Is it possible to remove an element in a vector in C?

Question

I've a problem with an exercise. It asks "Write a function that gets a vector and returns the vector removing any prime numbers". There's also a suggestion, "Remove an i-th element in a vector means move from the i+1-th element to the (N-1)-th element to cover the empty spot left from the deleted element". And I can't use any pointer.

I don't know how to do it.

Answer 1

Write a function that gets a vector

There is no such thing as a vector type in C. Probably it's an array.

And I can't use any pointer

hmmm... passing an array to a function in C is actually "passing a pointer" so it will be very difficult to avoid. But the function can be written so that it kind of hides the use of a pointer.

Now for the code, the function could look like:

size_t remove_primes(size_t size, int a[]);

That is a function taking an array and it's size as arguments and returns the new size after primes has been removed.

In code it could be something like:

size_t remove_primes(size_t size, int a[])
{
    size_t i = 0;
    while ( i < size)
    {
        if (is_prime(a[i]))
        {
            // It's a prime so move all element after i'th element to the left
            //
            // memmove(&a[i], &a[i+i], (size - 1 - i) * sizeof a[0]);
            //
            // or to hide use of pointers
            //
            for (size_t j = i+1; j < size; ++j) a[j-1] = a[j];

            --size;
        }
        else
        {
            ++i;
        }
    }
    return size;
}

or for better performance like:

size_t remove_primes(size_t size, int a[])
{
    size_t writer = 0;
    for (size_t reader = 0; reader < size; ++reader)
    {
        if ( ! is_prime(a[reader]))
        {
            // Not a prime so save the value
            a[writer] = a[reader];
            ++writer;
        }
    }
    return writer;
}