CS 40 - Lecture 5

Cay S. Horstmann

Lecture 4 Recap

• condition: an expression that is either true or false
• if-else decision: Either execute the statements in the if part or in the else part.
• while loop: Keep repeating the statements while a condition is true

Alice Install Survey

Please log in to ActiveLecture.org. Click Discuss, type A B C, click Send.

1. I already installed Alice
2. I brought my laptop today so I can get installation help
3. Help! I don't know how to get Alice to work at home

How Was the Lab Work on Tuesday?

1. Too easy/boring
2. Just right
3. Too hard/confusing

Thanks for your feedback!
Sample solutions are in http://www.horstmann.com/sjsu/cs40/schedule.html

Methods

• A method contains a sequence of instructions (e.g. move up, move right, move down)
• A method has a name (e.g. hop)
• A method belongs to an object (e.g. the toaster)
• To execute the statements, the method must be called (e.g. toaster.hop())

Example: SpiderRobot

• Load the First Encounter world again
• Note how long the my first world method is
• Note the Do together tile with the comment
  // spiderRobot moves forward as its legs walk
• Comment = English (not Alice code!) to help the human code reader
• These statements can be put into a spiderRobot method

Lab # 1: Walk Method

• Drag the Do together tile to the clipboard (top right corner of screen)
• Click on spiderRobot
• Click on create new method
• Name it walk
• Drag the clipboard into the walk method
• Go back to world.my first method and drag the Do together tile into the trash can
• Click on spiderRobot. Drag the walk tile into my first method.

Try It Out

1. Hit Play. What happens?
2. Why is this version of the program better than the original?
3. How many meters does the robot walk?
4. What happens if you place the walk tile twice?

Put answers to 1., 2., 3., 4. into http://ActiveLecture.org

Too Boring?

• Make the robot walk all the way to the rock, using the walk method and a while loop.

Parameters

• The walk method makes the robot walk 1 meter
• What if we want to walk 10 meters? 3.5 meters?
• Why not walk arbitrary distances?
• Make distance a parameter
• Parameter = input to method
• Other example of parameter: say method has parameter what (i.e. what to say)
  spiderRobot.say("Houston, we have a problem!")

Lab #2: Distance Parameter

• In spiderRobot.walk, click on create new parameter
• Call the parameter distance
• Make it a number
• Drag the parameter over the 1 meter value in the call to move
• Click Play. The robot moves 1 meter. (Look into my first method to see why.)
• In my first method, set parameter value to 5
• Click Play. The robot moves 5 meter...

Not So Fast!

• ...but it moves all 5 meter in a single second.
• You want to traverse 5 meters in 5 seconds
• But your method needs to work for any distance
• Fix this! (Hint: Click on more, select expression)

Too Boring?

• Make another method announce for the spiderRobot
• Your method should do the last three tiles in my first method: face the camera, turn the head red, say something
• The “something” should be a parameter (of type String)
• Make the robot announce “Houston, we have a problem!” followed by “The end.”, by calling the announce method twice.
• Still bored? Make the head color into a second parameter

Food for Thought

• In lecture 4, the evil step sister either moved to cookie and ate it, or it moved to cookie2 and ate that.
• What method could one make to simplify this code?
• What would the parameter be?
• What is the parameter type? Number, true/false, String, Color, Object?
• Try it out in your copious spare time...

Reminders

• Homework #1 is in eCampus