Writing python factorial function using recursion

问题

I am new to Python, I am trying to write a Python program to calculate and print factorials up to n! as shown using recursion.

However, I only get the following result:

Here is my code right now:

def factorial( n ):
    print("n n!")
    if n <=1: 
        print(str(n) + ' ' + str(n))
        return 1
    else:
        r = n * factorial( n - 1 ) 
        print(str(n) + ' ' + str(r))
        return r

What is wrong in my code?

回答1:

With recursion:

factorial = lambda n: n * factorial(n - 1) if (n > 1) else 1

This is similar to your function, except I don't print things out (because it slows it down and is unnecessary).

If you try to evaluate it, with, for example, the number 4:

factorial 4
4 * factorial(4 - 1)
4 * (3 * factorial(3 - 1))
4 * (3 * (2 * factorial(2 - 1)))
4 * (3 * (2 * 1)))
24

What this does is take the number n, then checks if it is greater than 1. If so, it returns n * the factorial of n - 1. If it is less than or equal to 1, then it returns 1.

回答2:

Okay, I figured out what is the problem. I used the Python Interactive Shell to run the program, so resulting in double printing of final result.

I put the program in a .py file and run it using command line and I finally got the correct result.

The code finally becomes:

def factorial( n ):
    if n <=1:   # base case
        print(str(n) + ' ' + str(n))
        return 1
    else:
        r = n * factorial( n - 1 )  # recursive call
        print(str(n) + ' ' + str(r))
        return r

print("n n!")
factorial(9)

回答3:

Without recursion:

def factorial(n):
    answer = 1
    for i in range(2, n+1):
        answer *= i
    return answer

With recursion:

def factorial(n):
    if n<=1:
        return 1
    else:
        return n*factorial(n-1)

来源：https://stackoverflow.com/questions/25916065/writing-python-factorial-function-using-recursion

标签

python

recursion

factorial