CS 40 - Lecture 4

Cay S. Horstmann

Lecture 3 Recap

• Top-down design: Break story into scenes, scenes into tasks, etc.
• Storyboard: Image on top, description, list of actors and actions below
• Code reading: Important skill. Lesser artists borrow, great artists steal...

How Can You Get Alice Outside Class?

Please log in to ActiveLecture.org. Click Discuss, type 1 2 3, click Send.

1. I already installed it
2. I brought my laptop today so I can get installation help
3. I will bring my laptop on Thursday so I can get help
4. I will install it myself over the weekend and use the eCampus discussion feature if I have problems
5. I need to use a computer in a lab that has Alice preinstalled

How Was the Lab Work on Tuesday?

Please respond in ActiveLecture.org

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

Thanks for your feedback!

Decisions, Decisions...

• One-time: If a condition is fulfilled, do x
• Repeated: While a condition is fulfilled, do x
• Condition is either true or false. E.g. "the red shoes are on sale"
• Condition may involve relational operators. E.g. "price of red shoes < price of black shoes"
• Note: Some operators look weird: ==, !=, <=, >=

If/Else

• Three parts
  • condition: an expression that is true or false
  • statement (or statement sequence) that executes when condition is true
  • "else" part: (optional), statement (or sequence) that executes when condition is false
• One of the two branches executes
• Afterwards, the statement following the if/else executes

Example: Eating the Closest Cookie

• Everyone knows that evil step sisters love eating cookies...
• ...but they are vain and don't want to get fat, so they only eat one per day...
• ...and they are lazy, so they eat the closest one

Lab #1

• Make a world with one evil step sister (People) and two cookies (Kitchen/Food)
• One cookie should be closer to the ESS
• In world.my first method, use an if/else to have the ESS move to the closer cookie and eat it
• To “eat” the cookie, turn its opacity to 0.
• Don't fuss with the details. We care about if/else, not realistic eating.
• Test that your code works: rearrange the cookies, hit Play again
• Feel free to use a different story. Everyone knows that a T.Rex loves eating toasters...
• Save as ess.a2w

Alice Tedium

Recipe for dragging the condition "if the distance from the ESS to cookie1 is less than the distance from the ESS to cookie2".

1. Drag an if/else tile. When forced to make a choice, select true from the menu.
2. Click on world, then functions in world's details
3. Drag the a < b tile into the if, replacing the useless true.
4. When forced to make choices for a and b, pick any values.
5. Now click on evilStepSister, then functions in evilStepSister's details
6. Now drag the distance to tile over the useless value to the left of the <
7. Repeat for the right side

While

• Two parts
  • condition: an expression that is true or false
  • statement (or statement sequence) that executes as long as condition is true
• Execution flow
  • Check condition
  • If true, execute statement(s)
  • Check condition again
  • If true, execute statement(s)
  • etc.
  • If condition is false, execute statement after while

Example: Waiting for the Toaster

• Everyone knows that T.Rex love eating toasters...
• ...they wait as toasters move across the desert...
• ...and when one comes close enough, they eat it...

Lab #2

• Make a lab with a Trex (Animals) and a toaster
• Drag a while tile. Leave the condition as true for now.
• In the loop, make the toaster move by a short distance.
• Play the world. The toaster should move across the desert, passing the Trex. If needed, adjust positions/directions.
• Change the while condition. Use one of the trex proximity functions. Eat the toaster if it is closer than 2 meter.
• Add code for eating. Again, realism isn't important.
• Where do you put that code? In the loop? After the loop?
• Probably the 2 meter didn't work out. Use trial/error to find a better value.
• Save as trex.a2w

Reminders

• Please submit two worlds (ess.a2w and trex.a2w) in eCampus
• Put your buddy's name in the comment field
• Homework #1 is in the eCampus schedule