Arrays

What is an Array?

An array is a collection of values.

The image below shows how we can think of an array named myFruits, with the values 'banana', 'apple', and 'orange' stored inside it.

Each value in an array has a position, called index, which starts at 0.

Below is how the myFruits array is created, using Python code:

The first value 'banana' is positioned at index 0 in the array.

What Can I Do With an Array?

Arrays make it easier to work with groups of values compared to using a separate variable for each value.

So instead of creating 3 variables:

fruit1 = 'banana'
fruit2 = 'apple'
fruit3 = 'orange'

We can just create an array:

myFruits = ['banana','apple','orange']

With an array, you can:

• Store a collection of numbers, words, or objects.
• Access any value using its index (position).
• Read, update, insert, or remove any of the array values.

See how an array can be created and used in the sections below.

Creating an Array

When creating an array we must specify the name of the array and the values inside it.

Here is how the myFruits array can be created using different programming languages:

myFruits = ['banana','apple','orange']

const myFruits = ['banana','apple','orange'];

String[] myFruits = {"banana","apple","orange"};

string myFruits[] = {"banana","apple","orange"};

In the Python code above:

• myFruits is the name of the array.
• The equal sign = stores the values on the right side into the array.
• The square brackets [ ] mean we are creating an array.
• 'banana','apple','orange' are the values inside the array, separated by comma.

Array Operations

Arrays can be read and manipulated in many different ways, here are some common things you can do with an array:

Go to the sections below to see how the code for these array operations look like in different programming languages.

Reading an Array Value

To read an array value, we use the array name with the index of the value we want to read in brackets, like this myFruits[0].

myFruits[0]

We must also use a command to write myFruits[0] to the console/terminal, so that we can actually see the result, and that is done a little different depending on the programming language.

myFruits = ['banana','apple','orange']

print(myFruits[0])

const myFruits = ['banana','apple','orange'];

console.log(myFruits[0]);

String[] myFruits = {"banana","apple","orange"};

System.out.println(myFruits[0]);

string myFruits[] = {"banana","apple","orange"};

cout << myFruits[0];

Updating an Array Value

To update a value in an array, we use the array name with the index position of the value we want to update, like this myFruits[0], and then we use the equal sign = to store a new value there.

myFruits[0] = 'kiwi'

This is how updating an array value at index 0 can be done in different programming languages:

myFruits = ['banana','apple','orange']

myFruits[0] = 'kiwi'

const myFruits = ['banana','apple','orange'];

myFruits[0] = 'kiwi';

String[] myFruits = {"banana","apple","orange"};

myFruits[0] = "kiwi";

string myFruits[] = {"banana","apple","orange"};

myFruits[0] = "kiwi";

Inserting an Array Value

To insert a value into an array, in addition to the existing values, we need:

• the array name
• a command to do the insert operation
• the value to be inserted

myFruits.insert('kiwi')

When we do insert this way, the new value is inserted at the end of the array.

The command to insert a value into an array varies a bit between the programming languages.

myFruits = ['banana','apple','orange']

myFruits.append('kiwi')

const myFruits = ['banana','apple','orange'];

myFruits.push('kiwi');

ArrayList<String> myFruits = new ArrayList<String>();
myFruits.add("banana");
myFruits.add("apple");
myFruits.add("orange");

myFruits.add("kiwi");

vector<string> myFruits = {"banana", "apple", "orange"};

myFruits.push_back("kiwi");

A value can also be added to a specific position in an array, using the index, like this:

myFruits = ['banana','apple','orange']

myFruits.insert(1,'kiwi')

const myFruits = ['banana','apple','orange'];

myFruits.splice(1,0,'kiwi');

ArrayList<String> myFruits = new ArrayList<String>();
myFruits.add("banana");
myFruits.add("apple");
myFruits.add("orange");

myFruits.add(1,"kiwi");

vector<string> myFruits = {"banana", "apple", "orange"};

myFruits.insert(myFruits.begin() + 1, "kiwi");

Removing an Array Value

An array value is removed by specifying the index where the value should be removed from.

myFruits.remove(2)

This is how an array value placed at index 1 can be removed in different programming languages:

myFruits = ['banana','apple','orange']

myFruits.pop(1)

const myFruits = ['banana','apple','orange'];

myFruits.splice(1,1);

ArrayList<String> myFruits = new ArrayList<String>();
myFruits.add("banana");
myFruits.add("apple");
myFruits.add("orange");

myFruits.remove(1);

vector<string> myFruits = {"banana", "apple", "orange"};

myFruits.erase(myFruits.begin() + 1);

A value can also be removed from the end of an array, without using the index (except for Java), like this:

myFruits = ['banana','apple','orange']

myFruits.pop()

const myFruits = ['banana','apple','orange'];

myFruits.pop();

ArrayList<String> myFruits = new ArrayList<String>();
myFruits.add("banana");
myFruits.add("apple");
myFruits.add("orange");

myFruits.remove(myFruits.size()-1);

vector<string> myFruits = {"banana", "apple", "orange"};

myFruits.pop_back();

Finding the length of an Array

You can always check the length of an array:

myFruits.length

This is how the length of an array is found in different programming languages:

myFruits = ['banana','apple','orange']

print(len(myFruits))

const myFruits = ['banana','apple','orange'];

console.log(myFruits.length);

ArrayList<String> myFruits = new ArrayList<String>();
myFruits.add("banana");
myFruits.add("apple");
myFruits.add("orange");

System.out.println(myFruits.size());

vector<string> myFruits = {"banana", "apple", "orange"};

cout << myFruits.size();

Looping Through an Array

See this page for an explanation of what a loop is.

Looping through an array means to look at every value in the array.

Here is how we can loop through the myFruits array using a for loop, printing every value:

for fruit in myFruits:
  print(fruit)

There is more than one way to loop through an array, but using a for loop is perhaps the most straight forward way that is also supported in all programming languages, like this:

myFruits = ['banana','apple','orange']

for fruit in myFruits:
  print(fruit)

const myFruits = ['banana','apple','orange'];

for (let fruit of myFruits) {
  console.log(fruit);
}

String[] myFruits = {"banana", "apple", "orange"};

for (String fruit : myFruits) {
  System.out.println(fruit);
}

string myFruits[] = {"banana", "apple", "orange"};

for (auto fruit : myFruits) {
  cout << fruit + "\n";
}

Another way to loop through an array is to use a for loop with a counting variable for the indexes, like this:

myFruits = ['banana','apple','orange']

for i in range(len(myFruits)):
  print(myFruits[i])

const myFruits = ['banana','apple','orange'];

for (let i = 0; i < myFruits.length; i++) {
  console.log(myFruits[i]);
}

String[] myFruits = {"banana", "apple", "orange"};

for (int i = 0; i < myFruits.length; i++) {
  System.out.println(myFruits[i]);
}

string myFruits[] = {"banana", "apple", "orange"};
int size = sizeof(myFruits) / sizeof(myFruits[0]);

for (int i = 0; i < size; i++) {
  cout << myFruits[i] + "\n";
}

Other things we can do with looping through arrays is to find out if "Bob" appears in an array of names, or we can loop through an array of groceries for example, to find the total sum we need to pay for them.

Below is an example of looping through an array of names, looking for "Bob".

listOfNames = ['Jones','Jill','Lisa','Stan','Bob','Alice']

for i in range(len(listOfNames)):
  print(listOfNames[i])
  if listOfNames[i] == 'Bob':
    print('Found Bob!')
    break

let listOfNames = ['Jones','Jill','Lisa','Stan','Bob','Alice'];

for (let i = 0; i < listOfNames.length; i++) {
  console.log(listOfNames[i]);
  if (listOfNames[i] === 'Bob') {
    console.log('Found Bob!');
     break;
  }
}

String[] listOfNames = {"Jones", "Jill", "Lisa", "Stan", "Bob", "Alice"};

for (int i = 0; i < listOfNames.length; i++) {
  System.out.println(listOfNames[i]);
  if (listOfNames[i] == "Bob") {
    System.out.println("Found Bob!");
    break;
  }
}

string listOfNames[] = {"Jones", "Jill", "Lisa", "Stan", "Bob", "Alice"};
int size = sizeof(listOfNames) / sizeof(listOfNames[0]);

for (int i = 0; i < size; i++) {
  cout << listOfNames[i] + "\n";
  if (listOfNames[i] == "Bob") {
    cout << "Found Bob!\n";
    break;
  }
}

In the code above, the break statement stops the loop once "Bob" is found. That is why "Alice" is not printed.

Strict Definition of an Array

Arrays found in modern languages like Python or JavaScript are flexible, meaning arrays can grow, shrink, and hold different types of values. Other programming languages, like C and Java, require arrays to be defined more strictly.

A more strict definition of an array means that in addition to being a collection of values, an array is also:

• fixed length
• same data type for all values
• stored contiguously in memory

Fixed length means that the array length (the number of values inside the array), cannot be changed.

When using the C programming language for example, if you have created an array of 4 values, the array length (4) is fixed and cannot be changed. So if you want to insert a 5th value at the end of your array, you must create a new array 5 values long, put in the original 4 values, and put the 5th value in the last place in new array where there is now place for it.

Same datatype means that all values in the array must be of the same type, so they must all be whole numbers for example, or decimal numbers, or characters, or strings, or some other data type.

Having the array stored contiguously in memory means that the values are stored right after each other in one block of memory, like a group of friends living right next to each other on the same street. Read more about how arrays are stored in memory here.

Using arrays in their strict form gives the user full control over how the program actually executes, but it also makes it hard to do certain things, and it is more prone to errors.

When in need for more flexible/dynamic array functionality in languages such as C or Java, developers often use libraries to help them get the expanded dynamic array functionality they are looking for.

In the code examples on this page, to achieve a dynamic array length so that we can insert and remove values, we have used Python Lists, JavaScript Arrays, Java ArrayList, and C++ Vectors.