Counting Total Number of Elements in a List using Recursion

Question

The Problem:

Count the number of elements in a List using recursion.

I wrote the following function:


def count_rec(arr, i):
    """
        This function takes List (arr) and Index Number
        then returns the count of number of elements in it 
        using Recursion.
    """
    try:
        temp = arr[i]  # if element exists at i, continue
        return 1 + count_rec(arr, i+1)
    except IndexError:
        # if index error, that means, i == length of list
        return 0

I noticed some problems with it:

RecursionError (when the number of elements is more than 990)
Using a temp element (wasting memory..?)
Exception Handling (I feel like we shouldn't use it unless necessary)

If anyone can suggest how to improve the above solution or come up with an alternative one, It would be really helpful.

Since Python has such a low recursion limit, you can't use recursion for this if you need to handle longer lists. — Barmar
– Barmar, Commented May 28, 2020 at 21:04
Btw, you can directly write arr[i] instead of temp = arr[i] — Mustafa Aydın
– Mustafa Aydın, Commented May 28, 2020 at 21:07
The recursion limit can also be changed: docs.python.org/3/library/sys.html#sys.setrecursionlimit. — Dr. V
– Dr. V, Commented May 28, 2020 at 21:09

Bobby Ocean · Accepted Answer · 2020-05-28 22:57:34Z

1

What you have is probably as efficient as you are going to get for this thought experiment (obviously, python already calculates and stores length for LIST objects, which can be retrieved with the len() built-in, so this function is completely unnecessary).

You could get shorter code if you want:

def count(L):
    return count(L[:-1])+1 if L else 0

But you still need to change python's recursion limit.

import sys; sys.setrecursionlimit(100000)

However, we should note that in most cases, "if else" statements take longer to process than "try except". Hence, "try except" is going to be a better (if you are after performance). Of course, that's weird talking about performance because recursion typically doesn't perform very well, due to how python manage's namespaces and such. Recursion is typically frowned upon, unnecessary, and slow. So, trying to optimize recursion performance is a littler strange.

A last point to note. You mention the temp=arr[i] taking up memory. Yes, possibly a few bytes. Of course, any calculation you do to determine if arr has an element at i, is going to take a few bytes in memory even simply running "arr[i]" without assignment. In addition, those bytes are freed the second the temp variable falls out of scope, gets re-used, or the function exits. Hence, unless you are planning on launching 10,000,000,000 sub-processes, rest assure there is no performance degradation in using a temp variable like that.

answered May 28, 2020 at 22:57

Bobby Ocean

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2 Comments

Gagan Deep Singh Over a year ago

when I changed the limit and gave it around 10000 elements, it hung up my pc.

Bobby Ocean Over a year ago

Again, recursion is not efficient. My computer takes 878 ms to resolve 10,000 recursions. If I try 100,000 recursions, my computer idles for like 30 seconds and then kills the process, producing "Killed" to the terminal. There is probably some global timers and other checks to help stop these deep recursions.

HackLab · Accepted Answer · 2020-05-28 21:07:00Z

1

you are prob looking for something like this

def count_rec(arr):
    if arr == []:
        return 0
    return count_rec(arr[1:]) + 1

answered May 28, 2020 at 21:07

HackLab

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Comments

maede rayati · Accepted Answer · 2020-05-28 21:48:34Z

1

You can use pop() to do it.

def count_r(l):
    if l==[]:
        return 0
    else:
        l.pop()
        return count_r(l)+1

edited May 28, 2020 at 21:48

answered May 28, 2020 at 21:07

maede rayati

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4 Comments

HackLab Over a year ago

I mean sure it works, but kind of still uses no recursive count len()

maede rayati Over a year ago

It is a recursive function. It uses an initial condition, and the same function to calculate the length. I have changed the name of the function from count to count_r to not confuse the native count function in python and the name of this function.

HackLab Over a year ago

The name was not my problem, my problem was that you computed, with every recursion, already the result in the line if len(l)==0:, the idea was to replace len

maede rayati Over a year ago

Good catch!! I have changed that.

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