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Algorithms and Strings

String bestCourse(String[] courses) {
for (String course : courses) {
if (course.contains("124")) {
return course;
}
}
return "";
}
System.out.println(bestCourse(new String[] {"CS 124", "ECE 110"}));
System.out.println(bestCourse(new String[] {"STAT 107", "CS 105"}));

This lesson is a lot of fun. We’ll integrate what we’ve learned recently about Strings, algortihms, and then functions. And then we’ll get some practice approaching a few new problems using Strings.

Odds and Ends
Odds and Ends

But 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:

int i = 10.8; // Can't assign a double literal to an int

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

double i = 10; // Can assign a int literal to a double

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!";

Practice: String Reverse

Created By: learncs.online Staff
/ 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!

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:

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

(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: String Flip Halves

Created By: learncs.online Staff
/ 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.

More Practice

Need more practice? Head over to the practice page.