Overview

• Preliminaries
  • Admin.
  • A few quick notes on HW 2
  • Questions on HW 3
• Leftover types topics.
• A few notes on textual output and input.
• Output.
• Input.
• Lab.

Admin

• The strange things that CS students discuss: "If two people are dating, are they boyfriend and girlfriend (or boyfriend and boyfriend) (or girlfriend and girlfriend) or (gender-netural-term-friend and ...)?"
• Reading for Monday (it's ready!)
  • Exceptions
• I've prepared a lab for today, but I don't know if there will be time.
• I hear that some of you are losing files. You should make it a practice to save early and often. You should also make it a practice to commit regularly. (Say, after each working procedure you write.)
• I'm in the midst of grading HW2. I should have it done by Monday. We'll discuss the averaging problem.
• Only about half the class has filled out the prologue (as of 8:30 p.m. on Thursday)
• EC opportunities:
  • CS Table, Friday: Trusting Trust.
  • CS Table, next Friday: Pair Programming.
  • More?

HW 2

• For the averaging problem, there are a variety of strategies

  • Promote the type of the parameters.

    • Many of you used double. We'll talk about why that's a bad idea.
      • Floating points approximate. Are you sure your code works correctly for every situation?
      • doubles are 64 IEEE floating-point numbers, they have about 52 bits, so we are okay
    • We'll also talk about why promotion may not be the best strategy. what if we have

    public static long average(long left, long right) { return (left + right) / 2; } // average

will this work?

    public static long average(long left, long right) {
        return (long) (((double) left + (double) right) / 2);
    } // average

how about this?

    public static long average(long left, long right) {
        return (left/2) + (right/2);
    } // average

nope, won't work if both are odd ... so

    public static long average(long left, long right) {
        // Compute the average w/o overflow
        long tmp = (left/2) + (right/2);
        // Handle introduced inaccuracies
        if (isOdd(left) && isOdd(right))
            return tmp+1;
        else
            return tmp;
    } // average

coming up: generalized average: taking an array of longs as input

• Java carefully specifies all of its numeric types longs are 64 bits, twos complement
  • A few of you changed the preconditions. I was pretty clear that I wanted you to rewrite the procedure, but I'll accept that as an approach (particularly since the testing lab suggests that approach for a similar problem).
  • Anything else?

HW 3

• Yes, exhaustive search is an acceptable initial technique for the last problem. (And no, it won't be efficient.)
  • You might also try a strategy in which you say "Can I do this with one coin, with two coins, with three coins, ...?"
• No, you don't need to set up a lot of test cases for the last problem.
• What should we do on splitCSV if the input is crappy (e.g., only one double quotation mark) - You can crash and burn.
• For the de-leet-ful problem, are all 3's e's? (Not quite, some are parts of B's, but there should be no 3's in your output.)

Leftover types topics

• Two dimensions:
  • Real vs. Int
  • primitive type, object corresponding to primitive type, or Big
• Primitive types: Mostly like C, except specified
  • ints are twos-complement
• But the primitive types aren't objects
  • And so we have java.lang.Integer and java.lang.Double
• And we want arbitrary precisions
  • java.math.BigInteger

A few notes on textual output and input

• Textual I/O is useful for files
• We use an OO approach to I/O

Output

• PrintWriter
  • print
  • println
  • format (like printf)
• PrintWriters buffer their output
  
  • need flush() to force outuput
• Can create with System.out

• Can create with files - new PrintWriter(new File('ooh.csv')) ;

  Input

• BufferedReader

  • read - read a character
  • readLine - read a line
• Can create with System.in
• Can create with files new BufferedReader(new FileReader(new File('ooh.csv')));

Lab

    public static void main(String[] args) throws Exception {
        // ...
    }