Unit 7
Conditional Branching

Pizza Toppings Time: 30 minutes

• Turn to the Design Recipe on Page 23 and grab a Design Recipe Worksheet.
• Suppose we've been hired by Luigi's Pizza to write a function that tells us the cost of different pizza pies. Let's use the design recipe to write this function. Have a student read the problem statement.
• I need a volunteer to be our function. Pick someone, and copy the contract as they answer. What is your name? cost Your Domain? String Your Range? Number.; cost: String -> Number
• Can someone from the class tell me how we should call this function? For example, "cost 'cheese'!" What will cost produce? Let's try this with other toppings...
• Now it's time to write down some examples. Can anyone raise their hands and tell me what I'd write? (EXAMPLE (cost "cheese") 9.00)
• What are some other examples for cost? What changes between them? The topping and the price returned! Make sure you label those.
• Do you notice something odd here? This is the first time that we've ever circled something in the second of the examples, which wasn't also circled in the first part. The price that's being produced changes, but the function never takes in the price!
• That's a hint that something special is going on, but let's see how much father the Design Recipe can take us...
• Now for the Function Header. What do I write here? (define (cost topping)
• The Function Body is next. But now we don't know what to write! We know that our examples behave differently from one another -- sometimes we want to return 9.00, other times it's 10.50, etc. So what do we do? Well, we could fill in all off those results. Let's do that...Make a large, 2-column table on under the Function Header.
  (define (cost topping)

  	10.50
  	9.00
  	11.25
  	10.25

  
  )
• But how do we know when we want to produce 9.00? When the topping is "cheese". When do we want to return 10.50? When the topping is "cheese".
• What we want is a way to go down each line, checking to see if the topping is the right one. If it is, we go on to finish the line. If not, we go on to the next one.
  • What's Domain of our function? String (according to the contract)
  • What's the type of "pepperoni"? String
  • What function compares two strings, and gives back a Boolean? string=?
  • What's the Racket code that compares the input topping to the string "pepperoni"? (string=? topping "pepperoni")
  • Now we can write that on our first line, as our first topping check. Can you do the rest?
• Have the students fill in the rest of the table...
  (define (cost topping)

  (string=? topping "pepperoni")	10.50
  (string=? topping "cheese")	9.00
  (string=? topping "chicken")	11.25
  (string=? topping "broccoli")	10.25

  
  )
• Each of these rows is called a condition. A condition has a test and a result. The computer goes down the code, one condition at a time, and will evaluate the first result for which the condition is true.
• Racket has a special function that lets us tell the computer to do this: cond. To use cond, you put square brackets around each of the branches, and write "cond" at the top:
  (define (cost topping) (cond [(string=? topping "pepperoni") 10.50] [(string=? topping "cheese") 9.00] [(string=? topping "chicken") 11.25] [(string=? topping "broccoli") 10.25] ) )
• Remind students that computers are very specific and can't make up new answers; we need to tell it what to do in case the user inputs an item that is not in our list. Let's add else. If it's not on the menu, we might still make that pizza for you, but it'll cost you! [else 10000000]
• Have students try it on the computers, adding new items on their own.
• If you have additional time, and would like to try another Cond challenge, check out the supplemental activity.

Player Movement Time: 25 minutes

• Great! Now that we know cond we can write update-player.
• Draw a screen on the board, and label the coordinates for a player, target and danger. Circle all the data associated with the Player.

• What is the player's starting x-coordinate?
• It's starting y-coordinate?
• What about after it moves? What's the new x and y? What has changed? And by how much? What happens when we press the down key? What should the new coordinates be then?
• Get students to tell you what update player should do...
• We want a function that will move up the screen when the user presses the up arrow and down when the user presses the down arrow.
• We've set up the computer to call update-player, passing in the player's y-coordinate and the name of the key pressed. The keypress will either be the string "down" or the string "up" (for now). What kind of data is the y-coordinate? What kind of data is the keypress?
• Make a table showing possibilities and results, walking students through it.
• With our pizza example, we had to deal with toppings that weren't on the menu. Now we need to deal with keys that aren't "up" or "down". How do we do that?
• On page 24, you'll find the problem statement for update-player. Grab a Design Recipe Worksheet, fill it out, and then write this function with your team.
• Students can also add "cheat codes", by adding Cond branches for other keys. For example, a student might add [(string=? key "c") 240], which causes the player to jump to the center of the screen if the c key is pressed.