Explain how this python while loop returns the reverse order of elements in a list

Question

Could someone explain to me how this python loop outputs the reverse order of elements in a list? This was part of a school assignment and I can't quite comprehend how this works.

I know this is a more difficult way of obtaining the reverse order of a list, but we were not allowed to use the reverse function and I could not seem to get list splicing to pass the autograder.

def reverse_list(stuff):
    '''Function that reverses the order of elements in a list'''
    i = 0
    n = len(stuff)-1
    while i<n:
        temp = stuff[i]
        stuff[i] = stuff[n]
        stuff[n] = temp
        i+=1
        n-=1
#test
stuff = [1,2,3,4]
reverse_list(stuff)
print(list) #Output [4,3,2,1]

temp isn't necessary; stuff[i], stuff[n] = stuff[n], stuff[i]. — chepner
– chepner, Commented Nov 9, 2019 at 18:15

azro · Accepted Answer · 2019-11-09 18:02:39Z

1

Using better names, and a print you may better understand

def reverse_list(stuff):
    begin = 0
    end = len(stuff) - 1
    while begin < end:
        temp = stuff[begin]
        stuff[begin] = stuff[end]
        stuff[end] = temp
        begin += 1
        end -= 1
        print(stuff)

For input [1, 2, 3, 4, 5, 6, 7, 8, 9] you'll see

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[9, 2, 3, 4, 5, 6, 7, 8, 1] # swap 1 and 9
[9, 8, 3, 4, 5, 6, 7, 2, 1] # swap 2 and 8
[9, 8, 7, 4, 5, 6, 3, 2, 1] # swap 3 and 7
[9, 8, 7, 6, 5, 4, 3, 2, 1] # swap 4 and 6

The way is, getting two indexes

one starting at the beginning and incrementing : begin
one startting at the end and decrementing : end

And each time on swap then using a third variable temp, and 2 by 2 you're going to swap all, until you reach the middle

answered Nov 9, 2019 at 18:02

azro

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