Copyright © 1997, 2013
Robert W. Hasker

Slightly less basic Java

• 3.11: assignments, initialization, initialization & declaration
• 3.14: more on constants
• 3.17: ++, --, +=, and similar operations
• 3.18: tracing code
• Testing

Discussion: why worry about spaces in output?

Assignment

• Write program to compute time to travel on city highway
  • Prompt for distance, compute minutes of travel time assuming 45 mph average
  • Rewrite to compute time to travel on back streets (new distance, assume average 23 mph)
• Assignment vs. initialization
  • Assignment: replace value
  • Initialization: set value first time (initial value)
  • Should always initialize variables at point of declaration unless the value is not going to be used.
  • Textbook: declare variables at top of main; this is a poor practice
• Additional assignment operators:
  • +=, *=, -=, etc.
  • ++, -- (foreshadow loops)

Tracing Code

• Step through above problem
• Note:
  • Stepping
  • Variable values
  • Breakpoints
  • Debug vs. run

Algorithms, Revisited

• Chapter 2
• Rewrite program using pseudocode
  • Pseudocode: "sort of" code
  • Algorithm: the structure of the solution
  • Implementation: the actual program written in Java (or another language)
• Note basic structure of this program: sequence
  • Simple, but very limited
  • Complexity is in the rules about the individual pieces: types, expressions, string operations, etc.
• Adding more complex behavior
  • Would like the program to recommend one path over another
  • Needs to make a decision
• Other types of decisions
  • Suggestions?
  • Input sanity check
    • One solution: print message and quit
    • Another: convert to positive
    • Which is actually better?
  • Add ability to describe traffic
• Review terms
  • Sequence, decision
  • Algorithm
  • Pseudocode
  • Implementation/code
• How to write a program
  • Get suggestions of some of the things that need to be done
  • Steps:
    1. Be sure you can hand-compute the results
    2. Write out an algorithm as pseudocode
    3. Write solution for part of problem
       • Fill out comment block at top right away!
    4. Test that portion works
    5. Repeat for other portions
    6. Test full solution, especially testing with obscure data
    7. Review initial assignment to make sure nothing missed

Loops

• Initial program sort of odd
  • Why just two branches?
  • If user enters a bad input, should we really just exit?
• Solution: introduce loops
  • For now: ask user how many paths to look at
  • Pseudocode for looping: repeat n times
  • Advanced: add something to keep track of best time

More modern input and output

• Section 3.25 of textbook
• Try:
  • import javax.swing.JOptionPane;
  • JOptionPane.showMessageDialog(null, "message")
  • JOptionPane.showInputDialog("prompt")
  • Integer.parseInt(JOptionPane.showInputDialog("prompt message"));

Exercises

• Write pseudocode to compute time of day in seconds
• Add error if time is invalid
• Read name, print first 3 letters if long enough, 1 letter otherwise
• Add list of numbers entered by user (where user states how many up front)

Review

• assignment, initialization, declaration
• tracing, breakpoint, debug vs. run
• algorithms, pseudocode, implementation
• sequence, decision, loop
• Note: a lot of terms!
  • Other terms to pay attention to: comment, input/output, variable, float, double, integer, character, string
• Being caught up:
  • Ch. 2, Ch. 3 through 3.25