## Thinking recursively as a problem-solving approach

Recursion is a problem-solving approach that involves a function calling itself to solve a sub-problem. It is a useful tool when creating algorithms as it simplifies the code’s structure, makes it more elegant, and often more efficient.

In this article, we will explore the ins and outs of recursion and discuss how to implement recursive functions in Python.

Recursion is essentially a way of thinking recursively to solve problems. It involves breaking down a problem into smaller sub-problems until the sub-problems become simple enough to solve directly.

This approach helps simplify the code structure, making it elegant and efficient.

### Algorithm for iterative present delivery

Consider an algorithm for delivering presents. An iterative algorithm would involve delivering each present to each child one by one until all the presents have been delivered.

This algorithm can be inefficient if there are many children.

### Algorithm for recursive present delivery

In recursion, the problem is broken down into smaller sub-problems, with an exit condition known as the “base case.” This base case keeps the function from infinitely looping. In the case of present delivery, a recursive algorithm would involve dividing the presents among the children in half, with each child receiving half of the presents.

The algorithm would then recursively divide the remaining presents among the children until each child receives a present.

## Formal definition of recursive functions

A recursive function is a function that calls itself during its execution. It is defined with a base case and a recursive case.

The base case is the condition under which the function stops calling itself, and the recursive case is the condition under which the function calls itself. A recursive function can be implemented with a “return” statement in the base case and a function call in the recursive case.

## Common structure of base case and recursive case

The base case is a defined exit condition that stops the recursion from continuing. The recursive case is the condition under which the function calls itself.

It must be set up in such a way that the input to the recursive call is a smaller subset of the data that was input to the previous call.

### Example of calculating n! recursively

Calculating n! (the factorial of n) recursively involves defining the function to call itself until it reaches the base case (which is 1! = 1).

In the recursive case, we multiply n with the factorial of the previous integer (n-1). When n reaches 1, the base case is triggered, and the function returns 1.

## Conclusion

Recursion is a powerful problem-solving approach that enables elegant and efficient code structures. It involves calling a function repeatedly until the problem is reduced to a simple enough form that can be solved directly.

When implemented correctly, it simplifies the code’s structure, making it elegant and efficient. In this article, we discussed the basics of recursion and how to implement recursive functions in Python.

Remember to always define the base case and recursive case of a recursive function to avoid infinite recursion.

## 3) Maintaining State During Recursion

When working with recursive functions, it’s important to understand how to maintain state throughout the recursive process. State refers to the variables that are being utilized or modified throughout the recursive function call.

To maintain state during recursion, we can either thread state through the function call or store the state in a global variable. Threading state refers to passing the current value that needs to be updated as a parameter to the recursive function call.

For example, suppose we have a function that recursively sums a list of numbers. In that case, we would keep track of the current value (i.e., the index of the list) and the accumulated sum, which is being updated during each call.

### A sample implementation of this function is shown below:

```
def sum_numbers_recursive(numbers, current_num=0, acc_sum=0):
# Base case
if current_num == len(numbers):
return acc_sum
# Recursive case
acc_sum += numbers[current_num]
current_num += 1
return sum_numbers_recursive(numbers, current_num, acc_sum)
```

In this example, we pass the current index of the list and the current accumulated sum as parameters to the recursive function call. These parameters are used to keep track of the state throughout the recursive process.

Alternatively, we can store state in the global scope, which refers to storing the state in a variable outside of the recursive function. This approach is useful when we need to maintain multiple variables’ state across different recursive calls.

For example, suppose we have a function that recursively calculates the factorial of a number. In that case, we would need to maintain the state of the current number and the current factorial throughout the recursive function calls.

### A sample implementation of this function is shown below:

```
factorial = 1
def calculate_factorial_recursive(num):
global factorial
if num == 1:
return factorial
factorial *= num
return calculate_factorial_recursive(num - 1)
```

In this example, we define the factorial variable in the global scope and modify it inside the recursive function call. This approach helps to maintain the current state across all recursive function calls.

## 4) Naive Recursion is Naive

Recursion can be an extremely useful tool, but naive implementation of recursive functions can lead to inefficient code. The Fibonacci sequence is an excellent example of this.

The Fibonacci sequence is a series of numbers where each number is the sum of the two preceding numbers, starting from 0 and 1. So, the sequence would look something like this: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …

We can define the sequence recursively with the recurrence relation F(n) = F(n-1) + F(n-2), and the base case F(0) = 0 and F(1) = 1. The naive implementation of the Fibonacci recursion function includes a recursive call for both F(n-1) and F(n-2), as shown below:

```
def naive_fibonacci(n):
# Base cases
if n <= 1:
return n
# Recursive case
return naive_fibonacci(n-1) + naive_fibonacci(n-2)
```

The issue with this naive implementation is that it leads to exponential complexity.

Each recursive call leads to two more calls, so the number of function calls quickly explodes. For example, to calculate F(5), the function calls would proceed as follows:

```
F(5) = F(4) + F(3)
F(4) = F(3) + F(2)
F(3) = F(2) + F(1)
F(2) = F(1) + F(0)
```

In this example, we can see that F(3) is being calculated twice, and F(2) and F(1) are being calculated thrice.

This redundancy leads to a lot of unnecessary function calls and slows down the program. To address the inefficiency of the naive implementation, we can use caching techniques like the lru_cache in Python.

Caching stores the result of a particular function call so that it can be reused for future calls with the same input parameters.

### The improved implementation of the Fibonacci recursion function using caching is shown below:

```
from functools import lru_cache
@lru_cache(maxsize=None)
def cached_fibonacci(n):
# Base cases
if n <= 1:
return n
# Recursive case
return cached_fibonacci(n-1) + cached_fibonacci(n-2)
```

By storing the result of a Fibonacci function call, we can reuse the result in future function calls with the same input parameter. This caching technique improves the performance of the function significantly and reduces the number of unnecessary function calls.

## Conclusion

In conclusion, understanding how to maintain state during recursion and implementing recursive functions efficiently are essential for writing high-performance recursive code. Threading state and storing state in global variables are two approaches for maintaining state during recursive function calls.

Additionally, caching techniques like the lru_cache function help to improve the performance of recursive functions by reducing the number of redundant function calls.

## 5) Conclusion

Recursion is a powerful problem-solving technique that simplifies code structure and often leads to more elegant and efficient algorithms. By dividing a problem into smaller sub-problems and solving them recursively, we can solve complex problems with ease.

To recap, recursion is a thinking approach that can be used to solve a wide range of problems. It involves breaking down a complex problem into smaller sub-problems and solving them recursively.

By defining a base case and a recursive case, we can set up a recursive function that calls itself with a smaller set of inputs and simplifies the problem. On a lighter note, we can apply recursion in humorous ways as well.

### One classic example of recursion in action is the following joke:

How do you tell a programmer’s child from a playground full of kids? – The programmer’s child will always try to solve the problem recursively by dividing the playground into smaller sub-problems.

This joke highlights how recursion is not only a problem-solving approach but also a mindset that can become ingrained in a programmer’s thinking process. In conclusion, recursion is a valuable tool for solving problems and simplifying code structures.

By understanding the principles of recursion and implementing it efficiently, we can create elegant and efficient code that solves complex problems with ease. But it’s also important to recognize when recursion isn’t the best approach and to have a sense of humor about the recursive mindset that it can create.

Recursion is a powerful problem-solving approach that involves dividing a complex problem into smaller sub-problems and solving them recursively. It simplifies the code structure, making it elegant and efficient, and has various use cases ranging from simple functions to complex algorithms.

To implement recursion efficiently, one must maintain state during recursion, use caching techniques to avoid redundant function calls, and understand when recursion is not the best approach. Recursion is not only a valuable tool, but it also ingrains a specific mindset, which can prove beneficial when solving problems.

Humorous examples of recursion indicate its wide usage and emphasize the importance of knowing and thinking recursively.