solidity two-dimentional dynamic array

Question

for dynamic array in solidity

uint[][] public test;

how can I insert the dynamic element? if test.length is 3, does this mean test[3][] or test[][3]? if I insert an element,

test.length++; len=test.length; test[len][test[len].length++]=my_int;

is this correct? thanks

Answer 1

Both of them work.

pragma solidity ^0.5.0;

contract Test {

    uint[3][] a;
    uint[][3] b;

    constructor () public {
    /// Error
    /// a[0] = [1, 2, 3, 4];
    /// Correct
        b[0] = [1, 2, 3, 4];

    /// Error
    /// b[4] = [1, 2];
    /// Correct
        a[4] = [1, 2];
    }
}

Which one is right depends on what you want to achieve. Do you want 3 arrays with an unkown number of members, or do you want an unknown number of arrays with 3 members each.

An example for the first option would be a vote where there are 3 parties. Each vote is added to one of the three parties.

An example for the second option would be a list of members in a club. For every member you have three entries: gender, age, location.

Answer 2

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.