ITEC 120 2007fall ibarland, jdymacek

home—info—exams—lectures—labs—hws
Recipe—Laws—lies—syntax—java.lang docs—java.util docs

lect02a
functions calling functions; `this`
no repeated code!

Last time, we left off last time having talked about several approaches to writing fridayPizzaArea, where (on Friday's) every order is delivered with a bonus free-foot-Friday 12″ pizza.

comparing solutions

We consider several different possible solutions to writing friPizzaArea. They all give the same answers, but some are better than others.

We see from our test cases, that the answer we want is the same as pizzaArea, with an extra 113 square inches. That naturally suggests:

double friPizzaArea_v1( double diam ) {
  return 3.14*(diam/2)*(diam/2)+113.04;
  }

Here is a barely-different version -- only some added spaces and parentheses -- yet it is decidably a bit better:

double friPizzaArea_v2( double diam ) {
  return (3.14*(diam/2)*(diam/2)) + 113.04;
  }

A related way of writing this would be:

double friPizzaArea_v3( double diam ) {
  return (3.14*(diam/2)*(diam/2))
       + (3.14*(12.0/2)*(12.0/2));
  }

• Version 2 is shorter.
• Version 3 makes it clear why the number 113.04 is used. Imagine if the offer were changed to only give away a 10″ pizza. The second would be easy to update; the first would take more effort.
• If there is an error/typo, it would be easier to spot in version 3.
• If somebody else were looking at the code (to modify it, or searching for the cause of a bug, or whatever), version 3 is a little clearer about what the code means to be computing.

Here is yet another variant:

double friPizzaArea_v4( double diam ) {
  return 0.785*(diam*diam + 144);
  }

This last version is the most different:

double friPizzaArea_v5( double diam ) {
  return pizzaArea(diam) + pizzaArea(12);
  }

• Version 5 makes it precisely clear what the code is computing.
• Version 5 is also clearly exactly correct (given that we've already tested pizzaArea).
• Suppose that Krusteaz Pizza Korporation, in its never-ending quest to be different from the competition, decides to start making all their pizzas triangular. How would our code have to change?
  Clearly, we'd have to re-write pizzaArea to have the area of a triangle, rather than a circle. If we'd used any of Versions 1-4, we'd also have to remember that those versions also depend on the exact shape of a pizza. But, Version 5 would not need any further update -- it is “robust”.

Introducing this

Alas, there is one problem Version 5. Do you remember, how we call a function, in Java? Our mantra:

To call a function: object   dot   functionName   openParen   argument

double friPizzaArea_v6( double diam ) {
  return this.pizzaArea(diam) + this.pizzaArea(12);
  }

Why re-use code?

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.

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

Exercise

Krusteaze Ink.'s diskriminating kustomers sometimes throw keggers cocktail parties, and want to inkuire how much it will kost to buy (say) five large pizzas. Or, two large pizzas. Or, seventeen larges, or …. PizzaServers know that the price of a pizza is (fairly enough) proportional to its size: three cents per square inch (a bargain already; no volume discounts²). Fill in the blanks to complete the program below:

class PizzaServer {

   // ...other functions omitted...

  /**
   * @param n the number of large pizzas being ordered.
   * @return The price of n large (16") pizzas, in dollars.
   *    costOfLarges( 0) =    0
   *    costOfLarges( 1) =~   6.029
   *    costOfLarges( 2) =~  12.06
   *    costOfLarges(17) =~ 102.57
   */
  ______ costOfLarges( ______ n ) {
    _____  _______________;
    }

  }

home—info—exams—lectures—labs—hws
Recipe—Laws—lies—syntax—java.lang docs—java.util docs

©2007, Ian Barland, Radford University Last modified 2007.Aug.27 (Mon)

Please mail any suggestions (incl. typos, broken links) to iba�rlandrad�ford.edu