Krdytkyu

I have written an implementation of Quicksort in Python. I am new to Python. Any suggestions for improvement or criticism on my use of Python?

def partition(a, lo, hi):

    i, j, v = lo+1, hi, a[lo]

    while(True):

        while(a[i] < v):

            i += 1

            if (i == hi): break    

        while(a[j] > v):

            j -= 1

            if (j == lo): break    

        if (i >= j): break            

        a[i], a[j] = a[j], a[i]    

    a[lo], a[j] = a[j], a[lo]

    return j



def sort(a, lo, hi):

    if (hi <= lo):

        return

    q = partition(a, lo, hi)

    sort(a, lo, q-1)

    sort(a, q+1, hi)

    assert isSorted(a, lo, hi)



def quick_sort(a):

    shuffle(a)

    sort(a, 0, len(a)-1)

    assert isSortedArray(a)



def isSorted(a, lo, hi):

    for i in range(lo, hi):

        if a[i+1] < a[i]:

            return False

    return True



def isSortedArray(a):

    for i in range(0, len(a)-1):

        if a[i+1] < a[i]:

            return False

    return True

When describing quicksort partitioning, your v is typically called the "pivot". The code would be clearer if you named the variable according to that convention.

You always choose a[lo] as the pivot. However, that produces pathological performance when the input array is already sorted.

I would prefer to see

while(a[i] < v):

      i += 1

      if (i == hi): break

… written as

while i < hi and a[i] < pivot:

    i += 1

Array index bounds usually work better when specified as inclusive-exclusive ranges, such that sort(a, lo, hi) means "sort a where lo ≤ index < hi". This is a common convention — you can see it in Python's range() and slicings. Also, Java's Arrays.sort(a, fromIndex, toIndex) works with inclusive-exclusive ranges.

Some nice properties of inclusive-exclusive ranges are:

• 
  hi - lo gives you the number of elements in the range.


• When creating a range for the entire array a, hi is just len(a). You save a "-1".


• When splitting [lo, hi) into two consecutive ranges, it becomes [lo, mid) and [mid, hi). You save a "-1".


• In Python, you can conveniently write for i in range(lo, hi) for the most common type of iteration. (Admittedly, iterating backwards is uglier, but it's less common.)