This Python program finds the sum of elements in a list using recursion.

Recursion is a programming technique where a function calls itself in order to solve a problem. To find the sum of elements in a list using recursion, we can break down the problem into smaller parts.

## Problem statement

You are given a list of integers. Write a Python program to calculate the sum of all the elements in the list using recursion.

## Python Program to Find the Sum of Elements in a List using Recursion

def recursive_sum(lst): # Base case: if the list is empty, return 0 if not lst: return 0 else: # Recursive case: sum the first element with the sum of the rest of the elements return lst[0] + recursive_sum(lst[1:]) # Example usage: my_list = [1, 2, 3, 4, 5] result = recursive_sum(my_list) print("The sum of elements in the list is:", result)

## How it works

To understand how the Python program to find the sum of elements in a list using recursion works, let’s break down the code step by step:

def recursive_sum(lst):

# Base case: if the list is empty, return 0

if not lst:

return 0

else:

# Recursive case: sum the first element with the sum of the rest of the elements

return lst[0] + recursive_sum(lst[1:])

**Function Definition**: We define a function called`recursive_sum`

that takes a single argument,`lst`

, which is the list of integers we want to find the sum of.**Base Case**: The function starts with a base case check:

if not lst:

return 0

- If the input list
`lst`

is empty (i.e., its length is 0), the function returns 0 immediately. This is the termination condition for the recursion. When we reach an empty list, we know there are no elements left to sum, so we return 0. **Recursive Case**: If the input list`lst`

is not empty, we enter the else block:

return lst[0] + recursive_sum(lst[1:])

`lst[0]`

represents the first element of the list. We add this element to the result.`recursive_sum(lst[1:])`

is where the magic of recursion happens. We call the`recursive_sum`

function again with the remaining part of the list, which is all elements except the first one (`lst[1:]`

).

Here’s a visualization of how the program works for the example `my_list = [1, 2, 3, 4, 5]`

:

- First Call:
`recursive_sum([1, 2, 3, 4, 5])`

returns`1 + recursive_sum([2, 3, 4, 5])`

- Second Call:
`recursive_sum([2, 3, 4, 5])`

returns`2 + recursive_sum([3, 4, 5])`

- Third Call:
`recursive_sum([3, 4, 5])`

returns`3 + recursive_sum([4, 5])`

- Fourth Call:
`recursive_sum([4, 5])`

returns`4 + recursive_sum([5])`

- Fifth Call:
`recursive_sum([5])`

returns`5 + recursive_sum([])`

Now, the base case is reached with an empty list `[]`

, so it returns 0.

The results are propagated back up the recursion stack, and the final result is calculated as follows:

1 + (2 + (3 + (4 + (5 + 0))))

1 + (2 + (3 + (4 + 5)))

1 + (2 + (3 + 9))

1 + (2 + 12)

1 + 14

15

So, the sum of elements in the list `[1, 2, 3, 4, 5]`

is indeed 15, as calculated by the recursive function.

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