CSC 207.01 2019S, Class 24: Pause for breath

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Quiz
• Code from the Interweb
• Testing our SLPQs
• Linked structures and iterators
• Array-based queues
• Testing average
• Average

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.)
• Oreos: Up to three

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

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

Extra credit (Wellness)

Extra credit (Wellness, Regular)

• 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.

Other good things

Questions

Quiz

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.

Testing SLPQs

• Key things
  • Try to think expansively
  • Practice, practice, practice
  • Take advantage of randomness - it can help you catch things you’ve never thought of
• Particular issues in SLPQs
  • Keep the queue in sorted order
  • Remove
• Writing a randomized test
  • Start with an ordered array of integers, generated randomly values[i] = values[i] + rand.nextInt(3);
  • Make a copy
  • Permute the copy for (int i = 0; i < copy.length; i++) { swap(copy, i, rand.nextInt(copy.length)); }
  • Add to the sorted queue
  • Compare to the order array of integers Iterator it = slpq.iterator(); for (int i = 0; i < values.length; i++) { assertTrue(it.hasNext()); assertEquals(values[i], it.next()); } assertFalse(it.hasNext());
  • Remove some values (even) Iterator it = slpq.iterator(); while (it.hasNext()) { if (it.next() % 2 == 0) { it.remove(); } // if } // while
  • Check that it worked for (int i = 0; i < values.length; i++) { if (values[i] %2 == 1) { assertTrue(it.hasNext()); assertEquals(values[i], it.next()); } // if } // for assertFalse(it.hasNext());
  • Add the even values back in
  • And check

SLPQs

Array-based queues

Testing average

Average