How to Calculate Sum of List Integers Using Recursion in Python?

Understanding Recursion in Python: Summing List Integers

Recursion is a programming technique where a function calls itself repeatedly to solve smaller instances of a problem until a base condition is reached. In Python, we can apply recursion to various tasks, including calculating sums of list integers.

Recursive Function: listSum

To define a recursive function listSum that takes a list of integers and returns their sum, we can break down the problem as follows:

• Base Condition: If the list is empty (list == []), the sum is 0.
• Recursive Step: If the list is not empty, the sum is the first element (list[0]) plus the sum of the remaining elements (listSum(list[1:])).

Simple Version:

<code class="python">def listSum(ls):
    # Base condition
    if not ls:
        return 0

    # First element + result of calling `listsum` with rest of the elements
    return ls[0] + listSum(ls[1:])</code>

Tail Call Recursion:

To improve efficiency, we can pass the current sum to the function parameter:

<code class="python">def listSum(ls, result):
    # Base condition
    if not ls:
        return result

    # Call with next index and add the current element to result
    return listSum(ls[1:], result + ls[0])</code>

Passing Around Index Version:

To avoid creating intermediate lists, we can pass the index of the current element:

<code class="python">def listSum(ls, index, result):
    # Base condition
    if index == len(ls):
        return result

    # Call with next index and add the current element to result
    return listSum(ls, index + 1, result + ls[index])</code>

Inner Function Version:

To simplify code, we can define a recursive inner function:

<code class="python">def listSum(ls):

    def recursion(index, result):
        # Base condition
        if index == len(ls):
            return result

        # Call with next index and add the current element to result
        return recursion(index + 1, result + ls[index])

    return recursion(0, 0)</code>

Default Parameters Version:

Using default parameters, we can simplify further:

<code class="python">def listSum(ls, index=0, result=0):
    # Base condition
    if index == len(ls):
        return result

    # Call with next index and add the current element to result
    return listSum(ls, index + 1, result + ls[index])</code>

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