Question

I have a dynamically created array of integers. Now I have to remove all elements which have index %3 == 0. (for example, 3, 6, 9, …). So, what is the best way to decrease array size? With malloc I can use realloc for the same part of memory, but what about new operator? What to do this way. Just slide all elements left, make zero to all another elements?

Answer 1

I’d simply allocate a new smaller array and then copy elements to it. Something like this (this includes the element at 0 index):

int* array = new int [original_size];

// fill array

size_t new_size = original_size - original_size / 3 - 1; // i think i got this right, untested
int* new_array = new int [new_size];

for (int i = 0, int j = 0; i < original_size; i++)
{
    if (i % 3 == 0)
    {
        new_array[j] = array[i];
        j++
    }
}

delete [] array;
array = new_array;
new_array = nullptr;

You can of course work in place and shift elements to the left. But you can’t delete a part of array that was allocated via new[].

Since this is an exercise, and you can’t use STL, why don’t you try to implement a simple vector class yourself?