CSC 207.01 2019S, Class 24: Pause for breath

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Quiz
• About the makeup exam
• Code from the Interweb
• Quick notes on average
• Testing our SLPQs
• Linked structures and iterators
• Array-based queues

Preliminaries

News / Etc.

• Detour questions:
  • Do you prefer the PM/AV-style diagrams or the SR-style diagrams?
  • Do you prefer the PM/AV-style labs or the SR-style labs?
• I will not post the answers we develop in class today. (I may post the tests; feel free to take photographs of eboard or whiteboard.)
• We’ll probably do the Array-based queues on Friday
• Oreos!

Upcoming work

• Assignment 6 due Thursday.
  • This is an individual assignment, rather than a group assignment.
  • You may consult with anyone you wish, provided you cite them.
• Readings for Friday: Lots!
  • List ADTs (may get updated)
  • “Lists with ‘current’ considered harmful”
  • java.util.List
  • java.util.ListIterator
• Makeup exam 1 due Thursday the 11th
  • Required prologue due Friday night
  • I will try to do the exam tonight and report back on times
  • I will distribute unit tests on Saturday
  • You only need do the prologue for the problems you intend to do on the makeup.

Extra credit

Extra credit (Academic/Artistic)

• Convo Thursday: Microbes: The links between soil, gut, and health
• Noura Mint Seymali Concert tonight at 7:30 p.m. in Herrick
• Isabelle Demers concert, Saturday at 3:00 p.m. in Herrick

Extra credit (Peer)

• Singers concert, April 7 at 2:00 p.m.
• Thursday the 11th, Environment group event. More to come.

Extra credit (Wellness)

Extra credit (Wellness, Regular)

• Painting with plants 7:30 p.m. in Younker Lounge
• 30 Minutes of Mindfulness at SHACS every Monday 4:15-4:45
• Any organized exercise. (See previous eboards for a list.)
• 60 minutes of some solitary self-care activities that are unrelated to academics or work. Your email reflection must explain how the activity contributed to your wellness.
• 60 minutes of some shared self-care activity with friends. Your email reflection must explain how the activity contributed to your wellness.

Extra credit (Misc)

• Wednesday the 10 at 4pm on Mac Field: Giant Laurel Leaf. (Free t-shirt!)
• Scarlet and Give Back Day next Wednesday/Thursday (I think). If you don’t have money to donate, let me know and I will give you $5 to donate.
• Public speaking workshop.
• Storytelling workshop.
• Kinetic sculpture competition.

Other good things

Questions

Suppose I have two arrays, of sizes n and m, and I have to iterate both. What’s the time complexity?

O(n + m) is the most common model.

O(max(n,m)) is also acceptable, although it’s the same

Quiz

• Sam should not write pseudocode, at least not late at night.

About the makeup exam

• Sam’s grading:
  • Check: Full credit
  • Check-minus: Not quite full credit (drop by 1/3 letter)
  • Half: Half credit
  • Zero: No credit
  • Add ‘em up, follow those policies

Code from the InterWeb

Taken from https://stackoverflow.com/questions/1930454/what-is-a-good-solution-for-calculating-an-average-where-the-sum-of-all-values-e

double mean(double[] ary) {
  double avg = 0;
  int t = 1;
  for (double x : ary) {
    avg += (x - avg) / t;
    ++t;
  }
  return avg;
}

• What works for double may not work for long.
• This still has an overflow problem.
• The comments on this solution even say that it has an overflow problem.

Taken from http://www.java2novice.com/data-structures-in-java/queue/dynamic-array-impl/

  private void increaseCapacity(){
         
        //create new array with double size as the current one.
        int newCapacity = this.queueArr.length*2;
        int[] newArr = new int[newCapacity];
        //copy elements to new array, copy from rear to front
        int tmpFront = front;
        int index = -1;
        while(true){
            newArr[++index] = this.queueArr[tmpFront];
            tmpFront++;
            if(tmpFront == this.queueArr.length){
                tmpFront = 0;
            }
            if(currentSize == index+1){
                break;
            }
        }
        //make new array as queue
        this.queueArr = newArr;
        System.out.println("New array capacity: "+this.queueArr.length);
        //reset front & rear values
        this.front = 0;
        this.rear = index;
    }

If you ever turn in code this crappy, you will get a zero.

Issues

• Declares a pointless variable.
• Prints out random error messages for now reason.
• A for loop masquerading as a while loop. Ugly! (Also really hard to analyze).
• Lots of +1 and -1, which suggests difficulty getting bounds right.
• Inconsistency in naming. Sometimes front, sometimes this.front.
• Includes redundant rear field.
• Doesn’t update capacity field.

Average (quick notes)

Testing SLPQs

Testing is a skill, and takes time to develop.

• You develop the skill by trying again and again and again.
• “I think I’m done”; push a little bit further.
  • “What is likely to be a bit strange?”
  • “What is pretty normal?”
• If you can’t think of things, randomized tests are your friend.
  • Cover things you don’t think of.
  • When they fail, require you to think a bit more about why, giving you a new set of tests to write.
• Randomized tests depend on the particular domain that you’re working with. Often: Start with an input with a known solution, randomize input in a way that doesn’t affect the solution, run the alg.
• Key issues in SLPQs
  • Does the queue end up in order?
  • Does remove work?
    • At the front (multiple times)
    • At the end (multiple times)
      • Neighboring
      • Not neighboring
    • In the middle (multiple times)
    • With subsequent additions

Start with something we know: A randomly generated array of integers in ascending order, with potential duplicates.

// Generate an array with random size and elements in increasing
// order.
int[] values = new int[1 + rand.nextInt(100)];
values[0] = 10 - rand.nextInt(20);
for (int i = 1; i < values.length; i++) {
  values[i] = values[i-1] + rand.nextInt(3);
} // for

// Randomly permute the array
int[] copy = Arrays.copyOf(values);
permute(copy);      // We have to write permute

// Add all the values to the queue
for (int val : copy) {
  q.add(val);
} // for

// And check that it's in the right order
Iterator<Integer> it = q.iterator();
for (int i = 0; i < values.length; i++) {
  assertTrue(it.hasNext());
  assertEquals(values[i], it.next());
} // for
assertFalse(it.hasNext());

// Systematically remove elements
Iterator<Integer> it = q.iterator();
while (it.hasNext()) {
  if (it.next() % 2 == 0) {
    it.remove();
  } // if element is even
} // it.hasNext()

// And check to make sure that it contains all of the odd elements
// of the original array
Iterator<Integer> it = q.iterator();

// And check that it's in the right order
Iterator<Integer> it = q.iterator();
for (int i = 0; i < values.length; i++) {
  if (values[i] % 2 == 1) {
    assertTrue(it.hasNext());
    assertEquals(values[i], it.next());
  }
} // for
assertFalse(it.hasNext());

// Add some elements back in
for (int val : copy) {
  if (val % 2 == 0) {
    q.add(val);
  } // 
}
// And check the result

Utilities

public static void permute(int[] values) {
  for (int i = 0; i < values.length; i++) {
    swap(values, i, rand.nextInt(values.length));
  }
} // permute

Note

• Look at some of the tests I wrote. (The style isn’t great, but the ideas may be useful.)

SLPQs

Start with pictures!

Array-based queues

Testing average

Average