CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2011

Q&A for Homework #2, due Monday 17 October

Question text in black, answers in blue.

• Question H2.1, posted 9 October: In Exercise 5.2 of L&C, you say that the four code fragments are supposed to be run independently, but this makes no sense -- fragments (a) and (d) are identical. Shouldn't they be run sequentially?

  Yes. I've changed the HW#2 assignment page to fix this, and you get an extra point on the final exam for catching the mistake first.

• Question H2.2, posted 15 October: What exactly do you mean by "reading the input string only once" in Questions 7 and 8? I would like to read the input, push each letter into the stack, and pop them out later and read them. Does that count as reading the letters twice?

  No, you are allowed to read them from the input string only once. If you put them in your internal memory and read them from there later, that is fine -- in fact the internal memory would not be of much use otherwise!

  Could I use a temporary string as my internal memory?

  No, your only internal memory is what I gave you -- one stack and one queue in Question 7, and one stack along in Question 8.

• Question H2.3, posted 15 October: I believe I have solved Question #3 with each basic operation taking only O(1) time. Could you please look at my code?

  I looked at it, and while your methods are all O(1) time you are not implementing a dropout stack at all but an ordinary stack. When a dropout stack exceeds its capacity it must remove its bottom element, but your code doesn't do that. As we've seen, a singly linked list has trouble removing elements from both ends.

  Could I use a doubly linked list?

  We will do that eventually, but for now no, just do the best you can with the LinearNode class that we have.

• Question H2.4, posted 15 October I was writing code for Question 8 and my compiler objected to w.length -- you can only get the length of a String by saying w.length( ) because length is a method.

  You're right -- this was a typo on my part which has now been corrected. As you say, we can get the length of an array with just x.length but String objects require x.length( ). You get a point on the final for catching this.

  By the way, I see that you've noticed that your compiler contains a very sophisticated validity checker for Java code, so that even if you are writing a homework assignment on paper that doesn't need to be compiled, it can be useful to write your in your normal Java environment where the compiler can check it. The only difficulty with this is that paper assignments often assume that particular other code is present and it might not be.

• Question H2.5, posted 15 October In Question 7, do you mean one piece of code that uses both a stack and a queue, or one piece that uses just a stack and another that uses just a queue.

  The first, both a stack and a queue. You use just a stack in Question 8, and in fact it can't be done with just a queue if you are just reading the input once from left to right. (This is sometimes proved in CMPSCI 401.)

Last modified 15 October 2011