Quick-sort using vector and iterator in STL source code

This snippet submitted by Jianbao Tao on 2013-03-07. It has been viewed 102013 times.
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  /*
 *
 * Quick-sort C++ snippet using vector and iterator in STL.
 *
 * By Jianbao Tao @ SSL, UC Berkeley.
 * 
 * Variable names follow conventions in CLRS.
 *    
 * Interface:
 * 1. partition(A, p, r): Return an index q so that each element in A[p:q-1] is less
 *                        than A[q] and each element in A[q+1:r] is greater than A[q].
 * 2. quickSort(A, p, r): No return.
 * 
 * Operation flow in *partition*:
 * 1. Randomly pick an index irandom in [p, r], and swap A[r] with A[irandom].
 * 2. Use the new A[r] to be the pivot.
 * 3. Make a marching index iless so that it points to the last element that is
 *    less than the pivot and A[ismall+1] is greater than the pivot.
 * 4. Loop over A[p:r-1] with index j. If j > iless and A[j] is less than the pivot,
 *    then swap A[j] and A[iless+1].
 * 5. After the loop, swap A[r] and A[iless+1]
 * 6. Return iless+1 as q, i.e., q is the new position of the pivot.
 * 
 * Operation flow in *quickSort*.
 * if p < r:
 *         q = partition(A, p, r);
 *         quickSort(A, p, q-1);
 *         quickSort(A, q+1, r);
 *
 * Compile command:
 *   clang++ -stdlib=libc++ -std=c++ -o a.out main.cpp
 *
 * Sample output:
 * --------------
 * Original order:
 * 32 37 16 33 57 81 14 4 73 3
 * Sorted order:
 * 3 4 14 16 32 33 37 57 73 81
 *
 */

#include <iostream>
#include <vector>
#include <random>
#include <ctime>

using std::cout;
using std::endl;
using std::vector;

//------------------------------------------------------------------------------
vector<int>::iterator partition(const vector<int> &A, 
                                const vector<int>::iterator &p,
                                const vector<int>::iterator &r) {
    // Get a random element within A[p:r].
    auto seed = clock() * clock() * clock();
    std::default_random_engine dre(seed);
    std::uniform_int_distribution<size_t> di(0, r - p);

    auto random_it = p;
    random_it = p + di(dre);

    // Swap values of random_it and r.
    auto tmp = *random_it;
    *random_it = *r;
    *r = tmp;

    auto pivot = *r;

    int iless = -1;
    for(int i = 0; i < r - p; i++) {
        if(*(p+i) <= pivot) {
            iless++;
            if(iless != i) {
                // Swap *(p+iless) and *(p+i)
                tmp = *(p+iless);
                *(p+iless) = *(p+i);
                *(p+i) = tmp;
            }
        }
    }

    // Swap *(p+iless+1) and *r
    *r = *(p+iless+1);
    *(p+iless+1) = pivot;

    return p + iless + 1;
}


//------------------------------------------------------------------------------
void quickSort(const vector<int> &A, const vector<int>::iterator &p,
                                     const vector<int>::iterator &r) {
    if(p < r) {
        auto q = partition(A, p, r);
        quickSort(A, p, q-1);
        quickSort(A, q+1, r);
    }
}

//------------------------------------------------------------------------------
int main(int argc, char *argv[]) {
    // Make a vector to hold a sample array.
    vector<int> A;

    // Set up random number generator.
    auto seed = clock() * clock() * clock();
    std::default_random_engine dre(seed); // engine
    std::uniform_int_distribution<int> di(0, 100); // distribution

    // Populate A with 10 random number in [0,100].
    int num = 10;
    for(int i = 0; i != num; i++) A.push_back(di(dre)); // Roll the die.

    // Original order.
    cout << \"Original order:\" << endl;
    for(auto it = A.begin(); it != A.end(); it++) 
        cout << *it << \" \";
    cout << endl;

    // Sort.
    auto p = A.begin();
    auto r = A.end() - 1;
    quickSort(A, p, r);

    cout << \"Sorted order:\" << endl;
    for(auto it = A.begin(); it != A.end(); it++)
        cout << *it << \" \";
    cout << endl;

    return 0;
}
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