mask_array Class
The latest version of this topic can be found at mask_array Class.
An internal, auxiliary template class that supports objects that are subsets of parent valarrays, specified with a Boolean expression, by providing operations between the subset arrays.
Syntax
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. Thus, 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