Explain recursive function for finding max element in array

Question

I decided to incorporate recursions into my code and for that I want to train using it. I have a recursive function which finds the max element in the given array.

def maxx(lst):
    if len(lst) == 1:
        return lst[0]
    else:
        m = maxx(lst[1:])
        return m if m > lst[0] else lst[0]

maxx([1,3,2])
> 3

I tried to draw a sequence of events and I can't get why the code above works? Because according to what I see it should be 2 not 3. Where is my mistake? Is there a method that correctly expands the recursion and thus helps you to understand its flow?

Check in your notes the content of the array in each recursion. When you get back the 2 from the last return, you compare it with array[0], which is 3 so you return 3, then you go up again and array[0] is 1 so 3 goes out again. P.S. It's confusing to read that you expect the code to throw out a 2 looking for max value :) — Alfabravo
– Alfabravo, Commented May 11, 2017 at 19:12

Andrew Long · Accepted Answer · 2017-05-11 20:08:35Z

2

Depth 0:

lst = [1,3,2]
m = maxx([3,2])

Depth 1:

lst = [3,2]
m = maxx([2])

Depth 2:

lst = [2]
returning 2

Back to Depth 1:

lst = [3,2]
m = 2
return m (2) if m > lst[0] (3) else return lst[0] (3)
returning 3

Back to Depth 0:

lst = [1,3,2]
m = 3
return m (3) if m > lst[0] (1) else return lst[0] (1)
returning 3

The reply by Alfabravo is correct, I think that's where you lost track of lst[0] as you went back up the tree.

answered May 11, 2017 at 20:08

Andrew Long

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