python function argument update

Question

I am working on a function to print the list elements in reverse order using recursion. I came up with the following code

    class Solution:
    def __init__(self):
        self.count=0
    def reverseString(self, s):

        def helper(s):
            """
            Do not return anything, modify s in-place instead.
            """
            print(s[:])
            if len(s)>1:
                s[0],s[len(s)-1]=s[len(s)-1],s[0]
                print('s[0]',s[0])
                print('s[len(s)-1]',s[len(s)-1])
                helper(s[1:len(s)-1])
        helper(s)

As you see, I am using print statements to debug the code. I get the following output

['h', 'e', 'l', 'p', 'o']
s[0] o
s[len(s)-1] h
['e', 'l', 'p']
s[0] p
s[len(s)-1] e
['l']
['o', 'e', 'l', 'p', 'h']

I see that my logic is working that there is something fundamental I am missing about variable update at local and global level. Can someone explain to me why I am swapping the first and last list element but my list output is not correct? I expect the output to be ['o', 'p', 'l', 'e', 'h'] On the other hand below modification seems to work fine

class Solution:
    def __init__(self):
        self.count=0
    def reverseString(self, s):

        def helper(left,right):
            """
            Do not return anything, modify s in-place instead.
            """
            print(s[:])
            if left<right:
                s[left],s[right]=s[right],s[left]
                print('s[0]',s[left])
                print('s[len(s)-1]',s[right])
                helper(left+1,right-1)
        helper(0,len(s)-1)

x=Solution()
s=["h","e","l","p","o"]
x.reverseString(s)
print(s)

['h', 'e', 'l', 'p', 'o']
s[0] o
s[len(s)-1] h
['o', 'e', 'l', 'p', 'h']
s[0] p
s[len(s)-1] e
['o', 'p', 'l', 'e', 'h']
['o', 'p', 'l', 'e', 'h']

I looked at the discussion Python inplace update of function arguments? and Immutable vs Mutable types which could possibly be related.

score 1 · Answer 1 · answered Mar 14 '20 at 20:13

1

Your code essentially swaps two elements together and in your last line of code, you are swapping only the first and last. Your code should find a way to swap all elements not just the first and last.

answered Mar 14 '20 at 20:13

Freezzee Shanding

11
2

solid.py · Answer 2 · 2020-03-14T21:19:58.473

I think something like the code below might do the trick.

Notice how the print is done after the recursive call, this is done on purpose, because when the call stack returns it executes all the calls in reverse order, which applies for the prints statements after the recursive calls.

def reverse_print(list):
    if list: # As long as the list is not empty proceed with the recursion / print.
        reverse_print(list[1:]) # Start from the next list element.
        print(list[0]) # The print statement is after the recursive call, on purpose.
    else: # The list is been reduced one element at a time until it reaches 0 length.
        return

s = ["h", "e", "l", "p", "o"]
reverse_print(s)

When run this prints the list of strings containing a single character in reverse order:

o p l e h

python function argument update

2 Answers2