CS 40 - Lecture 7

Cay S. Horstmann

Lecture 6 Recap

• voting machine: a machine that tallies votes
• computer science: impossible to verify that a computer program works correctly; failures must be expected
• public policy implication: must build in verification (random spot checks) and redundancy (independent paper trail)

Variables

• A variable holds a value
• The value can change over time
• Initializing the variable = setting the first value
• Instance variable is defined in object
• Local variable is defined in method

Example: Voting Machine

• Load the VotingMachine1 world again
• The voting machine needs to keep track of the votes
• Where are the variables?
• Answer in ActiveLecture.org

What Kind of Variables?

What are the variables in the voting machine?

1. Local variables
2. Instance variables

Answer in ActiveLecture.org

Lab # 1: Combo Lock

• Load the ComboLock1 world.
• Click Play, then hit the left and right arrow keys ← and →. What happens?
• Why does it happen? Hint: Close the Play window. Look at the events. Look at the methods.
• Answer in ActiveLecture.org

Track the Number

• We want to track the number under the red arrow.
• The combo lock doesn't know it. The dial is just a 3D image.
• Need a variable.
• Need an instance variable because we store object state (accessible by multiple methods)

Make an Instance Variable

• Click on the comboLock tile, then the properties tab in comboLock details. Click on create new variable.
• Name the variable dialValue. It is supposed to denote the current value of the dial.
• Click on the methods tab, then edit the up method.
• Click again on the properties tab and drag the dialValue tab into the up method.
• cont'd on next slide...

Make an Instance Variable (Cont'd)

• Pick comboLock.dialValue++ from the menu. (++ is shorthand for “increment” or “add 1”)
• Be sure to keep the properties tab visible and click Play. Hit the right arrow key →.
• What happens with the value of dialValue in the properties tab? (ActiveLecture.org)

Initializing

• If you carefully look at the dial and the value of the variable, what do you notice?
• How do you fix it? (ActiveLecture.org)

Changing Values

• Also implement the down method. Simply pick comboLock.dialValue--
• What happens when you keep hitting the left arrow key?
• That isn't so good. What do you do to fix it?
• Do you also need to put a fix in the up method?

Answers in ActiveLecture.org

Too Boring?

• To select a number in a real combo lock, you reverse direction when you reach the number.
• We want to remember the last selected number.
• We also need to track the direction so we can tell when it is reversing.
• Make two variables. direction is +1 when moving up, -1 when moving down. When do you set selectedNumber?
• How do you initialize direction and selectedNumber?

Why Local Variables?

• Consider the First Encounter world.
• Look at the horrid expression spiderRobot.body.backLeftLegBase.backLeftUpperJoint. It appears twice in a row (or does it?)
• It is better to store the expression in a variable backJoint. Then the calls become

  backJoint.turn(FORWARD, 0.1)
  backJoint.turn(BACKWARD, 0.1)

• Tip: Use local variables for complex expressions that occur more than once

Lab #2: Local Variables

• Start with the sample solution lab05.a2w
• Make a local variable by clicking on create new variable
• Initialize backJoint with the horrid expression
• Replace the horrid expression with backJoint twice
• Do the same with frontJoint
• Click Play to make sure it works
• If you have extra time, move all four joints.

Reminders

• Lab work in eCampus
• Is your bio/photo in eCampus?.
• Homework #2 is in, you guessed it, eCampus