Python recursion confusion

Question

def recursion(x):
    answer = 0
    if( x > 10 ):
        answer +=1
        return recursion( x - 1)
    return answer
recursion(15)

I'm just playing around with python and recursive functions and made this, but strangely having errors.

Why is this printing 0 instead of my expected answer of:

5

Answer 1

you need to add the recursive call to your answer, not return it

def recursion(x):
    answer = 0
    if( x > 10 ):
        answer = answer + 1 + recursion( x - 1)
    return answer

sometimes it is helpful to step through your code ... debuggers do this excellently, however you can also use pythontutor.com to visualize execution.

this will visually show you what happened with your old code and why it was broken

Answer 2

You need to make answer a global variable:

answer = 0

def recursion(x):
    global answer
    ...

Answer 3

Because the answer you define is in local scope .

It means the answer in this function call is not the same as in last call .

Your code should be

def recursion(x , answer):
    if( x > 10 ):
        answer +=1
        return recursion( x - 1 , answer)
    return answer
print(recursion(15 , 0))

Answer 4

What is happening is that every time you call recursively the function the value of answer is set to 0. You can change it to:

answer = 0

def recursion(x):
    global answer
    if( x > 10 ):
        answer += 1
        return recursion( x - 1)
    return answer

recursion(15)

And a better implementation would be:

def recursion(x):
    if( x > 10 ):
        return 1 + recursion(x - 1)
    return 0

Answer 5

Cuz your answer is a local variable,every time you call the function,there is a "new answer para",just like that:

recursion(15):answer=0 answer+=1

recursion(14):answer=0 answer+=1

...

recursion(10):answer=0 jump if then return the answer to recursion(11), then to recursion(12)...... the right way for your program should be:

answer = 0
def recursion(x):
    global answer
    if( x > 10 ):
        answer +=1
        return recursion( x - 1)
    return answer

Answer 6

The variable answer allways return 0.

A better solution would:

def recursion(x, answer):
    if( x > 10 ):
        answer +=1
        return recursion( x - 1, answer)
    return answer
recursion(15, 0)