How to create a recursive separator?-CodePudding

I am trying to learn recursion and am separating odd and even values in two lists and merging them to another list as below:

Code:

def separateNumbers(L):
    evenList = []
    oddList = []
    main = []
    if len(L)==0:
        return L
    if L[0] % 2 == 0:
        evenList.append(L[0])
        separateNumbers(L[1:])
    if L[0] % 2 == 1:
        oddList.append(L[0])
        separateNumbers(L[1:])
    main.append(evenList)
    main.append(oddList)
    return main

inputList = [1,2,3,4,5,6,7,8,9,10]
L = separateNumbers(inputList)
print(L)

Input:

L = [1,2,3,4,5,6]

Output:

[[1,3,5], [2,4,6]]

The even and odd arrays reset everytime the recursive function is called, how can I fix this?

Tried with inner function:

def separateNumbers(L):
    evenList = []
    oddList = []
    main = []
    def inner(L):
        if len(L)==0:
            return L
        if L[0] % 2 == 0:
            evenList.append(L[0])
            inner(L[1:])
        if L[0] % 2 == 1:
            oddList.append(L[0])
            inner(L[1:])
        main.append(evenList)
        main.append(oddList)
        return main
    a = inner(L)
    return a

Output:

[[2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 
10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9], [2, 4, 6, 8, 10], [1, 3, 5, 7, 9]]

CodePudding user response：

You don't need a nested function. try:

def separate_numbers(lst):
    if not lst: # empty list
        return [], []
    odd, even = separate_numbers(lst[1:]) # recursion call
    if lst[0] % 2: # if the first item is odd
        return [lst[0], *odd], even
    else: # if even
        return odd, [lst[0], *even]

lst = [1,2,3,4,5,6,7,8,9,10]
print(separate_numbers(lst)) # ([1, 3, 5, 7, 9], [2, 4, 6, 8, 10])

The function calls itself using the tail part of the input list, receiving two lists: odd for odd numbers and even for even numbers. Then it returns those lists, after attaching the head element lst[0] to one of the lists.