generate, generate_n

Algorithm

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Initialize a container with values produced by a value-generator class.

None

#include <algorithm>

template <class ForwardIterator, class Generator>
  void generate(ForwardIterator first, ForwardIterator last,
                Generator gen);

template <class OutputIterator, class Size, class Generator>
  void generate_n(OutputIterator first, Size n, Generator gen);

A value-generator function returns a value each time it is invoked. The algorithms generate and generate_n initialize (or reinitialize) a sequence by assigning the return value of the generator function gen to all the elements designated by iterators in the range [first, last) or [first, first + n). The function gen takes no arguments. (gen can be a function or a class with an operator () defined that takes no arguments.)

generate_n assumes that there are at least n elements following first, unless first is an insert iterator.

The generate and generate_n algorithms invoke gen and assign its return value exactly last - first (or n) times.

// 
// generate.cpp 
//
 #include <algorithm>
 #include <vector>
 #include <iostream>
 using namespace std;

 // Value generator simply doubles the current value 
 // and returns it 
 template <class T>
 class generate_val
 {
   private:
      T val_;
   public:
      generate_val(const T& val) : val_(val) {}
      T& operator()() { val_ += val_; return val_; }
 };

 int main()
 {
   int d1[4] = {1,2,3,4};
   generate_val<int> gen(1);

   // Set up two vectors
   vector<int> v1(d1,d1 + 4), v2(d1,d1 + 4);
   // Set up one empty vector
   vector<int> v3;

   // Generate values for all of v1
   generate(v1.begin(),v1.end(),gen);

   // Generate values for first 3 of v2
   generate_n(v2.begin(),3,gen);

   // Use insert iterator to generate 5 values for v3 
   generate_n(back_inserter(v3),5,gen);

   // Copy all three to cout
   ostream_iterator<int,char> out(cout," ");
   copy(v1.begin(),v1.end(),out);
   cout << endl;
   copy(v2.begin(),v2.end(),out);
   cout << endl;
   copy(v3.begin(),v3.end(),out);
   cout << endl;

   // Generate 3 values for cout
   generate_n(ostream_iterator<int>(cout," "),3,gen);
   cout << endl;

   return 0;
 }

Program Output

2 4 8 16
2 4 8 4
2 4 8 16 32
2 4 8

If your compiler does not support default template parameters, then you always need to supply the Allocator template argument. For instance, you have to write:

vector<int, allocator<int> >

instead of:

vector<int>

If your compiler does not support namespaces, then you do not need the using declaration for std.

Function Objects