Data structures

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Lijucay • 4/2/2024

In our last adventure, we explored the world of algorithms – the recipes that tell our computers how to solve problems.
Today, we’re diving into the fundamental building blocks of programming: data structures.

Data structures are like containers that hold and organize our information. They determine how efficiently we can access, manipulate, and store data. Just like a toolbox has different compartments for screwdrivers, hammers, and nails, programmers use a variety of data structures depending on the task at hand.

We will mainly focus on static arrays, dynamic arrays, stacks and queues here but remember, that there is more.

Static arrays

An array is a collection of variables of the same type. It can be addressed by its index.

An index starts at 0. That means, we start counting at 0, which would be the first item of the array.

Static arrays have a fixed size, which means that they cannot grow or shrink later.

Declaration without value assignments

When you declare an array without assigning values, you specify the type of data the array will hold and its size. Here’s the breakdown for Java:

// Declaration of a String array with a size of 10
String[] names = new String[10];

In this example:

  • String[]: Indicates that the variable names will hold an array of Strings.

  • new String[10]: Indicates that the array will have a size of 10

If you declare an array without assigning values, each entry will be filled with the default value of the data type (null for objects, 0 for integers, false for booleans, etc.).

Declaration with value assignments

When you declare an array and assign values directly, you list the values within curly braces ({}) after the variable’s name. Here is how it looks:

String[] names = {"Ken", "Nila", "Mika", "Elisa"}; // Declare an array with assigning values

In this example:

{“Ken”, “Nila”, “Mika”, “Elisa”}: Assigns these strings to the names array.

Get a value on a specific index

To retrieve a value from a specific index in the array, you use square brackets ([]) with the index inside. For example:

int[] numbers = {1, 2, 3, 4, 5, 6};
numbers[0]; // Retrieves the value on index 0 (which is 1)

Assigning values

To change the value at a specific index in the array, you again use square brackets ([]) with the index inside, and then assign the new value. Here is an example:

int[] numbers = {1, 3, 3, 4, 5, 6};
numbers[1] = 2; // Changes the value at index 1 from 3 to 2

Get the length of an array

In Java, you can get the length of an array by using the length property. Here is how you do it:

int[] numbers = {1, 2, 3, 4, 5, 6};
int size = numbers.length; // Returns 6, the length of the array

Dynamic arrays

In Java, ArrayList is a dynamic array implementation provided by the standard library.
Unlike static arrays, ArrayList can grow and shrink as needed.

Here is how you create one:

ArrayList<String> strings = new ArrayList<>();

In this example:

  • ArrayList: Indicates that strings will hold elements of type String.

  • new ArrayList<>(): Creates a new ArrayList instance.

Adding elements

You can add elements to the end of the ArrayList or at a specific index using the add method:

strings.add("world"); // Adds "world" to the end of the list
strings.add(0, "Hello"); // Adds "Hello" at index 0, pushing other elements forward
strings.add("!");

Removing elements

You can remove elements by their index or by their value:

strings.remove(1); // Removes the element at index 1
strings.remove("Hello"); // Removes the element with the value "Hello"

Accessing elements

You can access elements by their index using the get method:

String firstElement = strings.get(0); // Retrives the element at index 0

2-dimensional arrays

A two-dimensional array in Java is like a table with rows and columns. You specify the dimensions when you create it. Here’s how you create a 3x3 2D array of type integer

int[][] twoDArray = new int[3][3]

In this example:

  • int[][]: Indicates that twoDArray is a two-dimensional array of integers

  • new int[3][3]: Create a new 2D array with 3 rows and 3 columns

Initializing values

You can initialize values when creating the array…

int[][] twoDArray = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

… or assign them later

twoDArray[0] = {1, 3, 3};
twoDArray[0][1] = 2;

Accessing values

To access a value in the 2D array, you use two sets of square brackets ([rowIndex][columnIndex]):

int secondRowFirstVal = twoDArray[1][0] // Retrieves the value at row 1, column 0 (which is 4)

Accessing a row

You can also access an entire row as a 1D array:

int[] secondRow = twoDArray[1]; // Retrieves the entire second row (4, 5, 6)

Queue

A queue is a fundamental data structure, that follows the First-In-First-Out (FIFO) principle. It means that the element added first will be removed first. In Java, you can implement a queue using the Queue interface. In this example, we will use the LinkedList as an underlying implementation, but note that there are other implementations.

Here is how you create a queue of integers:

Queue<Integer> queue = new LinkedList<>();

In this example:

  • Queue: Indicates that queue is a queue that stores integers.

  • new LinkedList<>(): Creates a new linked list-based queue instance.

Adding elements

To add elements to the end of the queue, you use the add method:

queue.add(1);
queue.add(2);

In this example, 1 will be added first, followed by 2.

Removing elements (polling)

To remove and retrieve the first element from the queue, you use the poll method:

int firstElement = queue.poll(); // Retrieves and removes the first element from the queue

Subsequent calls to poll will remove and return the next element in the queue.

int secondElement = queue.poll(); // Retrieves and removes the second element from the queue

Stack

A stack is a fundamental data structure that follows the Last-In-First-Out (LIFO) principle. It means that the last element added to the stack will be the first one removed. In Java, you can implement a stack using the Stack class. Here is how you create a stack of integers:

Stack<Integer> stack = new Stack<>();

In this example:

  • Stack: Indicates that stack is a stack that stores integers.

  • new Stack<>(): Creates a new stack instance.

Adding elements

To add elements to the top of the stack, you use the push method

stack.push(1);
stack.push(2);

In this example, 1 will be added first, followed by 2.

Removing elements (popping)

To remove and retrieve the top element from the stack, you use the pop method:

int topElement = stack.pop() // Removes and returns the top element from the stack

Subsequent calls to pop will remove and return the next element from the top of the stack.

int nextElement = stack.pop(); // Removes and returns the next element from the stack

Now that’s it so far. You should now have a basic understanding of data structures, including arrays and stacks. Our next topic will be binary trees, so stay tuned!