©2007 Felleisen, Proulx, et. al.

5  Structural Recursion vs. Accumulators; Abstractions; Libraries

Portfolio Problems

Problem 15.10 on page 175 in the text dealt with shopping lists.

1. Design the method that produces a shopping list sorted by the brand names. The class String defines the following method:

   // this String is lexicographically before that String: positive result
   // this String is same as that String: result is 0
   // this String is lexicographically after that String: negative result
   int compareTo(String that);
   

2. Rewrite the methods howMany, brandList, and highestPrice using accumulator style.

5.1  Problem

Problem 4.5 asked you to design data definitions for files of images, text, and sounds. Problem 14.6 asked you to add methods timeToDownload, smallerThan, and sameName.

Design the appropriate abstract class that eliminates as much repetition in the code as possible. Solution

5.2  Problem

Problem 18.3. Solution

5.3  Problem

In problem 14.7 you designed methods unitPrice, lowerUnitPrice, and cheaperThan for the classes that represent the inventory in a grocery store.

Design the appropriate abstract class that eliminates as much repetition in the code as possible. Solution

5.4  Problem

Problem 19.1. Solution

Note:

If you feel that you really understand the concepts covered in these etudes, you may do only two out of four. However, make sure you know how to solve all of them.

Main Assignment — Part 1

5.1  Problem

One bad apple can spoil a bunch. In this problem we practice designing methods for lists of data. The classes LoA, MtLoA, and ConsLoA represent a list of apples, for each apple we record its weight, and whether it is good or spoiled. Design these classes.

Now design the following methods that deal with lists of apples:

1. Method countGood that counts all the good apples in the list of apples.

2. Method onlyGood that produces a list of all good apples in the list of apples.

3. Method biggest that finds the weight of the heaviest good apple in the list of apples.

4. Method sort that produces a list of apples sorted by their weight.
   

   Now rewrite the first three methods by using the accumulator style:

5. Design the method countGood2 that invokes countGoodAcc where
   countGoodAcc uses an accumulator to remember known values.

6. Design the method onlyGood2 that invokes onlyGoodAcc where
   onlyGoodAcc uses an accumulator to remember known values.

7. Design the method biggest2 that invokes biggestAcc where
   biggestAcc uses an accumulator to remember known values.

 Solution

Main Assignment — Part 2: Rat Race

The goal of this exercise is to design an interactive game. The rat lives in a cage and rambles around looking for something to eat. There are globs of stuff around - some are food and some are poison. If the rat does not eat for some time, he starves to death. If the rat finds a food glob, his life expectancy increases as he eats the food. If he finds poison, he dies instantly.

Rat’s life is simulated on a canvas. The rat moves in response to the key events - as the user hits the arrow keys, the rat moves in the corresponding direction. The world starts with a collection of globs of food and poison. As the timer ticks away, the rat gets hungrier and closer to death, unless he finds some food to eat.

5.1  Problem

Design the class(es) that represent the rat, and design the methods that simulate the rat’s behavior:

1. Analyze the information needed to represent the rat. Make examples (in English) of several instances of a rat. Then, design the class(es) that represent the rat in this game. And we do not have to remind you that ...

2. Design the method that consumes a String that represents a key event and produces a rat that has moved in the direction specified by the key event. Of course, that the rat only moves if one of the appropriate keys was hit.

3. Design the method canEat that determines whether the rat is close enough to the given location (where, presumably, some food or poison lies).

4. Design the method eatFood that produces a rat that consumed the given amount of food.

5. Design the method poison that produces a dead rat.

6. Design the method starve that produces a new rat, one hour hungrier that this rat, possibly a dead rat.

7. Design the method draw that displays the rat on the given Canvas.

8. Design the class RatWorld that contains one rat. Design the methods draw, erase, onKeyEvent that allow you to show the rat and control the moves using the arrow keys. You will need to implement the skeleton of the method onTick as well — you learned how to do it in the lab.

9. Now design the onTick method that just invokes the starve method in the class(es) that represent a rat. Additionally, if the rat is dead, the game ends by returning the result of the endOfWorld("The rat died.") method invocation.

 Solution

5.2  Problem

Design the classes that represents the globs of food or poison in the rat cage. Then add methods that simulate the rat’s interaction with the globs (finding a glob, eating it, etc.). Note, that the rat’s life expectancy increases proportionately to the amount of food in each food glob. However, any amount of poison is fatal to the rat.

1. Analyze the information needed to represent the globs. Make examples (in English) of several instances of a both kinds of globs. Then, design the classes that represent the globs in this game.

2. Design the method(s) foundGlob that determine whether a given rat has found this glob. (Remember the method canEat in the class(es) for rats?)

3. Design the method feed that produces a new instance of the given rat after he ate this glob. Remember the eatFood method to the class Rat?

4. Design the method newGlob that produces a new glob at random - either poisonous of a food glob of a random weight (food amount), at a random location within the given bounds.

5. Design the method draw that will display a glob in the given Canvas. Make sure that the player can distinguish visually between the food globs and poisonous globs. It would also be helpful, if the display indicated the relative sizes of the globs.

6. Add two globs to the class RatWorld that contains one rat. Modify the methods draw, erase to show the glob as well.

7. Now modify the method onTick so that if the rat is close to the glob, he will eat it.

8. Complete the game design, so that the rat moves in response to the key events (arrow keys), eats the food when close enough ot it, the eaten food is replaced, and the game ends if the rat ate poison or starved to death.

 Solution

5.3  Problem: Optional - Extra Credit

Design the class(es) that represent a list of globs and complete the game design.

1. Design the classes that represent a list of globs in this game.

2. Design the method draw that displays this list of globs in the given Canvas

3. Design the method feedRat that produces a new rat that consumed the first glob in this list that was close enough to the rat.

4. Design the method removeEaten that replaces the first glob in this list that was close enough to the rat, (so that the rat ate it), with a new randomly chosen glob randomly placed it in the field.

5. Modify the RatWorld so that instead of one glob, it contains a list of them. Modify the onTick method so that the rat now has a choice of globs among all globs in a list of globs.

 Solution