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Kotlin
Java

# Algorithms and Lists

interface SimpleList {
fun get(index: Int): Any?
fun set(index: Int, value: Any?)
fun remove(index: Int): Any?
}
class SimpleArrayList(private var values: Array<Any?>) : SimpleList {
override fun get(index: Int): Any? {
require(index in values.indices)
return values[index]
}
override fun set(index: Int, value: Any?) {
require(index in values.indices)
values[index] = value
}
override fun remove(index: Int): Any? {
TODO("Not implemented!")
}
override fun add(index: Int, value: Any?) {
TODO("Not implemented!")
}
override fun toString() = values.contentToString()
}
val list = SimpleArrayList(arrayOf("you", "are", "not", "alone"))

Welcome back! The rest of the course is incredibly exciting material. We’ll begin building and analyzing new data structures and algorithms, while also introducing new bits of Java syntax along the way.

## Algorithms and Data StructuresAlgorithms and Data Structures

Algorithms and data structures comprise the core conceptual concerns of computer science. Algorithms are how we do things. Data structures are how we represent things.

The two topics are intertwined. We will implement data structures to support certain algorithms. And we will design algorithms that utilize specific data structure capabilties.

### Algorithm AnalysisAlgorithm Analysis

As we proceed, we will spend more time talking about how long certain algorithms take and why—or performing algorithm analysis. To do this we use something called Big-O notation to describe the behavior of algorithms. Let’s define those terms:

Big-O notation is a mathematical notation that describes the limiting behavior of a function when the argument tends towards a particular value or infinity.

### Complexity CategoriesComplexity Categories

We’ll take a very high-level view of Big-O as we get started with algorithm analysis. Let’s provide an overview of the different complexity categories that we’ll learn to identify, and some of the code features that are associated with them.

## Array ListsArray Lists

To get some practice with algorithm analysis, over the next few lessons we’ll be implementing a data structure known as a list. You’ve already been working with Kotlin’s built-in `List`s, so this will give you a peek at how they are actually implemented.

Lists store a sequence of elements. We already know how to do that using arrays, and we can build an implementation of lists on top of an array. Let’s see how!

// Initial SimpleArrayList

### RemoveRemove

OK, this is good start. But so far all we have is a wrapper around an array! That’s not particularly interesting.

Indeed, the key difference between an array and list is that the size of the list can change. But doing this using a list that maintains are array internally requires more work. Let’s see how, starting with the `remove` operation. (You get to implement `add` as this lesson’s homework.)

// SimpleArrayList remove

## Solve: SimpleArrayList get and set (Practice)

Created By: Geoffrey Challen
/ Version: 2020.10.0

Let's begin building a simple list implementation that uses arrays to store the values. Create a class `SimpleArrayList` with a public constructor that initializes the list using a passed non-`null`able array of `Any?` references. Your array should be private.

Next, implement:

1. `fun get(Int): Any`, which takes an `Int` index and returns the `Any` at that index
2. `fun set(Int, Any?)`, which takes an `Int` index and an `Any` reference and sets that value at the index to the passed reference.

Both your `get` and `set` method should require that the index passed is valid for that SimpleArrayList. Here's an example of how your `SimpleArrayList` should work:

Don't overthink this! Both `get` and `set` should be two lines of code (including one for the `require`).

## List Method Algorithm AnalysisList Method Algorithm Analysis

Next, let’s take a look at our core list functions and see how they perform. We’re going to use our new big-O vocabulary and try to understand the performance of `get`, `set`, and `remove`.

// SimpleArrayList performance

Created By: Geoffrey Challen
/ Version: 2020.10.0

Let's write the `add` method for our `SimpleArrayList`. First, create a `SimpleArrayList` class with a single public constructor that initializes the list with a passed non-null array of `Any?` references. Call the array property `values`, and it should be publicly readable but not publicly writable. Also provide a method `size()` with that returns the current size of the list.

Now write the `add` method, which takes the position to add at as an `Int` as its first parameter and the `Any?` reference to add as its second. `add` should add the element to the list, increasing the size by one and shifting elements after the add position backward. You should assert that the passed position is valid for this list. But note that you should allow adding a new item to the end of the existing list.

When you are done, here is how your `SimpleArrayList` class should work:

## CS People: Mark DeanCS People: Mark Dean

Mark Dean was a pioneering Black American computer scientist, engineer, and inventor, who made important contributions to several computing technologies. He developed the ISA bus, an early computer standard allowing interconnection of hardware components. He also worked on computer graphics and the first chip to achieve a 1 GHz clock rate(1).

In recognition of his many accomplishments, Mark Dean was the first African-American to be named an IBM Fellow. Watch the following short video to learn more about Mark Dean:

## More Practice

Need more practice? Head over to the practice page.