Using a binary search algorithm as a basis to implement a binary insert

Question

I'm trying to implement a binary insert method.

Currently this method is very simple, it takes an argument, in a while loop it searches for an element that is bigger than the argument (in this case a String that is the last name of a person), it breaks once it finds it and shifts the rest of the array to the right. Then the argument is inserted at the position of breaking.

I tried changing it to one that would search for the insert position by borrowing from the binary search algorithm. However, I just can't get it to work.

Could you please let me know what I'm doing wrong? Here's my code:

public void insert(Person person) 
{

    String lastName = person.getLastName();

    int position = -1; // the position from which the array will be shifted to the right and where the argument will be inserted, will be assigned properly below

    int lowerBound = 0;
    int upperBound = numElems - 1;
    int currIt;

    if (numElems == 0)
        array[0] = person; // if array empty insert at first position
    else 
    {
        while (true) 
        {
            currIt = (lowerBound + upperBound) / 2; //the item to compare with

            int result2 = 0;

            int result1 = array[currIt].getLastName().compareTo(lastName);

            if (array[currIt+1] != null) // if the next element is not null, compare the argument to it
                result2 = array[currIt+1].getLastName().compareTo(lastName);

            if (currIt == 0 && result1 > 0) // this is when the argument is smaller then the first array element
            { 
                position = 0;
                break;
            }
            // if the position to insert is the last one, or we have found a suitable position between a smaller and a bigger element
            else if (  (result1 < 0 && (currIt == numElems-1))  ||  (result1 < 0 && result2 > 0)  ) {
                position = currIt+1;             
                break;
            }    
            else
            {
                if (result1 < 0)  // the place to put it should be in the upper half
                    lowerBound = currIt + 1; 
                else 
                    upperBound = currIt - 1; //in the lower half              
            }

        }
    }
    // position has been set to anything else other -1, then we have found our spot, probably a redundant check but useful for debugging
    if (position != -1)
    {
        //shift array to the right and insert element
        for (int j = numElems; j > position; j--)
        array[j] = array[j-1];
        System.out.println("Inserted an element");
        array[position] = person;
        numElems++;
    }

}

Can you provide the simplest example which demonstrates your problem? — Peter Lawrey
– Peter Lawrey, Commented Sep 6, 2012 at 13:17
If say we have the following: arr.insert(new Person("Charles", "Dickens", 23));arr.insert(new Person("Adam", "Bay", 29)); arr.insert(new Person("Ethan", "Fowler", 18)); I was hoping to arrange them by last name ("Bay"...); This works fine: String lastName = person.getLastName(); int i; for (i = 0; i < numElems; i++) if (array[i].getLastName().compareTo(lastName) > 0) break; for (int j = numElems; j > i; j--) array[j] = array[j-1]; array[i] = person; numElems++; But wanted to speed it up:) — Nobilis
– Nobilis, Commented Sep 6, 2012 at 13:33

DerMike · Accepted Answer · 2012-09-06 13:32:53Z

1

Your "probably [] redundant check" prohibits initial inserting. Position is -1 the first time.

Setting position to 0 at the top, should fix the problem.

edited Sep 6, 2012 at 13:32

answered Sep 6, 2012 at 13:26

DerMike

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