Creating a dynamic array of arrays using pointers

Question

I'm trying to create a dynamic array of arrays. So, for example, let's look at Pascal's Triangle:

1
11
121
1331
14641
...
...

This is basically an array of length N, that has at each index an array of i+1.

How exactly do we set that up?

I've tried a little bit using pointers.

I set up an array of pointers like such:

int *arr[N];

Then I need a pointer i to point to an array of i+1, so I did:

int *i = 0;
    for(int j = 0; j < N; j++){
        int numArr[j+1];
        arr[*i] = numArr;
        *i++;
    }

Am I going the right direction for this? Because I believe I'm supposed to allocate memory for this as I must use free() later. Would I use malloc() for each array initialization?

Yes, you must use malloc. If you've never used it before, I suggest you try allocating a single int, then once that works perfectly, try allocation an array of int, and once that works perfectly, take a crack at the problem above. (This is a good approach for any new technique.) — Beta
– Beta, Commented Sep 29, 2019 at 19:45
Why bother using a pointer for i? Just use int i and be done with it. (but yes, you will need to use malloc for numArr) — S.S. Anne
– S.S. Anne, Commented Sep 29, 2019 at 20:01

Adrian Mole · Accepted Answer · 2019-09-30 01:31:33Z

1

The code could be made extremely simple, if you know what you're doing:

int *arr = malloc(N * sizeof(int*));
int i;
for (i = 0; i < N; ++i) {
    arr[i] = malloc(sizeof(int) * (i + 1));
}

Of course, you'll need corresponding calls to free() further down the line, like this:

for (i = 0; i < N; ++i) {
    free(arr[i]);
}
free(arr);

edited Sep 30, 2019 at 1:31

answered Sep 29, 2019 at 21:09

Adrian Mole

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2 Comments

ESM Over a year ago

So free() would be called when the triangle is complete, and then I just use it once? Or do a for loop and free the memory of each allocated array?

Adrian Mole Over a year ago

You would only call free() when you've finished with the arrays. And yes, you would need a for loop (see edit).

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