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Solidity Tutorial: all about Array

In Solidity, Arrays are ordered list of items indexed numerically starting at 0.

Array types

Below, T is the element type and k is the array length / size. If k is equal to 5, then the array can hold a maximum of 5 values.

  • Fixed Size: T[k]
  • Dynamic Size: []
  • Multi-dimensional: T[][k]

Example: uint[][5] represents an array that include 5 other arrays of dynamic size uint.

(Draw an example of code here).

Note that in Solidity, the notation of multidimensional array is reversed compared to other programming languages.

How to define arrays ?

example: uint[][5] x Where x is the variable name.

Access elements within Fixed and Dynamic size arrays.

Like many other programming languages, you must specify the index between the square brackets.

Access elements within multi-dimensional arrays.

As said before, the notation is reversed, so you must access the element in the opposite direction of the declaration. Here is an example:

y = x[2][1]

Here, y represents the value of the 2nd uint inside the 3rd dynamic array located in the array x.

Arrays literals (inline arrays)

Arrays literals are arrays written like an expression. They do not get assigned to a variable right away.

// Base type (common types of the given elements)
// Here the type would be uint8[3]
function func1() public pure {

    // Conversion of the 1st element to uint (uin256) is necessary because
    // the type of every constant memory is uint8 by default.
    
    func2([uint(1), 2, 3]);
}

// memory type array that has a fixed size (uint3)
function func2(uint[3] _data) {
    // Some code here
}

Allocate memory Arrays

Use the new keyword.

function _memory_array(uint len) public {
    
    uint[] memory x = new uint[](7);
    bytes[] memory y = new bytes(len):;
    
    // Here we have x.length == 7 and y.length == len
    x[6] = 8;

}

Note that you can't change the size of memory arrays by assigning values to the member .length

You can't assign fix size memory arrays to dynamic size memory array !

(copy code from note book here)

Reference type members of Arrays

It exists to main members functions that you can perform on arrays in Solidity. These are also common to other programming languages that use arrays.

  • .length > return the number of elements the array holds.

Note that dynamic sized arrays may be resized in storage, not in memory, by modifying the length member.

  • .push(new_element) > append a new element at the end of the array. Returns the new length.

This member function is only available for dynamic sized arrays and bytes, not strings (not sure if you can't push an element in a fixed sized array).

What you can't do in Arrays with Solidity ?

It is not possible to use arrays of arrays within external functions.