A "2D" array is treated as an array of arrays. For example, if you needed a 10 x 2, it would be a uint[10] in which each row is itself a uint[2]. Either dimension can be dynamic.

A common mistake is to reverse the order of the indexes. For example, a dynamic list of uint[3] would be:

uint[3][]

Later, when you want to set, say the 2nd element of row 100, it would be:

myArray[100][2] because there is a uint[3] stored in row 100.

This may help you find your way around dynamic x dynamic. Remember, weird switcharoos happen if it seems like it's not working.

pragma solidity 0.5.0;

contract TwoD {
    
    uint[][] public test;
    
    function appendFirstOrderArray() public {
        uint[] memory element;
        test.push(element);
    }
    
    function appendSecondOrderElementAtRow(uint row, uint value) public {
        test[row].push(value);
    }
    
    function getFirstOrderLength() public view returns(uint) {
        return test.length;
    }
    
    function getSecondOrderLengthAtRow(uint row) public view returns(uint) {
        return test[row].length;
    }
    
}

Hope it helps.

A "2D" array is treated as an array of arrays. For example, if you needed a 10 x 2, it would be a uint[10] in which each row is itself a uint[2]. Either dimension can be dynamic.

A common mistake is to reverse the order of the indexes. For example, a dynamic list of uint[3] would be:

uint[3][] myArray;

Later, when you want to set, say the 2nd element of row 100, it would be:

myArray[100][2] because there is a uint[3] stored in row 100 and we want the 2nd element in that array.

This may help you find your way around dynamic x dynamic. Remember, weird switcharoos happen if it seems like it's not working.

pragma solidity 0.5.0;

contract TwoD {
    
    uint[][] public test;
    
    function appendFirstOrderArray() public {
        uint[] memory element;
        test.push(element);
    }
    
    function appendSecondOrderElementAtRow(uint row, uint value) public {
        test[row].push(value);
    }
    
    function getFirstOrderLength() public view returns(uint) {
        return test.length;
    }
    
    function getSecondOrderLengthAtRow(uint row) public view returns(uint) {
        return test[row].length;
    }
    
}

Hope it helps.