Exception When Calling Function In a Library that Returns an Array

Question

I have a solidity library that has a public function and it's returning a uint8[] array.

pragma solidity ^0.8.4;

library LibA {
    function getArray() public pure returns (uint8[] memory) {
        uint8[] memory arr;
        arr[0] = 100;
        ...

        return arr;
    }
}

Then I have a Solidity smart contract that references the library and calls the function trying to access the array defined in the library. The code looks like this.

pragma solidity ^0.8.4;

import "./LibA.sol";

contract SampleContract {

    function doSomething(uint8 _index) public pure returns (uint8) {
        return LibA.getArray()[_index];
    }
}

The code compiles just fine. I have a mocha unit test that is run using Hardhat to test the method and when I run the unit test I get the following exception.

Error: VM Exception while processing transaction: reverted with panic code 0x32 (Array accessed at an out-of-bounds or negative index)

I am quite new to Solidity and I'm having trouble understanding exactly what is the reason for the exception and how to resolve it. Some help would be greatly appreciated.

Answer 1

I've never worked with libraries before, however I believe the issue might be that you are trying to have a dynamically sized array in memory. This should be

uint8[] memory arr = new uint8[](arraySize);

in order to allocate the space in memory beforehand.

Answer 2

if you are using a dynamic array then instead of assigning the index, just push into the array.

pragma solidity ^0.8.4;

library LibA {
    function getArray() public pure returns (uint[] memory) {
        uint[] memory arr;

// change here

 arr.push(100);
        return arr;
    }
}