lect01c
a first program

Quickly recalling two of the expressions we typed in lab yesterday:

jo.pizzaArea(10) + jo.pizzaArea(2*6)
jo.pizzaArea( Math.sqrt(64) )

Remember the mantra for calling a method:

When calling a function: “object dot methodName openParen argument”

These two expressions further illustrate that the argument can be the result of a computation (2*6 or Math.sqrt(64)) and also that you can use the result of calling pizzaArea to do other stuff (like adding, in the first example).

Practice: Now it's your turn to write some functions.

Krusteaze Pizza is having a promotion:
Free foot Fridays: Order any pizza, and we'll throw in a 1-foot (12″) pizza for free!
Write friPizzaArea — a function which takes in (as a parameter) the diameter which the customer orders, and returns the total area of pizza they'll get. Remember that we have a procedure for writing the function; it's not a procedure which relies on intuition:
1. Test cases. We can try to test simple inputs -- like 0 or 2 -- first, to try to make our arithmetic easy.¹
  - Hmm .. friPizzaArea(0) = ?. Well, if somebody orders a zero-inch pizza on Friday (sneaky!), they'll receive their zero-inch pizza (total area: 0) plus a 12-inch pizza (total area: ... well, let's ask our expert: jo.pizzaArea(12) = 113.04), for a grand total of 113.04. Thus our test case is:
    jo.friPizzaArea(0) = jo.pizzaArea(12) = 113.04
    (We didn't have to do any multiplying ourselves, since we already know that jo can answer regular pizzaArea questions correctly.)
  - Okay, let's work on what the answer to friPizzaArea(2) should be. We'll, they'll get their regular 2″ pizza plus the free 12″ pizza. How much that, exactly? Well jo.pizzaArea(2) + jo.pizzaArea(12) = 3.14 + 113.04 = 116.18.
  - In addition to 0 and 2, we'll try one test case which is more typical — say, 16. Well, jo.friPizzaArea(16) = jo.pizzaArea(16) + jo.pizzaArea(12) = 200.96 + 113.04 = 314.0 (a curious ² answer, but correct.)
    Note how again, we don't have to do any squaring or re-type in ‘3.14’ ourselves, since we can use the shortcut of calling jo.pizzaArea.
2. Write the function:
  1. What is the function's signature — the first line: its return-type, name, and the type/name of the parameter?
  2. Generalize the arithmetic you did when doing test cases, into Java code.
  We'll place this code inside class PizzaServer, a few lines after (or before) pizzaArea.
3. Call the function, to test it. Remember, calling a function is different than writing a function.
Here are several solutions, compared
Write tuePizzaArea:
“20% Tuesdays”: order a pizza, and we'll bring you one with is 20% more area!

solution (The code between /* and */ are comments, and not actual code. The exact format of the comments will be discussed in Friday's lab.)
Finally, write wedPizzaArea:
Widening Wednesday — order any pizza, and they'll widen it by 10%!

solution

The key feature to these programs is when we want to compute the area of (say) a 12″ pizza, we don't need to write 3.14*(12/2)*(12/2); we can write pizzaArea(12). What are the advantages of this approach?

Robust to changes:
Suppose Krusteaze decides, as a gimmick, to change their pizzas from round to square, or even triangular. Certainly this would entail changing pizzaArea. But we wouldn't need to change friPizzaArea (nor tuePizzaArea, wedPizzaArea below).

This is a huge win! In practice, programs aren't written once; they are written, and then modified, tinkered with, and overhauled for years. Writing them so that they are easy to re-write is critical.
More understandable: When other programmers look at the code and see “3.14*(12/2)*(12/2)”, they need to figure out what the heck that code is intended to do. But if they see “pizzaArea(12)”, it's clear right away, what the purpose of that statement is.

This brings us to

The First Law of Programming: Do not repeat code, when you can re-use it.

¹ For any numeric function, 0 and 1 are obvious inputs. consider:

There once was a girl from Purdue Whose limerick stopped at line two.

Or the more interesting test,

There once was a boy from Verdun

All good computer scientists also know there is a similar limerick about Nero, it goes without saying. ↩

²Can you spot the Pythagorean theorem at work, in jo.pizzaArea(12) + jo.pizzaArea(16) = jo.pizzaArea(20)? ↩

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lect01ca first program

lect01c
a first program