolence, Commented May 2, 2018 at 7:15
Use warnings, for example ` warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=] printf("%d ", *(sizes + j));` — Ôrel
– Ôrel, Commented May 2, 2018 at 7:15
@Ultraviolence it's considered a code smell by many. of course you can make it work anyways -- just not a great idea, a different design would be better :) — user2371524
– user2371524, Commented May 2, 2018 at 7:25

Bob__ · Accepted Answer · 2018-05-02 14:09:33Z

1

One problem of OP's code is in the pointer arithmetic. Given:

int ***vectors;
*vectors = malloc(size * sizeof(int*));

This loop:

for (i = 0; i < size; i++)
{
    *(vectors + i) = NULL;
}

Would iterate over the next unallocated pointer to pointer to pointer to int, while what the OP needs is

for (i = 0; i < size; i++)
{
    *(*vectors + i) = NULL;     // or (*vectors)[i] = NULL;
}

The same holds in the following loops, where *(sizes + j) is used instead of *(*sizes + j) (or (*sizes)[j]).

answered May 2, 2018 at 14:09

Bob__

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1 Comment

Ultraviolence Over a year ago

thank you so much bob,this actually made the program not crash.can't thank you enough!

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