## digitalmars.D.learn - Basic Linear Algebra and D's Array Operation

• Andrew Edwards (47/47) May 18 2019 Sooo... I'm trying to learn this stuff so that I can fully grasp
• Alex (29/76) May 19 2019 Matrix operations are a pain in D. This is due to static arrays
• Andrew Edwards (8/40) May 19 2019 Can you do me a favor and extend it to cover the cases where V is
• Alex (10/32) May 19 2019 Not really. For a general matrix-matrix multiplication special
• James Blachly (2/7) May 19 2019 I have wondered this myself -- anyone?
```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 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 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 m = M[] * [];

Output:
Error: cannot implicitly convert expression M[] * [] of type
int[][] to int[]

int 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 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.

Thanks,
Andrew

P.S. Why do we still have two sets of documentations (,)
for the language? Which is the official one and when can we get
rid of the other?

 https://dlang.org/library/std/array.html
 https://dlang.org/phobos/std_array.html
```
May 18 2019
```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 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 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 m = M[] * [];

Output:
Error: cannot implicitly convert expression M[] * [] of
type int[][] to int[]

int 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 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 (,)
for the language? Which is the official one and when can we get
rid of the other?

 https://dlang.org/library/std/array.html
 https://dlang.org/phobos/std_array.html

```
May 19 2019
```On Sunday, 19 May 2019 at 10:07:35 UTC, Alex wrote:
On Sunday, 19 May 2019 at 06:34:18 UTC, Andrew Edwards wrote:
So the question is, how do I pull this off in D using just
builtin arrays and phobos?

Any assistance is appreciated.

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;
}
´´´

Can you do me a favor and extend it to cover the cases where V is
either an array or multidimensional array? The text I'm using has
exercise to implement a bunch of matrix operations so that would
help out a lot.

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

Yes, I'm aware. I'm actually am using Lubeck to accomplish much
of this stuff but when it comes to the implementation exercises,
I have to ditch the library and stick with vanilla D.
```
May 19 2019
```On Sunday, 19 May 2019 at 16:17:17 UTC, Andrew Edwards wrote:
´´´
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;
}
´´´

Can you do me a favor and extend it to cover the cases where V
is either an array or multidimensional array? The text I'm
using has exercise to implement a bunch of matrix operations so
that would help out a lot.

Not really. For a general matrix-matrix multiplication special
algorithms exist. ndslices use blas for calculation. For
different numeric types, things get involved very fast, not to
speak about special algorithms for special-formed matrices.
And I don't even know, which representation (row/column major)
you expect ;)

The operation itself is, however, a simple one. To implement a
basic version I would cite
http://rosettacode.org/wiki/Matrix_multiplication#D
```
May 19 2019
```On Sunday, 19 May 2019 at 17:13:11 UTC, Alex wrote:
The operation itself is, however, a simple one. To implement a
basic version I would cite
http://rosettacode.org/wiki/Matrix_multiplication#D

This is awesome. Thank you very much.

Andrew
```
May 19 2019    James Blachly <james.blachly gmail.com> writes:
```On 5/19/19 2:34 AM, Andrew Edwards wrote:
P.S. Why do we still have two sets of documentations (,) for the
language? Which is the official one and when can we get rid of the other?

 https://dlang.org/library/std/array.html
 https://dlang.org/phobos/std_array.html

I have wondered this myself -- anyone?
```
May 19 2019