2D matrix operation (subtraction)

[Ali Çehreli]

On 2/21/20 12:51 AM, Andre Pany wrote:
> Hi,
> 
> I have a 2D double array and I want to subtract from the first column a 
> value,
> is this possible with matrix operation in D?
> 
> ```
> void main()
> {
>      double[][] data = [[0.0, 1.4], [1.0, 5.2], [2.0, 0.8]];
> 
>      // subtract -2.0 from the first column for every value
> 
>      // Expected output
>      // data = [[-2.0, 1.4], [-1.0, 5.2], [0.0, 0.8]];
> 
> }
> ```
> 
> Kind regards
> André
> 
> 


I don't have experience with it but mir's ndslice is designed for this:

   https://github.com/libmir/mir-algorithm

Although it feels like something similar is probably already in Phobos, 
I've come up with the following solution just now for fun:

import std.stdio;
import std.algorithm;
import std.range;

// At least something similar to this exists in Phobos?
struct ElementReference(R) {
   ElementType!(ElementType!R) * p;

   ref reference() {
     return *p;
   }

   alias reference this;
}

struct Column(R) {
   R range;
   size_t n;

   auto empty() {
     return range.empty;
   }

   auto front() {
     return ElementReference!R(&(range.front[n]));
   }

   auto popFront() {
     return range.popFront();
   }
}

auto byColumn(R)(R range, size_t n) {
   return Column!R(range, n);
}

void main() {
   double[][] data = [[0.0, 1.4], [1.0, 5.2], [2.0, 0.8]];

   data.byColumn(0).each!(a => a -= 2.0);
   writeln(data);
}

Ali