How to initialize an array inside the function and push items into it?

Question

How to initialize an array inside the function and push items into it? I'm not gonna initialize outside the function.

Here's my code:

function tokenOfOwner(address owner) public virtual returns (uint256[] memory) 
{
    uint256[] storage _ownerAmount;
    uint256 amount = ERC721.balanceOf(owner);
    for(uint256 i = 0; i< amount ; i++)
    {
        _ownerAmount.push(_ownedTokens[owner][i]);
    }
    return _ownerAmount;
}

The error message shows:

TypeError: This variable is of storage pointer type and can be accessed without prior assignment, which would lead to undefined behaviour.

Answer 1

TypeError: This variable is of storage pointer type and can be accessed without prior assignment, which would lead to undefined behaviour.

Is perfectly normal as uint256[] storage _ownerAmount; is not initialized, meaning that it defaults to slot 0 possibly overwriting whatever is already occupying that slot.

However, looking at your code you probably don't need a dynamic storage array. Only dynamic storage arrays can be resized (pushed to / popped from), this makes sense when the number of elements in unknown, but that is not your case due to :

uint256 amount = ERC721.balanceOf(owner);

A more gas efficient implementation could rely only on static memory arrays with fixed size, such as :

function tokenOfOwner(address owner) public virtual returns (uint256[] memory) 
{
    uint256 amount = ERC721.balanceOf(owner);
    uint256[] memory _ownerAmount = new uint[](amount);
    for(uint256 i = 0; i< amount ; i++)
    {
        _ownerAmount[i] = _ownedTokens[owner][i];
    }
    return _ownerAmount;
}

Answer 2

I am not sure about the end-functionality of your code in the terms of what you are looking to accomplish (Gater all the values of all the tokens owner has?)

But this commented code should help, I tried to make it as simple as possible.

pragma solidity ^0.7.0;

contract Function{
 
 function tokenOfOwner(address owner) public virtual returns (uint256[] memory) 
{

    uint256 amount = ERC721.balanceOf(owner);
    _ownerAmount = new uint256[](amount); //Creates an array of "amount" length
    
    for(uint256 i = 0; i< amount ; i++)
    {  
        _ownerAmount[i] = //Plug in the value you wish to record;
    }
    return _ownerAmount;
}
  
   
}