mask_array

Library:  Numerics

Does not inherit

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• Assignment Operators
• Computed Assignment Operators
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Members

mask_array() operator%=() operator&=()

operator>>=() operator<<=() operator*=()

operator+=() operator-=() operator/=()

operator=() operator^=() operator|

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

#include <valarray>

namespace std {
  template <class T>
  class mask_array;
}

mask_array gives a masked view into a valarray. A mask_array can only be 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 mask_array to a valarray using either the valarray copy constructor or the assignment operator. Reference semantics are lost at that point.

namespace std {

  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>  // Includes valarray and
                       // provides stream inserter.

typedef std::valarray<int>    valarray_t;
typedef std::valarray<bool>   maskarray_t;

int main(void) {

  // Create a valarray of ints.
  valarray_t::value_type   ibuf[10] = {0,1,2,3,4,5,6,7,8,9};
  valarray_t               vi(ibuf, 10);

  // Create a valarray of bools for a mask.
  maskarray_t::value_type  mbuf[10] = {1,0,1,1,1,0,0,1,1,0};
  maskarray_t              mask(mbuf,10);

  // Print out the valarray<int>.
  std::cout << "original valarray<int> vi\n\n" << vi << "\n\n";

  // Print out the valarray<bool>.
  std::cout << "original valarray<bool> mask\n\n" << mask 
            << "\n\n";
  
  // Print a mask array.
  std::cout << "vi[mask]\n\n" << vi[mask] << "\n\n";
  
  // Double the values of the masked array and print out.
  vi[mask] += static_cast<valarray_t> (vi[mask]);
  std::cout << "vi[mask] += vi[mask]\n\n" << vi << std::endl;

  return 0;
}


Program Output:

original valarray<int> vi

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

original valarray<bool> mask

[1,0,1,1,1,0,0,1,1,0]

vi[mask]

[0,2,3,4,7,8]

vi[mask] += vi[mask]

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

slice, slice_array, valarray, gslice, gslice_array, indirect_array

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 26.3.8