Is this a correct implementation of quicksort?

Question

I would like to check if this is a correct implementation of QuickSort, It seems to be doing the job, but Am I missing out anything?

public class QuickSort implements Sorter {

public void sort(Comparable[] items) {
    QuickSort(items, 0, items.length - 1);
}

static void QuickSort(Comparable[] items, int a, int b) {
    int lo = a;
    int hi = b;
    if (lo >= hi) {
        return;
    }
    Comparable mid = items[(lo + hi) / 2];
    Comparable T;

    while (lo < hi) {
        while (items[lo].compareTo(mid)<0) {
            lo++;
        }
        while (mid.compareTo(items[hi])<0) {
            hi--;
        }
        if (lo < hi) {
            T = items[lo];
            items[lo] = items[hi];
            items[hi] = T;
        }
    }

    QuickSort(items, a, lo);
    QuickSort(items, lo == a ? lo + 1 : lo, b);

}

}

fixed:

private static void quickSort(Comparable[] items, int a, int b) {
    int i = a;
    int j = b;

    Comparable x = items[(a+ b) / 2];
    Comparable h;

    do {
        while (items[i].compareTo(x) < 0) {
            i++;
        }
        while (items[j].compareTo(x) > 0) {
            j--;
        }
        if (i <= j) {
            h = items[i];
            items[i] = items[j];
            items[j] = h;
            i++;
            j--;
        }
    } while (i <= j);

    if (a < j) {
        quickSort(items, a, j);
    }
    if (i < b) {
        quickSort(items, i, b);
    }
}

Answer 1

1 small point- there's a potential int overflow here:

(lo + hi) / 2

Answer 2

EDIT: My bad for missing the java tag, sorry... The below is C# generic quickSort... I'll leave it here anyway for .net readers...

It looks ok at first glance, but how about this generic one?

public class QuickSort<T> where T : IComparable<T>
{
    #region private variable to sort inplace
    readonly private IList<T> itms;
    #endregion private variable to sort inplace

    #region ctor
    private QuickSort() { } // Hide parameterless constructor
    /// <summary>
    /// Constructor requires IList<T> T must implement CompareTo() method.../>
    /// </summary>
    /// <param name="Lst">List<T> of elements to sort</param>
    public QuickSort(IList<T> Lst):this)() { itms = Lst; }
    #endregion ctor

    #region public sort method
    public void Sort() { Sort(0, itms.Count - 1); }
    /// <summary>
    /// Executes QuickSort algorithm
    /// </summary>
    /// <param name="L">Index of left-hand boundary of partition to sort</param>
    /// <param name="R">Index of right-hand boundary of partition to sort</param>
    private void Sort(long L, long R)
    {
        // Calls iSort (insertion-sort) for partitions smaller than 5 elements
        if (R - L < 4) iSort(L, R); 
        else
        {
            long i = (L + R) / 2, j = R - 1;
            // Next three lines to set upper and lower bounds
            if (itms[L].CompareTo(itms[i]) > 0) Swap(L, i);
            if (itms[L].CompareTo(itms[R]) > 0) Swap(L, R);
            if (itms[i].CompareTo(itms[R]) > 0) Swap(i, R);
            Swap(i, j);
            // --------------------------------
            T p = itms[j]; // p = itms[j] is pivot element
            i = L;
            while (true)
            {
                while (itms[++i].CompareTo(p) < 0) {}
                while (itms[--j].CompareTo(p) > 0) {}
                if (j < i) break;
                Swap(i, j);
            }
            Swap(i, R - 1);
            Sort(L, i);     // Sort  Left partition --  HERE WAS TYPO BUG
            Sort(i + 1, R); // Sort Right partition
        }
    }
    #endregion public sort method

    #region private Helper methods
    private void Swap(long L, long R)
    {
        T t = itms[L];
        itms[L] = itms[R];
        itms[R] = t;
    }
    private void iSort(long L, long R)
    {
        for (long i = L; i <= R; i++)
        {
            T t = itms[i];
            long j = i;
            while ((j > L) && (itms[j - 1].CompareTo(t) > 0))
            {
                itms[j] = itms[j - 1];
                j--;
            }
            itms[j] = t;
        }
    }
    #endregion private Helper methods
}

Answer 3

Take this opportunity to learn how to write a unit-test. (Google on "junit", for example). Generate large arrays and make sure that they are sorted properly, for example: arrays filled with random numbers, arrays filled with 0, 1, Integer.MAX_INT. Try to provoke things like integer overflow and other weird cornercases.

Answer 4

And here's a javascript version... QuickSort(a, comp, desc)

• a is of course the array to be sorted.
• comp is the compare function that has to take two values and return -1, 0 or +1 depending on how the 2 arguments should sort.

• desc is boolean to reverse the sort order.

  function QuickSort(a, comp, desc) {
     function defComp(L, R)  {return((L>R)? 1: (L<R)? -1: 0);}
      var cmp = (comp)? comp: defComp; 
      var siz = a.length;
      qSort(0, siz-1);
      if (desc) reverse();
      // ------------------
      function qSort(L, R) {
          if (R - L < 4) {iSort(L, R);} // insertion-sort--
          else {
              var i = parseInt((L+R) /2),  j=R-1;
              if (cmp(a[L], a[i]) > 0) swap(L,i);
              if (cmp(a[L], a[R]) > 0) swap(L,R);
              if (cmp(a[i], a[R]) > 0) swap(i,R);
              swap(i,j);
              // ------------------------------------------
              var p=a[j]; // p=a[j] is pivot
              i=L;
              while (true) {
                  while(cmp(a[++i], p) < 0); 
                  while(cmp(a[--j], p) > 0);    
                  if (j < i) break;
                  swap(i,j);
              }
              swap(i,R-1);
              qSort(L,i);   // Left  Partition
              qSort(i+1,R); // Right Partition
          }
      }
      function swap(L,R) {var t=a[L];  a[L]=a[R];  a[R]=t;}
      function reverse()
         {for(var i=0; i<parseInt(siz/2); i++) swap(i,(siz-i-1));}
      // insertion-sort 
      function iSort(L,R) {
          var j,t;
          for(var i=L; i<=R; i++) {
              t = a[i], j = i;
              while ((j>L) && (cmp(a[j-1], t) > 0))
                  {a[j] = a[j-1]; j--;}
              a[j] = t;
          }
      }
  }
  

Answer 5

public static void quicksort( Comparable [ ] a ) {

quicksort( a, 0, a.length - 1 );
}
private static final int CUTOFF = 10;
private static void quicksort( Comparable [ ] a, int low, int high ) {
if( low + CUTOFF > high )
insertionSort( a, low, high );
else {
int middle = ( low + high ) / 2;
if( a[ middle ].compareTo( a[ low ] ) < 0 )
swapReferences( a, low, middle );
if( a[ high ].compareTo( a[ low ] ) < 0 )
swapReferences( a, low, high );
if( a[ high ].compareTo( a[ middle ] ) < 0 )
swapReferences( a, middle, high );
swapReferences( a, middle, high - 1 );
Comparable pivot = a[ high - 1 ];
int i, j;
for( i = low, j = high - 1; ; ) {
while( a[ ++i ].compareTo( pivot ) < 0 )
;
while( pivot.compareTo( a[ --j ] ) < 0 )
;
if( i >= j )
break;
swapReferences( a, i, j );
}
swapReferences( a, i, high - 1
quicksort( a, low, i - 1 ); // Sort small elements
quicksort( a, i + 1, high ); // Sort large elements
 }
}
public static final void swapReferences( Object [ ] a, int index1, int index2 )
{
Object tmp = a[ index1 ];
a[ index1 ] = a[ index2 ];
a[ index2 ] = tmp;
}
private static void insertionSort( Comparable [ ] a, int low, int high ) {
for( int p = low + 1; p <= high; p++ ) {
Comparable tmp = a[ p ];
int j;
for( j = p; j > low && tmp.compareTo( a[ j - 1 ] ) < 0; j-- )
a[ j ] = a[ j - 1 ];
a[ j ] = tmp;
}
}

From http://java-blackberry.blogspot.com/2007/12/quick-sort-implementation-with-median.html