On Sunday, 19 May 2019 at 06:34:18 UTC, Andrew Edwards wrote:
Sooo... I'm trying to learn this stuff so that I can fully
grasp the content of Jens Mueller's 2019 DConf talk and its
applications in financial sector (forex and options/futures
trading). Unfortunately, I'm doing so using python but I'd like
to accomplish the same in D. Here goes:
Array (Vector) Algebra
auto V = [1, 2, 3];
auto v = V[] * 4; // v == [4, 8, 12]
Output:
Error: array operation V[] * 4 without destination memory not
allowed
Huh? What? That makes no sense, but okay... whatever.
int[3] v = V[] * 4; // v == [4, 8, 12]
v[] = v[] + v[]; // v == [8, 16, 24]
v[] = v[] / 2; // v == [4, 8, 12]
v[] = v[] - v[]; // v = [0, 0, 0]
so far so good. On to the fun stuff.
Multidimensional Array (Matrix) Algebra
auto M = [[1, 2, 3], [1, 2, 3], [1,2,3]];
int[3][3] m = M[] * 4;
Output:
Error: incompatible types for (M[]) * (4): int[][] and int
Okay, I'm lost on that one. But let's press on.
int[3][3] m = M[] * [[4]];
Output:
Error: cannot implicitly convert expression M[] * [[4]] of
type int[][] to int[]
int[3][3] m = M[] * [[4,4,4], [4,4,4], [4,4,4]];
Output:
Error: cannot implicitly convert expression M[] * [[4, 4, 4],
[4, 4, 4], [4, 4, 4]] of type int[][] to int[]
I'm so confused. Maybe it's the way I'm accessing M?
int[3][3] m = M[][] * 4;
Output:
Error: incompatible types for (M[]) * (4): int[][] and int
And... it begins again!!!
So the question is, how do I pull this off in D using just
builtin arrays and phobos?
Any assistance is appreciated.
Matrix operations are a pain in D. This is due to static arrays
and dynamic arrays are different types and don't mix a priori:
https://dlang.org/spec/arrays.html
Slice operations exist, but they are defined mainly for arrays
and not for arrays of arrays. Some basic operations (like
setting) are possible, but as you see, even multiplication fails.
You could reinvent something like
´´´
import std;
void main()
{
int[][] M = [[1,2,3],[1,2,3],[1,2,3]];
M.recursiveMultiplier(4);
writeln(M);
}
void recursiveMultiplier(T, V)(T arr, V val) @nogc
{
static if(isArray!(ElementType!T))
arr.each!(el => el.recursiveMultiplier(val));
else
arr[] = arr[] * val;
}
´´´
BUT:
As I saw, in the talk, NDarrays are referenced. The common
(widely used and recommended) way to use this functionality in D
is by using mir.ndslices.
http://docs.algorithm.dlang.io/latest/mir_ndslice.html
Thanks,
Andrew
P.S. Why do we still have two sets of documentations ([1],[2])
for the language? Which is the official one and when can we get
rid of the other?
[1] https://dlang.org/library/std/array.html
[2] https://dlang.org/phobos/std_array.html
