mask_array

Valarray helpers

• Summary
• Data Type and Member Function Indexes
• Synopsis
• Description
• Interface
• Constructors
• Assignment Operators
• Computed Assignment Operators
• Member Functions
• Example
• Warnings
• See Also

A numeric array class that gives a masked view of a valarray.

Member Functions
operator%=() operator&=() operator>>=() operator<<=() operator*=() operator+=()	operator-=() operator/=() operator=() operator^=()

#include <valarray>
template <class T>
class mask_array;

mask_array<T> gives a masked view into a valarray. mask_arrays are only produced by applying the mask subscript operator to a valarray. This subscript operator takes a valarray<bool> argument and produces a mask_array. Only the elements in the valarray whose corresponding elements in the valarray<bool> argument were true are selected by the mask_array. The elements in a mask_array are references to selected elements in the valarray (so changing an element in the mask_array really changes the corresponding element in the valarray). A mask_array does not itself hold any distinct elements. The template cannot be instantiated directly since all its constructors are private. However, you can copy a slice_array to a valarray using either the valarray copy constructor or the assignment operator. Reference semantics are lost at that point.

template <class T> class mask_array {
public:

  // types
  typedef T value_type;

  // destructor
  ~mask_array();

  // public assignment
  void operator= (const valarray<T>& array) const;
  // computed assignment
  void operator*= (const valarray<T>& array) const;
  void operator/= (const valarray<T>& array) const;
  void operator%= (const valarray<T>& array) const;
  void operator+= (const valarray<T>& array) const;
  void operator-= (const valarray<T>& array) const;
  void operator^= (const valarray<T>& array) const;
  void operator&= (const valarray<T>& array) const;
  void operator|= (const valarray<T>& array) const;
  void operator<<= (const valarray<T>& array) const;
  void operator>>= (const valarray<T>& array) const;
 
  // other
  void operator= (const T&) const; 

private:
  // constructors
  mask_array();
  mask_array(const mask_array<T>&);
  // operator =
  mask_array<T>& operator= (const mask_array<T>& array);
};

mask_array();
mask_array(const mask_array&);

All mask_array constructors are private and cannot be called directly. This prevents copy construction of mask_arrays.

void operator=(const valarray<T>& x) const;

Assigns values from x to the selected elements of the valarray that self refers to. Remember that a mask_array never holds any elements itself, it simply refers to selected elements in the valarray used to generate it.

mask_array<T>& 
operator=(const mask-_array<T>& x);

Private assignment operator. Cannot be called directly, thus preventing assignment between mask_arrays.

void operator*=(const valarray<T>& val) const;
void operator/=(const valarray<T>& val) const;
void operator%=(const valarray<T>& val) const;
void operator+=(const valarray<T>& val) const;
void operator-=(const valarray<T>& val) const;
void operator^=(const valarray<T>& val) const;
void operator&=(const valarray<T>& val) const;
void operator|=(const valarray<T>& val) const;
void operator<<=(const valarray<T>& val) const;
void operator>>=(const valarray<T>& val) const;

Applies the indicated operation using elements from val to the selected elements of the valarray that self refers to. Remember that a mask_array never holds any elements itself; it simply refers to selected elements in the valarray used to generate it.

void operator= (const T& x) const;

Assigns x to the selected elements of the valarray that self refers to.

//
// mask_array.cpp
//
#include "valarray.h" // Contains a valarray stream inserter
using namespace std;

int main(void)
{
  int ibuf[10] = {0,1,2,3,4,5,6,7,8,9};
  bool mbuf[10] = {1,0,1,1,1,0,0,1,1,0};

  // create a valarray of ints
  valarray<int>         vi(ibuf,10);

  // create a valarray of bools for a mask
  valarray<bool> mask(mbuf,10);

  // print out the valarray<int>
  cout << vi << endl;

  // Get a mask array and assign that mask to another array
  mask_array<int> msk = vi[mask];
  valarray<int> vi3 = msk;

  // print out the masked_array
  cout << vi3 << endl;

  // Double the masked values
  msk += vi3;

  // print out vi1 again
  cout << vi << endl;

  return 0;
}

Program Output

[0,1,2,3,4,5,6,7,8,9]
[0,2,3,4,7,8]
[0,1,4,6,8,5,6,14,16,9]

If your compiler does not support namespaces, then you do not need the using declaration for std.

valarray, slice_array, slice, gslice, gslice_array, indirect_array