https://wiki.dlang.org/Dense_multidimensional_arrays#Static_arrays describes a way to create static arrays:

int[3][3] matrix = [
    [ 1, 2, 3 ],
    [ 4, 5, 6 ],
    [ 7, 8, 9 ]
];

However my complains that I can't implicitly create static arrays from dynamic arrays.

private T[R1][C2] loopMul(ulong R1, ulong C1, ulong R2, ulong C2, T)(
        auto ref T[R1][C1] matrixA, auto ref T[R1][C2] matrixB) if (C1 == R2) {
    T[R1][C2] result;
    for (ulong r = 0; r < R1; ++r) {
        for (ulong c = 0; c < C2; ++c) {
            T toAdd = 0;
            for (ulong n = 0; n < C1; ++n) {
                toAdd += matrixA[r][n] + matrixB[c][n];
            }
            result[r][c] = toAdd;
        }
    }
}

void main() {
    import std;

    scope (success)
        std.writeln("loopMul -- ok");

    // assert([[0, -1, 2], [4, 11, 2]].loopMul!(2, 3, 3, 2, int)([[3, -1], [1, 2], [6, 1]]) == [
    //         [11, 0], [35, 20]
    //         ]);
    const int[2][3] matA = [[0, -1, 2], [4, 11, 2]];
    const int[3][2] matB = [[3, -1], [1, 2], [6, 1]];
    const int[2][2] matC = [[11, 0], [35, 20]];
    assert(matA.loopMul(matB) == matC);
}


/// I would share the D online editor link but `shorten` button doesn't do anything
onlineapp.d(24): Error: cannot implicitly convert expression [[0, -1, 2], [4, 11, 2]] of type int[][] to const(int[2])[]
onlineapp.d(25): Error: cannot implicitly convert expression [[3, -1], [1, 2], [6, 1]] of type int[][] to const(int[3])[]
onlineapp.d(27): Error: template onlineapp.loopMul cannot deduce function from argument types !()(const(int[2][3]), const(int[3][2])), candidates are:
onlineapp.d(1):        loopMul(ulong R1, ulong C1, ulong R2, ulong C2, T)(auto ref T[R1][C1] matrixA, auto ref T[R1][C2] matrixB)

What's causing this?

On Friday, 31 January 2020 at 12:37:43 UTC, Adnan wrote:
> What's causing this?

You mixed up the array lengths:

const int[3][2] matA = [[0, -1, 2], [4, 11, 2]];
const int[2][3] matB = [[3, -1], [1, 2], [6, 1]];

matA is an SA containing <2> elements of type int[3].
matB is an SA containing <3> elements of type int[2].

On Friday, January 31, 2020 5:43:44 AM MST MoonlightSentinel via Digitalmars-d-learn wrote:
> On Friday, 31 January 2020 at 12:37:43 UTC, Adnan wrote:
> > What's causing this?
>
> You mixed up the array lengths:
>
> const int[3][2] matA = [[0, -1, 2], [4, 11, 2]];
> const int[2][3] matB = [[3, -1], [1, 2], [6, 1]];
>
> matA is an SA containing <2> elements of type int[3].
> matB is an SA containing <3> elements of type int[2].

Specifically, the dimensions are read outwards from the variable name, so on the left-hand side, that means that they go right-to-left, whereas on the right-hand side, they go left-to-right. This is consistent with how it works with types in C/C++ except that there, they put the dimensions for static arrays on the right-hand side of the variable name, meaning that while you have to read stuff like pointer types from left-to-right in C/C++, you don't have to do that with static arrays. Ultimately, what D is doing is consistent but confusing.

e.g. For C/C++

int** foo; // A pointer to a pointer to an int
int foo[5][2]; // A 5 dimensional array of two dimensional arrays of int
foo[4][1] = 7;

and for D:

int** foo; // A pointer to a pointer to an int
int[2][5] foo; // A 5 dimensional array of two dimensional arrays of int
foo[4][1] = 7;

For C/C++, you often don't realize how the rules work until you have to read function pointers, because they put the static array lengths no the right-hand side, and they actually allow you to put stuff like const in multiple places instead of only in the place where it would be right right-to-left. e.g. if the rule were followed strictly,

const int i = 0;

wouldn't be legal in C/C++. Rather, it would have to be

int const i = 0;

In reality, both work, but people end up using the first one. So, ultimately, it adds to the confusion when dealing with more complex tyypes. D doesn't have that problem, but since it used parens with type qualifiers, it forces const to go on the left, making it less consistent. e.g.

const int i = 0;

or

const(int) i = 0;

So, neither C/C++ nor D is entirely consistent, but the basic rule is that types are read outwards from the variable name, which is why you get the weirdness with static array dimensions in D.

- Jonathan M Davis