C++ Tutorial/STL Algorithms Modifying sequence operations/generate n

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Demonstrating fill, fill_n, generate, and generate_n Standard Library methods.

<source lang="cpp">#include <iostream>

  1. include <algorithm>
  2. include <vector>

using namespace std; char nextLetter(); int main() {

  vector< char > chars( 10 );
  fill( chars.begin(), chars.end(), "5" );
  fill_n( chars.begin(), 5, "A" );
  generate( chars.begin(), chars.end(), nextLetter );
  generate_n( chars.begin(), 5, nextLetter );
  cout << endl;
  return 0;

} char nextLetter() {

  static char letter = "A";
  return letter++;

}</source>

generate_n and int sequence

<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 <list>
  3. include <algorithm>

using namespace std; /* PRINT_ELEMENTS()

* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/

template <class T> inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="") {

   typename T::const_iterator pos;
   std::cout << optcstr;
   for (pos=coll.begin(); pos!=coll.end(); ++pos) {
       std::cout << *pos << " ";
   }
   std::cout << std::endl;

}

class IntSequence {

 private:
   int value;
 public:
   // constructor
   IntSequence (int initialValue)
    : value(initialValue) {
   }
   // ""function call""
   int operator() () {
       return value++;
   }

}; int main() {

   list<int> coll;
   IntSequence seq(1);    // integral sequence starting with 1
   // insert values from 1 to 4
   // - pass function object by reference
   //     so that it will continue with 5
   generate_n<back_insert_iterator<list<int> >,
              int, IntSequence&>(back_inserter(coll),    // start
                                 4,      // number of elements
                                 seq);   // generates values
   PRINT_ELEMENTS(coll);
   // insert values from 42 to 45
   generate_n (back_inserter(coll),      // start
               4,                        // number of elements
               IntSequence(42));         // generates values
   PRINT_ELEMENTS(coll);
   // continue with first sequence
   // - pass function object by value
   //     so that it will continue with 5 again
   generate_n (back_inserter(coll),      // start
               4,                        // number of elements
               seq);                     // generates values
   PRINT_ELEMENTS(coll);
   
   // continue with first sequence again
   generate_n (back_inserter(coll),      // start
               4,                        // number of elements
               seq);                     // generates values
   PRINT_ELEMENTS(coll);

}</source>

1 2 3 4
1 2 3 4 42 43 44 45
1 2 3 4 42 43 44 45 5 6 7 8
1 2 3 4 42 43 44 45 5 6 7 8 5 6 7 8

Provide std::generate_n with predicate

<source lang="cpp">#include <iostream> using std::cout; using std::endl;

  1. include <algorithm>
  2. include <vector>
  3. include <iterator>

char nextLetter(); int main() {

  std::vector< char > chars( 10 );
  std::ostream_iterator< char > output( cout, " " );
  // generate values for first five elements of chars with nextLetter
  std::generate_n( chars.begin(), 5, nextLetter );
  cout << "\n\nVector chars after generating letters A-J:\n";
  std::copy( chars.begin(), chars.end(), output );
  cout << endl;
  return 0;

} // generator function returns next letter (starts with A) char nextLetter() {

  static char letter = "A";
  return letter++;

}</source>

Vector chars after generating letters A-J:
A B C D E

Use generate_n to insert five random numbers into a list

<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 <cstdlib>
  2. include <iostream>
  3. include <vector>
  4. include <deque>
  5. include <list>
  6. include <set>
  7. include <map>
  8. include <string>
  9. include <algorithm>
  10. include <iterator>
  11. include <functional>
  12. include <numeric>

/* PRINT_ELEMENTS()

* - prints optional C-string optcstr followed by
* - all elements of the collection coll
* - separated by spaces
*/

template <class T> inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="") {

   typename T::const_iterator pos;
   std::cout << optcstr;
   for (pos=coll.begin(); pos!=coll.end(); ++pos) {
       std::cout << *pos << " ";
   }
   std::cout << std::endl;

} /* INSERT_ELEMENTS (collection, first, last)

* - fill values from first to last into the collection
* - NOTE: NO half-open range
*/

template <class T> inline void INSERT_ELEMENTS (T& coll, int first, int last) {

   for (int i=first; i<=last; ++i) {
       coll.insert(coll.end(),i);
   }

} using namespace std; int main() {

   list<int> coll;
   // insert five random numbers
   generate_n (back_inserter(coll),      // beginning of destination range
               5,                        // count
               rand);                    // new value generator
   PRINT_ELEMENTS(coll);

}</source>

41 18467 6334 26500 19169

Use generate_n to list to Random Values

<source lang="cpp">#include <algorithm>

  1. include <vector>
  2. include <list>
  3. include <iostream>

using namespace std; int main (){

   list <int> l (10);
   generate_n (l.begin (), 5, rand);
   cout << "Elements in the list of size: " << l.size ();
   list <int>::const_iterator li;
   
   for ( li = l.begin (); li != l.end (); ++ li )
       cout << *li << " ";
   return 0;

}</source>

Use std::generate_n to fill a vector by number

<source lang="cpp">#include <iostream> using std::cout; using std::endl;

  1. include <algorithm>
  2. include <vector>
  3. include <iterator>

char nextLetter(); int main() {

  std::vector< char > chars( 10 );
  std::ostream_iterator< char > output( cout, " " );
  // generate values for first five elements of chars with nextLetter
  std::generate_n( chars.begin(), 5, nextLetter );
  cout << "\n\nVector chars after generating letters A-J:\n";
  std::copy( chars.begin(), chars.end(), output );
  cout << endl;
  return 0;

} // generator function returns next letter (starts with A) char nextLetter() {

  static char letter = "A";
  return letter++;

}</source>

Vector chars after generating letters A-J:
A B C D E