Held: Monday, 24 February 2014

Back to Outline 20 - Reasoning About Loops with Loop Invariants. On to Outline 22 - Detour: Anonymous Inner Classes.

Summary

We consider issues from the first examination.

Related Pages

• Exam 1
• EBoard 21 (Source 21) (HTML 21) (PDF 21)

Overview

• Problem 1 - Testing.
• Problem 2 - Averaging.
• Problem 3 - Stringing.
• Problem 4 - Replacing.
• Additional issues.
• Makeup.

Administrivia

• Exam 1 will be returned at the end of class.
• MathLAN may or may not be acting up again. But we have no lab today, so it's okay.
• Reading for Tuesday: Anonymous inner classes.
• Homework 5 is due next Wednesday (March 5), but you may need two weeks to get it done.
• Our graders are working hard, but they've discovered just how much time it takes to grade well.
• Extra credit:
  • Town hall meeting Tuesday at noon or 7pm.
  • CS Extras: The new CS Curriculum.
  • More?

General Issues

• Generally less good than I would have hoped.
• Formatting is important.
• The form of your tarballs is important.

Problem 1: Testing

Here are some of the things I expected:

• Systematic tests: Using loops to build a variety of related arrays whose average you should know.
• Broad categories:
  • Positive numbers.
  • Negative numbers.
  • Positive and negative numbers.
    • With a positive average.
    • With a negative average.
• Edge cases: Rounding
  • Rounding up for negative sums.
  • Rounding down for positive sums.
  • Not just rounding of half cases.
• Edge cases: Extreme values
  • Multiple copies of Long.MAX_VALUE
  • Multiple copies of Long.MIN_VALUE
  • Average of those values.
  • Near those values.

Additional issues:

• If you're writing repetitious code, why aren't you writing helpers?
• Separate sets of tests probably belong in separate procedures. It makes it easier to see multiple issues.

Problem 2: Average

• No doubles!
• If you have a running average and a running remainder, can the remainder overflow?

Problem 3: Stringify

• It might have been helpful to write a helper that does the conversion in memory, before writing to the file.

• What do you see as the difference between

   String line = infile.readLine();
   while (line != null)
     {
       ...
       line = infile.readLine();
     } // while
  

  and

   String line;
   while ((line = infile.readLine()) != null)
     {
       ...
     } // while
  

• What do you see as the difference between

   ch = line.charAt(i);
   if (ch == '\t')
     ...
   else if (ch == '\n')
     ...
   else if (ch == '\\')
     ...
   else if (ch == '\"')
     ...
   else
     ...
  

  and

   switch (ch = line.charAt(i)
     {
       case '\t':
         ...;
         break;
       case '\n':
         ...;
         break;
       case '\\':
         ...;
         break;
       case '\"';
         ...;
         break;
       default:
         ...;
         break;
     } // switch
  

Problem 4

• Some of you used String.replace. I thought the problem was pretty clear that that approach was not permitted.

• Some of you used (approximately)

  int i = 0;
  while (i < contents.size-pattern.length())
    {
      if (this.matchesAt(i, pattern))
        {
          this.remove(i, i + pattern.length());
          this.prepend(i, replacement);
          i += pattern.length();
        } // if match at position i
      else // if no match at position if
        {
          i++;
        } // if no match at position i
    } // while
  

• This solution is potentially quadratic in the size of the input (e.g., "aaaaaa".replace("a","b"))

Makeup

Although I said that I was not going to allow a makeup exam, there were enough problems on problems 2 and 4 that I will allow you to make those problems up for partial credit.