mask_array class

An internal, auxiliary class template that supports objects that are subsets of parent valarray objects, specified with a Boolean expression, by providing operations between the subset arrays.

Remarks

The class describes an object that stores a reference to an object va of class valarray<Type>, along with an object ba of class valarray<bool>, which describes the sequence of elements to select from the valarray<Type> object.

You construct a mask_array<Type> object only by writing an expression of the form va[ba]. The member functions of class mask_array then behave like the corresponding function signatures defined for valarray<Type>, except that only the sequence of selected elements is affected.

The sequence consists of at most ba.size elements. An element J is included only if ba[J] is true. There are as many elements in the sequence as there are true elements in ba. If I is the index of the lowest true element in ba, then va[I] is element zero in the selected sequence.

Example

// mask_array.cpp
// compile with: /EHsc
#include <valarray>
#include <iostream>

int main( )
{
   using namespace std;
   int i;

   valarray<int> va ( 10 );
   for ( i = 0 ; i < 10 ; i += 2 )
      va [ i ] =  i;
   for ( i = 1 ; i < 10 ; i += 2 )
      va [ i ] =  -1;

   cout << "The initial operand valarray is:  ( ";
      for ( i = 0 ; i < 10 ; i++ )
         cout << va [ i ] << " ";
   cout << ")." << endl;

   // Use masked subsets to assign a value of 10
   // to all elements grrater than 3 in value
   va [va > 3 ] = 10;
   cout << "The modified operand valarray is:  ( ";
      for ( i = 0 ; i < 10 ; i++ )
         cout << va [ i ] << " ";
   cout << ")." << endl;
}

Output

The initial operand valarray is:  (0 -1 2 -1 4 -1 6 -1 8 -1).
The modified operand valarray is:  (0 -1 2 -1 10 -1 10 -1 10 -1).

Requirements

Header: <valarray>

Namespace: std

See also

Thread Safety in the C++ Standard Library