Arrays

Introduction

Arrays are data structures that contain a list of sequenced data. They are often organized close together in computer’s memory or storage to optimize for iteration. Arrays are one of the most fundamental data structures in programming. They serve as a building block for many other advanced data structures and algorithms. By the end of this tutorial, you’ll understand what arrays are, how they work, and their various applications.

What is an array?

At its core, an array is a collection of elements identified by index or key. The elements in an array are stored consecutively, which means that they are placed next to each other in memory.

Characteristics of Arrays

1. Fixed Size: Once you declare an array’s size, it can’t be changed.
2. Element Index: Every element in an array has an index, starting from 0 (for most programming languages).
3. Homogenous: All elements in an array are of the same type (e.g., text, number, object).

Not all programming languages have the same implementation for an array, so check the documentation for the programming language you are using for more details and limitations.

Using Arrays

Why Use Arrays?

Arrays are beneficial due to their simplicity and efficient access times. If you know the index of the element of the array, you can access it very quickly. This is often performed in constant time, denoted as O(1). If you aren’t familiar with this description of runtime, see our tutorial on Big-O notation.

Declaration

In many languages, arrays can be both declared and initialized as follows:

let arr = [10, 20, 30, 40];

arr = [10, 20, 30, 40]

int[] arr = {10, 20, 30, 40};

Accessing Elements

To access an element within an array, simply use the index of the element you’re looking for.

let element = arr[2] // Returns 30

element = arr[2] # Returns 30

int element = arr[2]; // Returns 30

Common Operations on Arrays

1. Insertion: Adding an element can be costly as you might need to shift elements. For example, if you have an array with 5 elements, and you want to insert an element at index 2 (in the middle of the array), then you will have to shift all elements over by one before you insert the new element. This would be performed in linear time, denoted as O(n).
2. Deletion: Similarly, removing an element can lead to shifting of elements.
3. Search: Finding an element by value (rather than index) can take linear time, denoted as O(n). This is because you’ll need to look at potentially every value in the array before you find the value you’re seeking.
4. Update: Changing an element at a particular index is fast and can be performed in constant time, O(1).

Example

We’ll wrap up this tutorial by demonstrating how to use an array in code. For this example, we’ll demonstrate creating an array of numbers, and introduce an algorithm which finds the largest number in an array.

let arr = [5, 10, 30, 18, 2, 7, 9];
let largestNumber = arr[0]; // Note that this algorithm assumes arr has at least one value
for (int i = 0; i < arr.length; i++) {
  if (largestNumber < arr[i]) {
      largestNumber = arr[i];
  }
}
console.log(largestNumber); // Prints 30

arr = [5, 10, 30, 18, 2, 7, 9]
largestNumber = arr[0] # Note that this algorithm assumes arr has at least one value
for num in arr:
  if largestNumber < num:
      largestNumber = num
print(largestNumber) # Prints 30

int[] arr = {5, 10, 30, 18, 2, 7, 9};
int largestNumber = arr[0]; // Note that this algorithm assumes arr has at least one value
for (int i = 1; i < arr.length; i++) {
  if (largestNumber < arr[i]) {
      largestNumber = arr[i];
  }
}
System.out.println(largestNumber); // Prints 30