Algorithms and Strings

Created By: Geoffrey Challen
/ Updated: 2021-09-09

Today's 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

But first, we have a few new things to cover.

Type Conversions

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

Unlike some other languages, Kotlin will not automatically convert a value even if you might think it could be done safely:

So let's look at how we can force Kotlin to perform certain type changes when necessary.

Discuss type casting using toInt or toDouble.

Strings Are Immutable

One of the important things to understand about Kotlin'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:

Demonstrate how String operations don't change their contents.

Array<> and arrayOf

We'll be working with arrays of Strings in this lesson. Given that previously, when we used an array of Ints we declared it as IntArray and initialized it using intArrayOf, and when we used an array of Doubles we declared it as DoubleArray and initialized it using doubleArrayOf, you might assume that an array of Strings would be a StringArray and intialized using stringArrayOf:

But not quite! Instead, the actual syntax is a bit different:

On the left we see the type Array<String>, which indicates that this is an array that contains Strings. The type of the contents of the array is inside the angle brackets: <String>. On the right we see the generic array initializer for Kotlin, which you can use to create arrays of any type.

As usual, we don't need to specify the type explicitly on the left:

How does Kotlin know what type of array it is? Based on the arguments to arrayOf! Since they are all Strings, the type is inferred as Array<String>.

For now we'll work with these arrays without worrying too much about what's going on under the hood. Just be careful with arrayOf, since if you mix the types of the arguments things won't work out as expected:

String Algorithms

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

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.

Show how to write a function that searches a String for a character. Sketch in comments first and then replace with code.

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: String.toInt, which allows us to convert a String to an Int.

Walk through some examples using the CSV data. Use the same approach as previously: sketch in comments, then replace with code. First, parse it using String.lines() and String.split. Discuss toInt. Then do something fun with data: total number of tests, or day with the most tests, or whatever.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough