How to free the final element of a malloc'd array in C?

Question

Say I initialize an array of 5 integer elements like this:

int *Q = malloc(sizeof(int) * 5);

for (int i = 0; i < 5; i++) {
    Q[i] = i;
}

The array looks like: {0, 1, 2, 3, 4}. Now if I shift everything along by 1 position:

Q++;

The array looks like: {1, 2, 3, 4, #}, where # is some garbage value.

Is there a way to free the final element so it's not stored in the array?

I tried this:

free(Q[4]);

But I know this won't work because free() can only operate of the whole chunk of memory allocated for Q.

Is there a better way to shift everything along? The resulting array should look like: {1, 2, 3, 4}.

Would it be a good idea to realloc() Q after every shift?

Answer 1

realloc() can change the size of an allocated chunk of memory, which will do the job for you. Note that this cannot be used to "free" arbitrary elements of an array, but only one(s) on the end.

How good an idea it is to do this depends on a number of factors, none of which you have provided.