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Showing content from https://cplusplus.com/reference/valarray/mask_array/ below:

class template

<valarray>

template <class T> mask_array;

Valarray mask selection

It references the elements in the valarray object that are selected by a mask (a valarray<bool>), and overloads the assignment and compound assignment operators, allowing direct access to the elements in the selection.

The type is convertible to a valarray (see valarray constructor), producing a new object with copies of the referred elements.

Objects of this type are obtained as the return value of a call to

. They cannot be directly

nor copied. It is declared as:

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template <class T> class mask_array {
public:
  typedef T value_type;
  void operator=   (const valarray<T>&) const;
  void operator*=  (const valarray<T>&) const;
  void operator/=  (const valarray<T>&) const;
  void operator%=  (const valarray<T>&) const;
  void operator+=  (const valarray<T>&) const;
  void operator-=  (const valarray<T>&) const;
  void operator^=  (const valarray<T>&) const;
  void operator&=  (const valarray<T>&) const;
  void operator|=  (const valarray<T>&) const;
  void operator<<= (const valarray<T>&) const;
  void operator>>= (const valarray<T>&) const;
  void operator=(const T&);
  ~mask_array();
private:
  mask_array();
  mask_array(const mask_array&);
  mask_array& operator= (const mask_array&);
};

Objects of this type are obtained as the return value of a call to

. They cannot be

, but can be copied. It is declared as:

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template <class T> class mask_array {
public:
  typedef T value_type;
  void operator=   (const valarray<T>&) const;
  void operator*=  (const valarray<T>&) const;
  void operator/=  (const valarray<T>&) const;
  void operator%=  (const valarray<T>&) const;
  void operator+=  (const valarray<T>&) const;
  void operator-=  (const valarray<T>&) const;
  void operator^=  (const valarray<T>&) const;
  void operator&=  (const valarray<T>&) const;
  void operator|=  (const valarray<T>&) const;
  void operator<<= (const valarray<T>&) const;
  void operator>>= (const valarray<T>&) const;
  void operator=(const T&) const;

  mask_array() = delete;
  mask_array(const mask_array&);
  const mask_array& operator= (const mask_array&) const;
  ~mask_array();
};

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// mask_array example
#include <iostream>     // std::cout
#include <cstddef>      // std::size_t
#include <valarray>     // std::valarray

int main ()
{
  std::valarray<int> foo (10);
  for (int i=0; i<10; ++i) foo[i]=i;        //  0  1  2  3  4  5  6  7  8  9

  std::valarray<bool> mymask (10);
  for (int i=0; i<10; ++i)
    mymask[i]= ((i%2)==1);                  //  f  t  f  t  f  t  f  t  f  t

  foo[mymask] *= std::valarray<int>(10,5);  //  0 10  2 30  4 50  6 70  8 90
  foo[!mymask] = 0;                         //  0 10  0 30  0 50  0 70  0 90

  std::cout << "foo:";
  for (std::size_t i=0; i<foo.size(); ++i)
    std::cout << foo[i] << ' ';
  std::cout << '\n';

  return 0;
}

foo: 0 10 0 30 0 50 0 70 0 90

slice_array

Valarray slice selection (class template)

gslice_array

Valarray gslice selection (class template)

indirect_array

Valarray indirect selection (class template)

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