Use the random_shuffle STL function in Visual C++

2022-05-08

This article describes how to use the random_shuffle Standard Template Library (STL) function in Visual C++.

Original product version: Visual C++
Original KB number: 156994

Required header

<algorithm>

Prototype

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 original version in the header file. They 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 to (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).
// of Microsoft Product Support Services,
// Software Core Developer Support.
// Copyright (c) 1996 Microsoft Corporation. All rights reserved.
//////////////////////////////////////////////////////////////////////

// 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, allocator<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:


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  }

References

For more information about the random_shuffle function, visit random_shuffle (STL Samples).

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