What's the correct way to reference elements in an array?

Question

I'm going through the Ethereum Pet Shop Truffle tutorial and found this in the test section:

address adopter = adoption.adopters(8);

A previous section had used adopters[petId] = msg.sender;

Why does one use [] and the other ()?

Answer 1

The tutorial says that,

address[16] public adopters; is a public state variable. Therefore, according to the Solidity docs,

The compiler automatically creates getter functions for all public state variables.

What you are seeing here: address adopter = adoption.adopters(8); is the tutorial accessing the automatically generated getter for the public adopters state variable.

Here is a simple example I created, to explain this for you:

Put this in the Solidity online compiler to see the outputs are as expected.

Answer 2

Where you see the square brackets, an array element is being accessed from within the same contract. This is a direct read or write to the array.

Where you see the curved brackets, it's usually a function call, most commonly made from outside the contract.

If an array is marked public, the compiler automatically creates an accessor function that takes the index you're calling as a parameter. This is what's happening with adoption.adopters(8); The adoption contract has a function called adopters, which is the equivalent of:

function adopters(uint idx) public constant {
    return adopters[idx];
}

You also occasionally see curved brackets with an array in a different context, where it is being used to initialize an in-memory array of a particular length. The following will create an in-memory array of 8 addresses:

address[] memory adopters = new address[](8);

Answer 3

The () is a function call.

In the tutorial, you'll see that the adopters array is public:

address[16] public adopters;

In Solidity, all public attributes of a contract automatically result in a getter with the same name being created; that is what you are seeing being called.