std::mask_array - cppreference.com

#include <iostream>
#include <valarray>

int main() 
{
    std::valarray<int> data = {0,1,2,3,4,5,6,7,8,9};

    std::cout << "Initial valarray: ";
    for(int n: data) std::cout << n << ' ';
    std::cout << '\n';

    data[data > 5] = -1;
    // the type of data>5 is std::valarray<bool>
    // the type of data[data>5] is std::mask_array<int>

    std::cout << "After v[v>5]=-1:  ";
    for(int n: data) std::cout << n << ' ';
    std::cout << '\n';
}

Initial valarray: 0 1 2 3 4 5 6 7 8 9 
After v[v>5]=-1:  0 1 2 3 4 5 -1 -1 -1 -1