c++ delete last element of dynamic array

Question

In c++ I took a dynamic array of n elements

int* a = new int[n];

After shifting left all element of that array, the last element, i.e a[n-1] is useless, and I want to delete it. And after shifting right I need to delete the first element of array, and to have a pointer to second element, i.e I need to make an array with length of n-1. How can I do that?

Answer 1

You need to allocate a new array and copy elements of the original array to the new array.

Here is a demonstrative program

#include <iostream>
#include <utility>
#include <algorithm>

size_t shift_left( int * &a, size_t n )
{
    if ( n )
    {
        int *p = new int [n-1];

        std::copy( a + 1, a + n, p );

        std::swap( a, p );

        delete []p;
    }

    return n == 0 ? n : n - 1;
}

size_t shift_right( int * &a, size_t n )
{
    if ( n )
    {
        int *p = new int [n-1];

        std::copy( a, a + n - 1, p );

        std::swap( a, p );

        delete []p;
    }

    return n == 0 ? n : n - 1;
}

int main() 
{
    size_t n = 10;      
    int *a = new int[n] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    for ( const int *p = a; p != a + n; p++ )
    {
        std::cout << *p << ' ';
    }

    std::cout << '\n';

    n = shift_left( a, n );

    for ( const int *p = a; p != a + n; p++ )
    {
        std::cout << *p << ' ';
    }

    std::cout << '\n';

    n = shift_right( a, n );

    for ( const int *p = a; p != a + n; p++ )
    {
        std::cout << *p << ' ';
    }

    std::cout << '\n';

    delete []a;

    return 0;
}

Its output is

0 1 2 3 4 5 6 7 8 9 
1 2 3 4 5 6 7 8 9 
1 2 3 4 5 6 7 8 

You can change the functions the following way. When the passed value of n is equal to 1 when just free the original pointer and set it to nullptr.

For example

size_t shift_left( int * &a, size_t n )
{
    if ( n )
    {
        if ( n == 1 )
        {
            delete []a;
            a = nullptr;
        }
        else
        {
            int *p = new int [n-1];

            std::copy( a + 1, a + n, p );

            std::swap( a, p );

            delete []p;
        }               
    }

    return n == 0 ? n : n - 1;
}

As an alternative you can use the standard container std::vector and its member function erase.

Or you can consider using std::valarray.