Initializing a Container with Random Numbers

Problem

You want to fill an arbitrary container with random numbers.

Solution

You can use either the generate or generate_n functions from the <algorithm> header with a functor that returns random numbers. See Example 11-13 for an example of how to do this.

Example 11-13. Initializing containers with random numbers

#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>
#include <cstdlib>

using namespace std;

struct RndIntGen
{
  RndIntGen(int l, int h)
    : low(l), high(h)
  { }
  int operator( )( ) const {
    return low + (rand( ) % ((high - low) + 1));
  }
private:
  int low;
  int high;
};

int main( ) {
  srand(static_cast<unsigned int>(clock( )));
  vector<int> v(5);
  generate(v.begin( ), v.end( ), RndIntGen(1, 6));
  copy(v.begin( ), v.end( ), ostream_iterator<int>(cout, "\n"));
}

The program in Example 11-13 should produce output similar to:

3
1
2
6
4

Discussion

The standard C++ library provides the functions generate and generate_n specifically for filling containers with the result of a generator function. These functions accept a nullary functor (a function pointer or function object with no arguments) whose result is assigned to contiguous values in the container. Sample implementations of the generate and generate_n functions are shown in Example 11-14.

Example 11-14. Sample implementations of generate and generate_n

template<class Iter_T, class Fxn_T>
void generate(Iter_T first, Iter_T last, Fxn_T f) {
  while (first != last) *first++ = f( );
}

template<class Iter_T, class Fxn_T>
void generate_n(Iter_T first, int n, Fxn_T f) {
  for (int i=0; i < n; ++i) *first++ = f( );
}