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Don't understand recursion? No worries, this conversation between the boss and the secretary will make you understand it in no time!!!

#python脚本103
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Recursion is a key concept in programming that often confuses beginners. It involves a function calling itself to break down complex problems into smaller subproblems until a base case is reached, where the function stops calling itself and returns a result. A classic example is the factorial function, which has a base condition and a recursive call. The document also illustrates recursion through a story involving a boss, a secretary, and a series of phone calls that reflect the recursive nature of communication, where each character passes on a message until the situation changes and the cycle restarts. The Python implementation simulates this story using a recursive function that prints each character's message and demonstrates the flow of communication.

Recursion is an unavoidable key concept in programming learning, and many beginners often feel confused when they first encounter it. So, what exactly does recursion mean?

Recursion is a method where a function calls itself. Typically, recursion breaks down a complex problem into smaller subproblems until a base case is reached (i.e., it no longer calls itself and directly returns a result).

A recursive function usually has two main parts:

  1. Base condition: When a certain condition is met, recursion stops, and the function no longer calls itself.

  2. Recursive call: The function calls itself while solving the problem, but the scale of the problem gradually decreases.

A classic example of recursion is calculating the factorial, defined as:

image

def factorial(n):    
    if n == 1:  # Base condition: return 1 when n is 1 
        return 1   
    else:       
        return n * factorial(n - 1)  # Recursive call

Next, let's use a real-life example to understand recursion:

The boss calls the secretary:

"Get ready for the weekend! We are going on a business trip for two days."

The secretary immediately calls her husband:

"Dear, I am going on a business trip with the boss, you have to take care of yourself for the next two days."

The husband smirks and calls his girlfriend:

"Baby, my wife is on a business trip, come over to my place, we can have some fun!"
The girlfriend's eyes light up and she quickly informs the boy she tutors:

"You are free this weekend, the tutoring is canceled!" The boy jumps up happily and immediately calls his dad:

"Dad, I can finally spend this weekend with you!" The boss hears his son's words and instantly softens, picking up the phone to call the secretary

and says:
"The business trip is canceled, I want to spend the weekend with my son."

The secretary sighs and calls her husband:
"We're not going, we'll stay home this weekend."

The husband is anxious and quickly messages his girlfriend:
"Sorry, my wife is not going on the trip, the plan is canceled!"

The girlfriend helplessly calls the boy:
"Sorry, the tutoring continues, hurry up and prepare the tuition."

The boy looks distressed and calls his dad:
"Sorry Dad, I still have to go to class..."

The boss sighs deeply and silently picks up his phone to call the secretary again:
"Well, get ready for the business trip..."

Implementing the above story in Python:

def phone_story(state):
    characters = [
        "The boss calls the secretary and says: 'Get ready for the weekend, we are going on a business trip.'",
        "The secretary calls her husband and says: 'I am going on a business trip with the boss for two days, you need to take care of yourself.'",
        "The husband calls his girlfriend and says: 'My wife is on a business trip, come over, we can have some fun.'",
        "The girlfriend calls the boy she tutors and says: 'You are free this weekend, no tutoring.'",
        "The boy calls his father and says: 'Dad, we can finally spend this weekend together.'",
        "The father (boss) calls the secretary and says: 'The business trip is canceled. I want to spend the weekend with my son.'",
        "The secretary calls her husband: 'I'm not going.'",
        "The husband calls his girlfriend: 'Sorry, my wife is not going.'",
        "The girlfriend calls the boy: 'You need to pay the tuition.'",
        "The boy calls his father and says: 'Sorry Dad, I have to go to class.'"
    ]
    
    print(len(characters))
    print(state)
    
    if state == len(characters):
        print("The father calls his secretary, the story loops back to the beginning...\n")
        return
    
    # Progressing layer by layer
    print(characters[state])  # Recursive call, simulating each person passing the phone step by step
    phone_story(state + 1)  # Backtracking, characters start changing decisions
    print(characters[len(characters) - state - 1])

# Start from the first phone call in the story
phone_story(0)
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