Session 1, Page 10

Views: What the hell is that?

Before we have a look at other matrix types, let me introduce views. The term view is used in FLENS to denote references to matrix or vector slices. And obviously it is adopted from the field of database systems.

So here a first example for

a vector x,
another vector c storing a copy of a slice of x.
a vector view y referencing a slice of x,
a vector view z referencing a slice of y.
Vector views can be used like regular vectors.
Modifying a view will modify the original vector,
but (of course) copies will not be modified.

s01_t07_vectorviews.cc

  #include <flens/flens.h>
  #include <iostream>
  
  using namespace flens;
  using namespace std;
  
  typedef DenseVector<Array<double> >                DEVector;
  
  int
  main()
  {
      DEVector x(6);
      x = 1, 2, 3, 4, 5, 6;
  
      // views
      DEVector::View y = x(_(2,6));
      DEVector::View z = y(_(1,2,5));
  
      // copy
      DEVector c = x(_(2,6));
  
      cout << "Vectors x,c and views y, z:" << endl;
      cout << " x = " << x << endl;
      cout << " c = " << c << endl;
      cout << " y = " << y << endl;
      cout << " z = " << z << endl;
      cout << endl;
  
  
      // modify views:
      y(2) = 123;
      z(3) = 456;
      cout << "y(2) = 123;" << endl;
      cout << "z(3) = 456;" << endl;
      cout << endl;
  
      cout << "Here again x, c, y and z:" << endl;
      cout << " x = " << x << endl;
      cout << " c = " << c << endl;
      cout << " y = " << y << endl;
      cout << " z = " << z << endl;
  }

So what's this DEVector::View?

DEVector is a typedef for
```
    DenseVector<Array<double> >
```
defined above in the program.
DEVector::View is a typedef for
```
    DenseVector<ArrayView<double> >
```

Now, this can be read as follows:

Both are DenseVector types using different implementations.
Array is an implementation using its own memory,
ArrayView is an implementation referencing memory managed by someone else.

So why use this DEVector::View?

It is possible to implement a further implementations VeryCoolArray also storing elements in its own memory.
Beside this an implementation VeryCoolArrayView suited to reference slices of VeryCoolArray can be provided.

Then

      DenseVector<VeryCoolArray<double> >::View

will be a typedef for

      DenseVector<VeryCoolArrayView<double> >

This allows generic programming:

      template <typename I>
      void
      dummy(DenseVector<I> &x)
      {
          // ...
      }

which is independent of the implementation used for DenseVector.

Here the output

Output of s01_t07_vectorviews

  Vectors x,c and views y, z:
   x = [6]
  1.000000  2.000000  3.000000  4.000000  5.000000  6.000000
  
   c = [5]
  2.000000  3.000000  4.000000  5.000000  6.000000
  
   y = [5]
  2.000000  3.000000  4.000000  5.000000  6.000000
  
   z = [3]
  2.000000  4.000000  6.000000
  
  
  y(2) = 123;
  z(3) = 456;
  
  Here again x, c, y and z:
   x = [6]
  1.000000  2.000000  123.000000  4.000000  5.000000  456.000000
  
   c = [5]
  2.000000  3.000000  4.000000  5.000000  6.000000
  
   y = [5]
  2.000000  123.000000  4.000000  5.000000  456.000000
  
   z = [3]
  2.000000  4.000000  456.000000

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