Algorithms and Strings

Debugging Challenge

Debugging Challenge

But let’s start with some debugging practice!

Odds and Ends

Odds and Ends

First, we have a few new things to cover.

Casting

Casting

When we started working with variables and Java’s eight primitive types, we observed that there were certain types of assignments that would fail. For example:

But, there are also other kinds of assignments from one type to another that will succeed:

Let’s discuss why that is, and how to can force Java to perform certain type changes when necessary.

int i = 10.8;

double i = 10;

Strings Are Immutable

Strings Are Immutable

One of the important things to understand about Java’s Strings is that none of the methods that we can call on them change that String. Instead, they always return a new String that may be modified in some way. Let’s see that in action:

String first = "I'm a string!";

Homework Restricted to Current CS 124 Students

A publicly-accessible version of this content is available at learncs.online.

Practice: String Reverse

Created By: Geoffrey Challen

/ Version: 2020.9.0

Write a function called reverse. It should accept a single String argument and return that String, reversed! There are several ways to approach this problem—so have fun!

For this problem we're expecting only a method.

Don't define a class, include modifiers like static or public, add import statements, or add code outside the method declaration.

WorkingPrevious

String Algorithms

String Algorithms

Now let’s have some fun and write a few new algorithms that work on Strings!

String Character Search

String Character Search

First, let’s try and write a function that determines if a String contains a particular character. We’ll sketch our our algorithm first, explore some potentially useful String methods, and construct and test a solution.

// Write a function that searches a `String` for a character

Structured String Parsing

Structured String Parsing

Frequently when computers work with text data, we are processing data that was itself created by a machine. Or set up for machines to easily process.

One example is data stored in comma-separated-value format. Let’s say we wanted to track how many people were tested for some random respiratory illness each day. We might save that data in a String that looked like this:

(Note that this is not real data!)

Let’s experiment with how we might work with this kind of data. We’ll use some existing String methods and a few new ones. We’ll also use another unfamiliar method: Integer.parseInt, which allows us to convert a String to an int.

// This is a new way of enclosing `Strings` that was just added to Java!

// The walkthrough talks about it

String data = """

2020-09-02,5402

2020-09-03,12042

2020-09-04,4637

2020-09-05,89054

2020-09-06,2033

2020-09-07,10238

2020-09-08,76452

2020-09-09,8902""";

System.out.println(data);

Homework Restricted to Current CS 124 Students

A publicly-accessible version of this content is available at learncs.online.

Practice: String Flip Halves

Created By: Geoffrey Challen

/ Version: 2021.8.0

Write a method name flipHalves that returns a passed String rearranged as follows. You should move the second half of the string to the front, and the first half to the back. So given the String "CS" you would return "SC", and given the String "testme" you would return "tmetes". However, if the length of the String is odd the middle character should remain in place. So given the String "CS124" you would return "241CS". If the passed String is empty, just return the empty String.

You will definitely want to review the substring String method, and also spend some time experimenting with it before you begin crafting your solution. You also may want to consider odd and even length Strings separately.

For this problem we're expecting only a method.

Don't define a class, include modifiers like static or public, add import statements, or add code outside the method declaration.

WorkingPrevious

Homework Restricted to Current CS 124 Students

A publicly-accessible version of this content is available at learncs.online.

Homework: Hawaiian Ws

Created By: Geoffrey Challen

/ Version: 2022.7.0

Words from languages that we are unfamiliar with can be difficult to pronounce correctly. Phonetic pronunciation guides can help make them more accessible to us. In this problem we'll complete one part of a pronunciation guide for words in the Hawaiian language.

Consonants in the Hawaiian language are pronounced similarly to the English versions. The only exception is 'w', which is pronounced as a "v" if it follows an 'i' or 'e' and pronounced as "w" otherwise.

Complete a method named rewriteW which, given a non-null String, rewrites all the "w"s according to the rule above. For example, given the String "wahine" you would return "wahine", since the "w" is not preceded by an 'i' or 'e'. Given the String "iwa" you would return "iva", since the "w" is preceded by an 'i'. You should treat upper and lowercase characters equivalently, and your return String should be all lowercase.

For this problem we're expecting only a method.

Don't define a class, include modifiers like static or public, add import statements, or add code outside the method declaration.

WorkingPrevious

CS People: Joy Buolamwini

CS People: Joy Buolamwini

Joy Buolamwini is a self-described poet of code who, through her art, voice, and research, is drawing attention to important social implications of the widespread use of artificial intelligence. Watch her speak for herself, in words both beautiful and powerful:

If you have a few more minutes, watch her TED Talk on algorithmic bias—what she calls “the coded gaze”.

More Practice

Need more practice? Head over to the practice page.