Re: Cannot implicitly convert expression [[0, -1, 2], [4, 11, 2]] of type int[][] to const(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
Re: Cannot implicitly convert expression [[0, -1, 2], [4, 11, 2]] of type int[][] to const(int[2])[]
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].
Cannot implicitly convert expression [[0, -1, 2], [4, 11, 2]] of type int[][] to const(int[2])[]
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?
