QuickSort Algorithm just for a exercise
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Thread: QuickSort Algorithm just for a exercise

  1. #1
    Join Date
    Jun 2004
    Location
    UK
    Posts
    1

    QuickSort Algorithm just for a exercise

    Just tried creating a quick sort algorithm in c# to sort an array of integers - from a long distant memory.

    Yes I could of just used the in built quicksort or better still just copied and pasted somebody elses off google.. But anyway, I have noticed at times will not sort the list entirely..

    Just wondered where the problem is exactly?

    Please bare in mind that it has been a long time since I wrote code, so dont be too brutal lol.

    Code:
            static public int[] quickSort(int[] sortArray, int left, int right)
            {
                // if there are less than 2 elements in array or comparison no need to sort
                if ((sortArray.Length < 2) || ((right-left) < 2))
                {
                    return sortArray;
                }
            
                // set random number for pivot
                Random rand = new Random();
                int pivotIndex = rand.Next(left, right);
    
    
                int l = left, r = right;
                
                while (l < r)
                {
                    //leave data that is already in good in relation to index
                    while (sortArray[l] < sortArray[pivotIndex])
                    {
                        l++;
                    }
                    while (sortArray[r] > sortArray[pivotIndex])
                    {
                        r--;
                    }
    
                    // swap elements
                    if (sortArray[l] >= sortArray[r])
                    {
    
                        int temp = sortArray[l];
                        sortArray[l] = sortArray[r];
                        sortArray[r] = temp;
    
                        //continue to seek
                        l++;
                        r--;
                    }
                }
    
                // recursive call to sort either side of pivot
                quickSort(sortArray, left, pivotIndex);
                quickSort(sortArray, pivotIndex, right);
    
    
                return sortArray;
    
            }

  2. #2
    Join Date
    Jan 2009
    Posts
    596

    Re: QuickSort Algorithm just for a exercise

    Code:
            static public int[] quickSort(int[] sortArray, int left, int right)
            {
                // if there are less than 2 elements in array or comparison no need to sort
                if ((sortArray.Length < 2) || ((right-left) < 2))
                {
                    return sortArray;
                }
            // rest of code...
            }
    Suppose you call this on a two element array, so left == 0 and right == 1. Then (right-left) == 1, i.e. is less than 2. Your code will just return in this case without sorting the 2 elements.

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