A full guide to Arrays in Solidity

In this guide we are going to learn everything about Arrays in Solidity and how to create arrays, add and remove elements from arrays in Solidity.

How to create arrays

To create arrays in Solidity, you have 2 options. You can either create a fixed size array or an array with a dynamic size.

If you're sure that your array will have a specific size, it's best to create a fixed size array so you use less storage and your types are more clearly defined.

Fixed size arrays

To create an array with a fixed size, you have to specify that in the definition of the type of the variable:

address[5] myAddresses;

That way, the myAddresses array can only have 5 or less items.

And if you do:

address[5] myAddresses = new address[](5)

You will initialise your array with 5 default values.

Dynamic size arrays

To create an array with a dynamic size, all you have to do is... not specify any size!

address[] myAddresses;

That way, myAddresses has a dynamic size and you can put as many items as you want inside of it.

How to add elements to Arrays, remove elements and update values

Add elements

To add elements, you can use the push function. It will add a new element at the end of the array:

uint256[] numbers;

function addNumber() public {
    numbers.push(5)
}

Remove elements

To remove elements, you can use the pop function to remove the last element of the array

uint256[] numbers;

function removeLastNumber() public {
    numbers.pop()
}

And to remove an element at a specific index, you can use the delete keyword:

uint256[] numbers;

function removeThirdElement() public {
    delete numbers[2]
}

Update a value

To update an item, you can use the index of that item and directly change its value:

uint256[] numbers;

function updateThirdElement() public {
    numbers[2] = 5
}

Get the length of an array

To get the length of an array you can use the length property:

uint256[] numbers;

function getLength() public view returns (uint256[]) {
    return numbers.length;
}

Differences between memory and storage arrays

The last thing to have in mind is that you cannot use the push and pop functions on arrays that are created with the memory keyword (or calldata).

You can only use them on arrays that use the storage keyword (explicitly or not). If you define a property at the top of your smart contract, it uses the storage by default so the array methods are available.

uint256[] myNumbers;

// does not work
function addElementToArray(uint256[] memory array) public {
    array.push(5) // won't work
}

// does work
function addElementToArray(uint256[] storage array) public {
    array.push(5) // will work
}

// does work
function addToNumbers() public {
    myNumbers.push(5) // will work
}

And that's it 🎉

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