CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2011

Q&A for Homework #3, due in lecture Wednesday 9 November

Question text in black, answers in blue.

Please note several corrections of L&C's questions on the assignment page.

• Question H3.1, posted 7 November: I think the code in L&C's Exercise 6.1 will throw an exception, but they (and you) want to know what happens after that. Is it possible they want the ordered list to have the highest number first? In that case the code works without an exception.

  That's a good idea for a save, but it's pretty clear from their code that they want the lowest number first. What you should do here is say when the exception will occur, but continue as though every line with a remove call is in a block like:

  
       try{
           X.remove(...);}
       catch (ElementNotFoundException e) { }
    

  so that the remove acts more like a poll.

• Question H3.2, posted 7 November: I'm confused by your Question 6, to give a recursive definition of a Java identifier. Do you mean for me to write a recursive boolean method that will take a String as input and return true if it is a valid identifier?

  No, that would be a sensible thing to ask, but it is not what I am asking. The model here is the recursive definition of a list on page 186 of L&C -- a list is a number (the base case) or a list, followed by a comma and a number (the recursive case). A String in Java is either the empty string "" (the base case), or a String followed by a character. You should be able to give a similar definition for identifiers.

• Question H3.3, posted 7 November: Is it possible to use "null" as a parameter for the find method in the array implementation of a list? I would like to find the first null spot in the array and return the index.

  Let's see what the method on page 156 will do if you call it with parameter null. The first time you check an element, it says target.equals(list[scan]), and this will throw a NullPointerException if target is null. They could have said list[scan].equals(target) instead, but this would have thrown an exception whenever the scan reached a null entry of the list. Of course, you could write your own separate findNull method that would do this specific job.

Last modified 7 November 2011