"Student Not Found" Problem

[trantommyd]

I'm trying to display a message that informs the user that the name he entered was not found in the list of students. I wrote a binary search to go through the list of names to compare to the search key.

		while(repeat)
		{
			searchEntry = JOptionPane.showInputDialog("Enter the last name of the student you wish to see the grade of: ");
			if(searchEntry.length() == 0)
			{
				System.exit(0);
			}
			int low = 0;
			int high = studentArray.length - 1;
		
			while(high >= low)
			{
				int mid = (low+high)/2;
				if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) < 0)
				{
					high = mid - 1;

				}
				else if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) > 0)
				{
					low = mid + 1;
				}
				else if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) == 0)
				{
					JOptionPane.showMessageDialog(null, studentArray[mid].firstName + " " + studentArray[mid].getLastName() 
														+ " received a grade of " + studentArray[mid].getNumberGrade());
					high = -1;
				}
				else
				{
					JOptionPane.showMessageDialog(null, "Name not found.");
				}
			}
		}

Here, I'm using an if-else-if statement, but it is not working the way I want it to.

[Fou-Lu]

The first question to ask - has studentArray been sorted prior to this point?
I'll take a look at this when I get home as well.

[trantommyd]

Yes, the array of objects called studentArray has already been sorted in alphabetical order according to last name.

[Fou-Lu]

Looks alright to me; you'll never get to the else condition though. The search entry will always be <, = or > the current iteration, regardless of if it exists in the list.
If you make it through the list without a result, then you have no entry found.

			int low = 0;
			int high = studentArray.length - 1;
		
			Student result = null;
			while(high >= low)
			{
				int mid = (low+high)/2;
				if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) < 0)
				{
					high = mid - 1;

				}
				else if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) > 0)
				{
					low = mid + 1;
				}
				else if(searchEntry.compareToIgnoreCase(studentArray[mid].getLastName()) == 0)
				{
					result = studentArray[mid];
					high = -1;
				}
			}
			if (result != null)
			{
				System.out.println("Student found: " + result);
			}
			else
			{
				System.out.println("No student with last name: " + searchEntry);
			}

The algorithm is otherwise correct.

[trantommyd]

So how exactly would I make a message display only if no match is found? Delete the else statement, and then make an if outside of the while loop? But what could I make as the condition of this if statement?

[Fou-Lu]

The if would just become if (result == null) to indicate that no match is found.

[trantommyd]

you sir, are awesome. Thank you very much.