Python中的QuickSort。 阵列故障增加

Question

I am trying to implement quicksort in python. Problem is how to increment/decrement value of i/j in array a . I know that I should write i=i+1 and there are no such thing like i++ in python, but I don't understand in which way I should do this. I am newbie, here is my code.

def quicksort(a,lo,hi):
    if(hi<=lo):
        return
    i = lo - 1
    j = hi
    v = a[hi]

    while True:
        while(a[++i] < v):
            pass

        while(v < a[--j]):
            if(j==lo):
                break
        if(i>=j):
            break
        t = a[i]
        a[i] = a[j]
        a[j] = t

    t = a[i]
    a[i] = a[hi]
    a[hi] = t
    quicksort(a, lo, i - 1)
    quicksort(a, i + 1, hi)

Answer 1

in python, you cannot assign and get the value, that's an intentional limitation to avoid issues with typos, find the proper sequence point...

You have no choice than to "emulate" the C-ported code:

    while(a[++i] < v):
        pass

    while(v < a[--j]):
        if(j==lo):
            break

(note that both constructs generate an infinite loop because:

++i == i

and

--j == j

(applying unary plus any number of times or unary minus an even number of times gives the same number, see Why Don't Two Plus Operators Throw an Error (eg, 1 + + 2) )

So change to:

    i += 1
    while(a[i] < v):
        i += 1

    j -= 1
    while(v < a[j]):
        if(j==lo):
            break
        j -= 1

Answer 2

The following construct does not work the same way in Python as it does in C++:

while(a[++i] < v):

as well as this one:

while(v < a[--j]):

The way you change the code is the following:

def quicksort(a,lo,hi):
    if(hi<=lo):
        return
    i = lo - 1
    j = hi
    v = a[hi]

    while True:
        i += 1
        while(a[i] < v):
            i += 1
            pass

        j -= 1
        while(v < a[j]):
            j -= 1
            if(j==lo):
                break
        if(i>=j):
            break
        t = a[i]
        a[i] = a[j]
        a[j] = t

    t = a[i]
    a[i] = a[hi]
    a[hi] = t
    quicksort(a, lo, i - 1)
    quicksort(a, i + 1, hi)