WTF Solidity: 6. Array & Struct

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Recently, I have been revisiting Solidity, consolidating the finer details, and writing "WTF Solidity" tutorials for newbies.

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In this lecture, we will introduce two important variable types in Solidity: array and struct.

Array

An array is a variable type commonly used in Solidity to store a set of data (integers, bytes, addresses, etc.).

There are two types of arrays: fixed-sized and dynamically-sized arrays.：

• fixed-sized arrays: The length of the array is specified at the time of declaration. An array is declared in the format T[k], where T is the element type and k is the length.

    // fixed-length array
    uint[8] array1;
    byte[5] array2;
    address[100] array3;

• Dynamically-sized array（dynamic array）：Length of the array is not specified during declaration. It uses the format of T[], where T is the element type.

    // variable-length array
    uint[] array4;
    byte[] array5;
    address[] array6;
    bytes array7;

Notice: bytes is special case, it is a dynamic array, but you don't need to add [] to it. You can use either bytes or bytes1[] to declare byte array, but not byte[]. bytes is recommended and consumes less gas than bytes1[].

Rules for creating arrays

In Solidity, there are some rules for creating arrays：

• For a memory dynamic array, it can be created with the new operator, but the length must be declared, and the length cannot be changed after the declaration. For example：

    // memory dynamic array
    uint[] memory array8 = new uint[](5);
    bytes memory array9 = new bytes(9);

• Array literal are arrays in the form of one or more expressions, and are not immediately assigned to variables; such as [uint(1),2,3] (the type of the first element needs to be declared, otherwise the type with the smallest storage space is used by default).

• When creating a dynamic array, you need an element-by-element assignment.

    uint[] memory x = new uint[](3);
    x[0] = 1;
    x[1] = 3;
    x[2] = 4;

Members of Array

• length: Arrays have a length member containing the number of elements, and the length of a memory array is fixed after creation.
• push(): Dynamic arrays have a push() member function that adds a 0 element at the end of the array.
• push(x): Dynamic arrays have a push(x) member function, which can add an x element at the end of the array.
• pop(): Dynamic arrays have a pop() member that removes the last element of the array.

Example:

Struct

You can define new types in the form of struct in Solidity. Elements of struct can be primitive types or reference types. And struct can be the element for array or mapping.

    // struct
    struct Student{
        uint256 id;
        uint256 score; 
    }

    Student student; // Initially a student structure

There are 4 ways to assign values to struct:

    //  assign value to structure
    // Method 1: Create a storage struct reference in the function
    function initStudent1() external{
        Student storage _student = student; // assign a copy of student
        _student.id = 11;
        _student.score = 100;
    }

Example:

     // Method 2: Directly refer to the struct of the state variable
    function initStudent2() external{
        student.id = 1;
        student.score = 80;
    }

Example:

    // Method 3: struct constructor
    function initStudent3() external {
        student = Student(3, 90);
    }
    
    // Method 4: key value
    function initStudent4() external {
        student = Student({id: 4, score: 60});
    }

Summary

In this lecture, we introduced the basic usage of array and struct in Solidity. In the next lecture, we will introduce the hash table in Solidity - mapping。