Question

I am learning how to work with classes. I have made two classes and one is a list of cars. However, I need to modify the add function so that it will add cars sorted by price. The problem I am having is that it will send the cheapest car to the beginning but kill the rest of the list. Here is my code for the add…

public void add_car(the_cars new_car)
        {// Method to add cars to list
            if (count == 0)
            {// If this is the first car 
                first = new_car;
                last = new_car;
                count = 1;
            }
            else
            {// If it is not the first car
                if (new_car.getPrice() < first.getPrice())
                {// If price of new car is lower than first car
                    last = first;
                    first = new_car; // new car becomes first car
                }
                else
                {
                    while (new_car.getPrice() > last.getPrice() || last.next != null)
                    {
                        last.next = new_car; // Null value now equal to car
                        last = new_car;
                    }
                }


                count++;

Answer 1

You have identified the cases correctly:

1. Is the list still empty?
2. Should the item be added at the beginning?
3. If not: after which item do we need to insert?

You implemented the first case alright.

The second case is wrong: Inserting in the beginning means to change the first element to new_car, but new_car.Next needs to point to the previous first element, otherwise you lose the link.

The third case is also wrong: You need go towards the end of the list until either you’ve reached the last element (which is then the one you need to insert after) or until you find an element the successor of which has a greater price and insert after that.

The reason why I can put the while condition like that is that if current != last I can be sure that there’s a current.Next, otherwise it would be last by definition. The reason I need a temporary iteration element is that if I modify first I lose the entry point to the list.

If have not tested the following code, but it should give you a clue and if it doesn’t work, single-step debugging will help you.

public void add_car(the_cars new_car)
{// Method to add cars to list
    if (count == 0)
    {// If this is the first car 
        first = new_car;
        last = new_car;
        count = 1;
    }
    else
    {// If it is not the first car
        if (new_car.getPrice() < first.getPrice())
        {// If price of new car is lower than first car
            new_car.Next = first; // Insert the first car as the first element
            first = new_car;
        }
        else
        {
            // Create temporary iteration element
            the_cars current = first;

            // Find the car
            while (current != last && new_car.getPrice() >= current.Next.getPrice())
                current = current.Next;

            // Insert after the given element
            new_car.Next = current.Next;
            current.Next = new_car;

            // Also you may need to update last to match the new end
            if (current == last)
                last = new_car;
        }

        count++;
    }
}