The following code example is taken from the book
The C++ Standard Library - A Tutorial and Reference
by Nicolai M. Josuttis, Addison-Wesley, 1999
© Copyright Nicolai M. Josuttis 1999
#include <iostream>
#include <valarray>
using namespace std;
// print three-dimensional valarray line-by-line
template<class T>
void printValarray3D (const valarray<T>& va, int dim1, int dim2)
{
for (int i=0; i<va.size()/(dim1*dim2); ++i) {
for (int j=0; j<dim2; ++j) {
for (int k=0; k<dim1; ++k) {
cout << va[i*dim1*dim2+j*dim1+k] << ' ';
}
cout << '\n';
}
cout << '\n';
}
cout << endl;
}
int main()
{
/* valarray with 24 elements
* - two groups
* - four rows
* - three columns
*/
valarray<double> va(24);
// fill valarray with values
for (int i=0; i<24; i++) {
va[i] = i;
}
// print valarray
printValarray3D (va, 3, 4);
// we need two two-dimensional subsets of three times 3 values
// in two 12-element arrays
size_t lengthvalues[] = { 2, 3 };
size_t stridevalues[] = { 12, 3 };
valarray<size_t> length(lengthvalues,2);
valarray<size_t> stride(stridevalues,2);
// assign the second column of the first three rows
// to the first column of the first three rows
va[gslice(0,length,stride)]
= valarray<double>(va[gslice(1,length,stride)]);
// add and assign the third of the first three rows
// to the first of the first three rows
va[gslice(0,length,stride)]
+= valarray<double>(va[gslice(2,length,stride)]);
// print valarray
printValarray3D (va, 3, 4);
}