我需要帮助，用Python及其在此递归语句中的作用来绕过return语句

Question

No this isn't homework but it is on our study guide for a test. I need to understand the role the return statement plays and the role recursion plays. I don't understand why the function doesn't break after x = 1.

def thisFunc(x):
    print(x)
    if x>1:
         result=thisFunc(x-1)
         print(result)
    return x+1

Sorry, I understand how elementary this is but I could really use some help. Probably why I can't find an explanation anywhere...because it's so simple.

edit: Why does it print out what it does and what and why is the value of x at the end? sorry if I'm asking a lot I'm just frustrated

Answer 1

When you enter the function with a value n>1 it prints the current value, and then calls it's self with n-1 . When the inner function returns it returns the value n - 1 + 1 which is just n . Hence, the function prints out the value n twice, once before the inner recursion and once after.

If n == 1 , which is the base case, the function only prints 1 once and does not call it self again (and hence does not get result back to print). Instead it just returns, hence why 1 is only printed once.

Think of it like an onion.

calling thisFunc(n) will result in

n
# what ever the output (via print) of thisFunc(n-1) is
n 

Answer 2

I don't understand why the function doesn't break after x = 1.

But it does:

>>> ================================ RESTART ================================
>>> x = 1
>>> def thisFunc(x):
    print("Function called on x-value: ", x)
    if x > 1:
        result = thisFunc(x-1)
        print(result)
    return x+1

>>> thisFunc(x)
Function called on x-value:  1
2
>>>

edit: Why does it print out what it does and what and why is the value of x at the end? Well, it prints it out because you're telling it to. Try following the value of x as you go through the function ("x is one, one is not bigger than 1; return 1+1. Ok. [new case] x is two, two is bigger than 1..." and so on).

return and recursion are part and parcel of programming; return statements designates the end of a function (even if you might have several lines more of code) and they also pass data back to whatever asked them for it. In your case you're asking "what happens when x is 1, given these rules?"; the returned data is your answer.

Recursion is simply the matter of letting the function call itself, should it (you) need to. You simply tell the program that "hey, as long as x is bigger than 1, call this function [that just so happens to be the same function initially called] on it and let it do its thing". To get a better understanding of your function I'd suggest that you add the line "Function called on x-value: " to the first print statement inside the function, or at least something that lets you identify which printed line is x and which is result .

For a more in-depth explanation on recursion, I recommend Recursion explained with the flood fill algorithm and zombies and cats