Initialising multimensional arrays

Question

This doesn't work in the contract body (a storage declaration):

uint8[][3] foo = [ [1,2,3], [], [0] ];

The error being:

TypeError: Unable to deduce common type for array elements

But neither does this work:

uint8[][3] foo = [ uint8[]([1,2,3]), uint8[]([]), uint8[]([0]) ];

With the error:

TypeError: Explicit type conversion not allowed from "uint8[3] memory" to "uint8[] memory".

And also, this doesn't work:

uint8[][3] foo = [ uint8[]([uint8(1),uint8(2),uint8(3)]), uint8[]([]), uint8[]([uint8(0)]) ];

with the same error as above.

But why? In the second example, the array literals are not uint8[3], they are uint8[] - which should match what the array declaration has (the intention being to create an array of 3 elements of uint8[]).

Is it even possible to initialise a multidimensional array of that shape in Solidity?

Answer 1

In Solidity, you can't directly initialize a storage array with variable-length inner arrays on storage declaration.

However, you can create a memory array in a function and store it in a storage array.

Here is an example:

contract ArrayExample {
    uint8[3][] public foo;

    constructor() {
        // Initialize the array
        foo.push([1, 2, 3]);
        foo.push([0, 0, 0]);
        foo.push([4, 5, 6]);

    }

    function getFoo() public view returns(uint8[3][] memory) {
      return foo;
    }

    function getFooByIndex(uint i) public view returns(uint8[3] memory) {
      return foo[i];
    }
}

edit: I'm not entirely sure what your specific goal is, but I've provided a possible solution below that may help you:

contract ArrayExample {
    uint8[][3] public foo;

    constructor() {
        // Initialize the array
        foo[0] = [uint8(1), 2, 3];
        foo[1] = new uint8[](0);
        foo[2] = [uint8(0)];
    }

    function getFoo() public view returns(uint8[][3] memory) {
      return foo;
    }

    function getFooByIndex(uint i) public view returns(uint8[] memory) {
      return foo[i];
    }
}