Predicate Version of random_shuffle (STL Sample)

The sample code below illustrates how to use the predicate version of the random_shuffle STL function in Visual C++.

Required Header:
<algorithm>

template<class RandomAccessIterator, class Predicate> inline
     void random_shuffle(RandomAccessIterator first, RandomAccessIterator last, Predicate pred)

Note: The class/parameter names in the prototype do not match the version in the header file. Some have been modified to improve readability.

Description:
The random_shuffle algorithm shuffles the elements of a sequence [first..last) in a random order. The predicate version uses the pred function to generate the indices of the elements to swap. The pred has to be a function object that takes a parameter n and returns an integral random number in the range 0 - (n - 1). The predicate version of random_shuffle uses operator= to perform swaps.

Sample Code:

//////////////////////////////////////////////////////////////////////
//
// Compile options needed: /GX
//
// random_shuffle.cpp: Illustrates how to use the predicate version
//                      of the random_shuffle function.
//
// Functions:
//
//    random_shuffle: Shuffle the elements in a random order.
//
//      Rand: Given n, generates an integral random number in the
//          in the range 0 - (n - 1).
//////////////////////////////////////////////////////////////////////

// disable warning C4786: symbol greater than 255 character,
// okay to ignore
#pragma warning(disable: 4786)

#include <iostream>
#include <algorithm>
#include <functional>
#include <vector>

using namespace std;


// return an integral random number in the range 0 - (n - 1)
int Rand(int n)
{
    return rand() % n ;
}

void main()
{
    const int VECTOR_SIZE = 8 ;

    // Define a template class vector of int
    typedef vector<int > IntVector ;

    //Define an iterator for template class vector of strings
    typedef IntVector::iterator IntVectorIt ;

    IntVector Numbers(VECTOR_SIZE) ;

    IntVectorIt start, end, it ;

    // Initialize vector Numbers
    Numbers[0] = 4 ;
    Numbers[1] = 10;
    Numbers[2] = 70 ;
    Numbers[3] = 30 ;
    Numbers[4] = 10;
    Numbers[5] = 69 ;
    Numbers[6] = 96 ;
    Numbers[7] = 100;

    start = Numbers.begin() ;   // location of first
                                // element of Numbers

    end = Numbers.end() ;       // one past the location
                                // last element of Numbers

    cout << "Before calling random_shuffle:\n" << endl ;

    // print content of Numbers
    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;

    // shuffle the elements in a random order.
    // the pointer_to_unary_function adapter converts a function to a
    // function object.
    random_shuffle(start, end, pointer_to_unary_function<int, int>(Rand));

    cout << "After calling random_shuffle:\n" << endl ;

    cout << "Numbers { " ;
    for(it = start; it != end; it++)
        cout << *it << " " ;
    cout << " }\n" << endl ;
}

 

Program Output is:

Before calling random_shuffle

Numbers { 4 10 70 30 10 69 96 100  }

After calling random_shuffle

Numbers { 10 30 4 70 96 100 69 10  }