Why cannot insert elements dynamically into 2D memory array

Question

I have some issues with understanding exactly how the arrays and memory works in solidity, but I found this nice question that kind of help me to understand some parts - Solidity 2D in-memory array with size defined by a variable

I see how the 2D array can be initialized, but why we have to mention the exact size for the first dimension (X axis)?

In my case, I know precisely how big the array can be, so it is fine, but what if I do not know how many elements I can have on X axis?

Answer 1

Actually, that's not true. You could have the size of all dimensions in the array be determined at runtime as follows:

pragma solidity ^0.8.15;

contract Demo {
    // Creates an array with height `n` and width `m`. 
    function makeArr(uint n, uint m) public pure returns (int[][] memory) {
        // Define the array with first dimension being of size n.
        int[][] memory arr = new int[][](n);
        // Initialize all the array rows.
        // Notice that each row is basically a 1D array of size `m`.
        for(uint i; i < n; i++) {
            arr[i] = new int[](m);
        }

        // At this point, you have a fully initialized 2D array of size n x m.
        // You could access and set each element in the array.
        // For example, here is how to set the value for the element [0][0]:
        arr[0][0]=10;
        return arr;
    }
}