# Lab 3: Recursion

Recursion is a powerful design technique. Because it can be a difficult concept to master, we concentrate on employing it in small programs before using it in larger programs. Therefore, this week’s lab is structured as a set of small problems that can be solved separately from one another.

The goals of this lab are:

• to practice writing recursive programs;
• to solve a variety of interesting algorithmic problems; and
• to train your brain to begin to think recursively.

Recursive solutions can often be formulated in just a few concise, elegant lines. A small error in a recursive method gets magnified by the many recursive calls, and so an incorrect method can have somewhat surprising behavior. Rest assured, all of the problems in this lab have short solutions. Nevertheless, finding that solution may take you some time–don’t rush!

## Partner Lab

This is a partner lab. You may work with one other person of your choosing, or if you are looking for an extra challenge you may work entirely by yourself. Although you are permitted to jointly develop code with your partner, each of you must independently submit your code. No copying of code is permitted. You must independently retype any code you develop with your partner.

Indicate your partnering arrangement (including those flying solo) by filling out the following form.

You would help finding a partner, please indicate so on the form and we will do our best to find you one.

## The Tao of Recursion

Recursive solutions to computational problems always consist of two parts:

1. a “base” case that determines how the program terminates, and
2. a “recursive” case that (a) does some work, and (b) reduces the problem toward the base case.

Once you learn to think recursively, recursive solutions to problems seem very intuitive. Spend some time on these problems and you’ll be much better prepared when you are faced with more sophisticated recursive problems. We will tackle some of these more sophisticated problems later in the semester.

Note that all of your solutions in this lab must be recursive. Notably, that means that you must not use any kind of looping construct, like for or while loops. Additionally, you should not need to create any new Java classes to solve these problems, although you may do so if you find them useful. You must also use the methods with the given parameters and return types: if you want to use additional parameters, or a different return type, you must define a helper method.

Finally, your code must use the Assert methods from structure5 that we learned about in class. Be sure to include at least one pre-condition assertion and at least one post-condition assertion in each method definition. Pre/post-condition assertions in helper methods are recommended but not required. Be sure to modify the method’s comments to include the pre/post-conditions in plain English.

## PRE-LAB Step 0: Warm-up Problems

Given the structure of this lab, a full design document is not required this week. However, as always, you should read through the lab carefully and think about how you will structure your solutions. If possible, sketch a written design for the warm-up problems described below, and bring it to lab on Wednesday.

Brainstorming can be very useful when learning to think recursively. You can work on the pre-lab warm-up problems before lab with a larger group, even if you do not work with that group during the rest of the lab.

## PRE-LAB Warm-up Problem 0.1: Digit Sum

Write a recursive method digitSum that takes a non-negative integer and returns the sum of its digits. For example, digitSum(1234) returns 1 + 2 + 3 + 4 = 10. Your method should take advantage of the fact that it is easy to break a number into two smaller pieces by dividing by 10 (i.e., 1234/10 = 123 and 1234%10 = 4). For these methods, we do not need to define any classes. Therefore, you can declare them to be static methods and call them directly from your main method:

public static int digitSum(int n) { ... }


## PRE-LAB Warm-up Problem 0.2: Subset Sum

Subset sum is an important and classic problem in computer theory. Given a set of integers and a target number, your goal is to find a subset of those numbers that sum to the target number. For example, given the set {3, 7, 1, 8, -3} and the target sum 4, the subset {3, 1} sums to 4 and therefore the result would be true. On the other hand, if the target sum were 2, the result would be false since there is no subset that sums to 2. The prototype for this method is:

public static boolean canMakeSum(int[] setOfNums, int targetSum)


Assume that the array contains setOfNums.length numbers (i.e., it is completely full). Note that you are not asked to print the subset members, just return true or false. You will likely need a helper method to pass additional information through the recursive calls. What additional information would be useful to track?

## Lab Programs

For each problem below, you must thoroughly test your code to verify it correctly handles all reasonable cases. For example, for the “Digit Sum” warm-up, you could write test code to call your method in a loop to allow the user to repeatedly enter numbers that are fed to your method until you are satisfied. Testing is necessary to be sure you have handled all the different cases. You can leave your testing code in the file you submit—there is no need to remove it.

#### Problem 1: Counting Cannonballs

Spherical objects, such as cannonballs, can be stacked to form a pyramid with one cannonball at the top, sitting on top of a square composed of four cannonballs, sitting on top of a square composed of nine cannonballs, and so forth. Write a recursive method countCannonballs that takes as its argument the height of a pyramid of cannonballs and returns the number of cannonballs it contains. The prototype for the method should be as follows:

public static int countCannonballs(int height)


#### Problem 2: Palindromes

Write a recursive method isPalindrome that takes a String and returns true if it is the same when read forwards or backwards. For example,

isPalindrome("mom")   → true
isPalindrome("cat")   → false
isPalindrome("level") → true


The prototype for the method should be as follows:

public static boolean isPalindrome(String str)


You may assume the input string contains no spaces.

Special case: Is the empty string a palindrome?

#### Problem 3: Balancing Parentheses

In the syntax of most programming languages, there are characters that occur only in nested pairs, called bracketing operators. Java, for example, has these bracketing operators:

( . . . )
[ . . . ]
{ . . . }


In a properly formed program, these characters will be properly nested and matched. To determine whether this condition holds for a particular program, you can ignore all the other characters and look simply at the pattern formed by the parentheses, brackets, and braces. In a legal configuration, all the operators match up correctly, as shown in the following example:

{ ( [ ] ) ( [ ( ) ] ) }


The following configurations, however, are illegal for the reasons stated:

( ( [ ] )      → The line is missing a close parenthesis.
) (            → The close parenthesis comes before the open parenthesis.
{ ( } )        → The parentheses and braces are improperly nested.


Write a recursive method

public static boolean isBalanced(String str)


that takes a String str from which all characters except the bracketing operators have been removed. The method should return true if the bracketing operators in str are balanced, which means that they are correctly nested and aligned. If the string is not balanced, the method returns false.

Although there are many other ways to implement this operation, you should code your solution so that it embodies the recursive insight that a string consisting only of bracketing characters is balanced if and only if one of the following conditions holds:

The string is empty.
The string contains "()", "[]", or "{}" as a substring and is still balanced if you remove that substring.


For example, the string "[(){}]" is shown to be balanced by the following chain of calls:

isBalanced("[(){}]")   →
isBalanced("[{}]")   →
isBalanced("[]")   →
isBalanced("")   → true


(Hint: Using the above example, can you reason backwards about how the code might be structured?)

#### Problem 4: Subsequences

Write a method:

public static void subsequences(String str)


that prints all subsets of the letters in str. The printed string should be a comma-separated list of subsequences. For example, the string "ABC" yields the possible subsequences, "", "A", "B", "C", "AB", "AC", "BC", "ABC". Therefore, the subsequences method should print

,A,B,C,AB,AC,BC,ABC


The order of the subsequences does not matter. You may find it useful to write a helper method

protected static void subsequenceHelper(String str, String soFar)


that is initially called with subsequenceHelper(str, ""). The variable soFar keeps track of the characters currently in the subsequence you are building. To process str you must:

• build all subsequences containing the first character (which you do by including that character in soFar), and
• build all subsequences not including the first character.

Continue until str has no more characters in it.

#### Problem 5: Print In Binary

Computers represent integers as sequences of bits. A bit is a single digit in the binary number system and can therefore have only the value 0 or 1. The table below shows the first few integers represented in binary:

binary decimal
0 0
1 1
10 2
11 3
100 4
101 5
110 6

Each entry in the left side of the table is written in its standard binary representation, in which each bit position counts for twice as much as the position to its right. For instance, you can demonstrate that the binary value 110 represents the decimal number 6 by following this logic:

place value   → 4   2   1
×   ×   ×
binary digits → 1   1   0
↓   ↓   ↓
4 + 2 + 0 = 6


Binary is a base-2 number system instead of the decimal (base-10) system we are familiar with. Write a recursive method

public static String printInBinary(int number)


that returns the binary representation for a given integer as a String. For example, calling printInBinary(3) would return "11". Your method may assume the integer parameter is always non-negative.

(Hint: You can identify the least significant binary digit by using the modulus operator with value 2 (ie., number % 2). For example, given the integer 35, the value 35 % 2 = 1 tells you that the last binary digit must be 1 (ie., the number is odd), and division by 2 gives you the remaining portion of the integer (17). Remember, to “build up” a string, you can start with the empty string, "", and append new strings to it using the += operator.)

#### Problem 6: Extending Subset Sum

Write two modified versions of the canMakeSum method:

1. Change the method to print the members in a successful subset if one is found. Do this without adding any new data structures (i.e. don’t build a second array to hold the subset). Just use the unwind of the recursive calls.

 public static boolean printSubsetSum(int[] nums, int targetSum)

2. Change the method to report not just whether any such subset exists, but the count of all such possible subsets. For example, in the set shown earlier, the subset {7, -3} also sums to 4, so there are two possible subsets for target 4. You do not need to print all of the subsets.

 public static int countSubsetSumSolutions(int[] nums, int targetSum)


## Lab Deliverables

Your repository should contain the following files:

lab03_recursion-{USERNAMES}/
Recursion.java


The Recursion.java file contains starter code, and you should write all of your functions inside that file. We do not need your .class files, as we will compile your code ourselves. Be sure that your code compiles before you submit it!

Do not forget to include pre and post assertions to check pre- and post-conditions.

• Verify your changes on GitLab. Navigate in your web browser to your private repository on GitLab. It should be available at https://evolene.cs.williams.edu/cs136-labs/<YOUR-USERNAME-HERE>/lab03-recursion.git