quicksort in python always return a unsorted array-CodePudding

i defined a quicksort in python.It printed a sorted list, but the return list is unsorted. [1, 5, 7, 8] #this is result of print(array) in quicksort [7, 5, 8, 1] #this is result of testing it's so weired

def quickSort(array):
    if len(array) <= 1:
        return array

    p = 0  #pivot
    pointer = p   1
    left = []
    pivot = []
    right = []
    pivot.append(array[p])
    
    #devide array into 3 arrays
    while pointer < len(array):
        if array[pointer] < array[p]:
            left.append(array[pointer])
        elif array[pointer] > array[p]:
            right.append(array[pointer])
        else:
            pivot.append(array[pointer])
            #pointer moves towards right
        pointer  = 1
    
    print(left)
    print(pivot)
    print(right)
    array = quickSort(left)   pivot   quickSort(right)
    print(array)
    #concatenation of the 3 arrays
    return (array)
    
array = [7,5,8,1] 
quickSort(array)
print(array)

CodePudding user response：

In the event that you pass arguments like whole numbers, strings or tuples to a function, the passing is like call-by-value because you can not change the value of the immutable objects being passed to the function.

Example:-

string = "Yash"
def test(string):
    string = "Yash Mehta"
    print("Inside Function:", string)

test(string)  #Yash Mehta
print("Outside Function:", string) #Yash

you can store the answer by making another variable which store list..

def quickSort(array):
    if len(array) <= 1:
        return array
    p = 0  #pivot
    pointer = p   1
    left = []
    pivot = []
    right = []
    pivot.append(array[p])
    
    #dvide array into 3 arrays
    while pointer < len(array):
        if array[pointer] < array[p]:
            left.append(array[pointer])
        elif array[pointer] > array[p]:
            right.append(array[pointer])
        else:
            pivot.append(array[pointer])
            #pointer moves towards right
        pointer  = 1
    
    print(left)
    print(pivot)
    print(right)
    array = quickSort(left)   pivot   quickSort(right)
    print(array)
    #concatenation of the 3 arrays
    return array
    
array = [7,5,8,1] 
sorted_array=quickSort(array)
print(sorted_array)

Output:

[5, 1]
[7]
[8]
[1]
[5]
[]
[1, 5]
[1, 5, 7, 8]
[1, 5, 7, 8]

CodePudding user response：

You are confusing pointer and reference in Java (or C# or C ) language. Your problem is the functional programming paradigm. Python supports 3 paradigms simultaneously. Therefore, when you give an argument to a function, the value of the argument does not normally change.

You need to assign function return to variable to use it.

def quickSort(array):
    if len(array) <= 1:
        return array

    p = 0  # pivot
    pointer = p   1
    left = []
    pivot = []
    right = []
    pivot.append(array[p])

    # devide array into 3 arrays
    while pointer < len(array):
        if array[pointer] < array[p]:
            left.append(array[pointer])
        elif array[pointer] > array[p]:
            right.append(array[pointer])
        else:
            pivot.append(array[pointer])
            # pointer moves towards right
        pointer  = 1

    print(left)
    print(pivot)
    print(right)
    array = quickSort(left)   pivot   quickSort(right)
    print(array)
    # concatenation of the 3 arrays
    return array


array = [7, 5, 8, 1]
array = quickSort(array)
print(array)
# or
response_array = quickSort(array)
print(response_array)

CodePudding user response：

# a is an array 
def quickSort(a):
    if len(a) <2 : return a
    else:
        pivot = a[0]
        less = [i for i in a[1:] if i <= pivot]
        print("less :", less)
        greater = [i for i in a[1:] if i > pivot]
        print("greater : " , greater)
        return quickSort(less)   [pivot]   quickSort(greater)
print(quickSort([10, 5, 2,100, 3,7,9]))