The following code example is taken from the book
The C++ Standard Library - A Tutorial and Reference, 2nd Edition
by Nicolai M. Josuttis, Addison Wesley Longman, 2012
Copyright © 2012 by Pearson Education, Inc. and Nicolai M. Josuttis
#include <iostream>
#include <valarray>
using namespace std;
// print valarray as two-dimensional array
template <typename T>
void printValarray (const valarray<T>& va, int num)
{
for (int i=0; i<va.size()/num; i++) {
for (int j=0; j<num; j++) {
cout << va[i*num+j] << ' ';
}
cout << endl;
}
cout << endl;
}
int main()
{
// create valarray for 12 elements
valarray<double> va(12);
// initialize valarray by values 1.01, 2.02, ... 12.12
for (int i=0; i<12; i++) {
va[i] = (i+1) * 1.01;
}
printValarray(va,4);
// create array of indexes
// - note: element type has to be size_t
valarray<size_t> idx(4);
idx[0] = 8;
idx[1] = 0;
idx[2] = 3;
idx[3] = 7;
// use array of indexes to print the ninth, first, fourth, and eighth elements
printValarray(valarray<double>(va[idx]), 4);
// change the first and fourth elements and print them again indirectly
va[0] = 11.11;
va[3] = 44.44;
printValarray(valarray<double>(va[idx]), 4);
// now select the second, third, sixth, and ninth elements
// and assign 99 to them
idx[0] = 1;
idx[1] = 2;
idx[2] = 5;
idx[3] = 8;
va[idx] = 99;
// print the whole valarray again
printValarray (va, 4);
}