C++/Valarray/valarray slice

valarray slice

<source lang="cpp">

/* 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.
*/

1. include <iostream>
2. 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

*/        
 </source>