Can't assign value to the 0 index of an array inside a function

Question

So, I tried 3 simple codes on array

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

contract test{
    uint[] public x;

    function testing() public {
        x[0] = 20;
    }
}

This throws an error but if i use x.push(20) instead of x[0] = 20 then it works OR.....

if I declare the array as

uint[3] public x; 

and then use x[0] = 20 inside the function

then it works .... OR..... if I write it as :

uint[] public x = [1,2,3];

and use x[0] = 20 inside the function

then also it works... I dont understand the mechanism behind it

Answer 1

That's because your array is of size zero when you declare it like

uint[] public x;

So zeroth index doesn't exist and it throws an error. When you define the array with an explicit length it doesn't give any errors because then zeroth index does exist. If you want to use a dynamic array try using:

x.push()