valarray slice

/* The following code example is taken from the book * "The C++ Standard Library - A Tutorial and Reference" * by Nicolai M. Josuttis, Addison-Wesley, 1999 * * (C) Copyright Nicolai M. Josuttis 1999. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */

#include <iostream>

#include <valarray>
using namespace std; 

// print valarray line-by-line
template<class 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()
{
   /* valarray with 12 elements
     * - four rows
     * - three columns
     */
   valarray<double> va(12); 

   // fill valarray with values
   for (int i=0; i<12; i++) {
       va[i] = i;
   } 

   printValarray (va, 3);

   // first column = second column raised to the third column
   va[slice(0,4,3)] = pow (valarray<double>(va[slice(1,4,3)]),
                           valarray<double>(va[slice(2,4,3)])); 

   printValarray (va, 3); 
   // create valarray with three times the third element of va
   valarray<double> vb(va[slice(2,4,0)]);

 
   // multiply the third column by the elements of vb
   va[slice(2,4,3)] *= vb;
   printValarray (va, 3); 
   // print the square root of the elements in the second row
   //printValarray (sqrt(valarray<double>(va[slice(3,3,1)])));

   // double the elements in the third row
   va[slice(2,4,3)] = valarray<double>(va[slice(2,4,3)]) * 2.0;

   printValarray (va, 3);
}

/*

0 1 2
3 4 5
6 7 8
9 10 11

1 1 2
1024 4 5
5.7648e+006 7 8
1e+011 10 11

1 1 4
1024 4 10
5.7648e+006 7 16
1e+011 10 22

1 1 8
1024 4 20
5.7648e+006 7 32
1e+011 10 44

*/