使用递归函数的阶乘。跟踪代码但无法弄清楚为什么会出现错误

Question

def factorial_recursive(n):
    while n > 1:
        factorial = factorial * n
        factorial_recursive(n - 1)
    return factorial
num = input("Please enter the number whose factorial you want to find ")
num = int(num)
factorial = 1
if num == 0:
    print("The factorial of 0 is 1")
elif num < 0:
    print("The factorial of a negative number cannot be computed")
elif num == 1:
    print("The factorial of 1 is 1")
else:
    print("The factorial is", factorial_recursive(num))

错误信息：

UnboundLocalError                         Traceback (most recent call last)
<ipython-input-28-c3621871e051> in <module>()
     16 else:
     17     factorial = 1
---> 18     print("The factorial is", factorial_recursive(num))

<ipython-input-28-c3621871e051> in factorial_recursive(n)
      2 def factorial_recursive(n):
      3     while n > 1:
----> 4         factorial = factorial * n
      5         factorial_recursive(n - 1)
      6     return factorial

UnboundLocalError: local variable 'factorial' referenced before assignment

我看过使用递归函数求一个数的阶乘的工作代码。我试图自己做，我已经跟踪了每一步的代码，但无法弄清楚为什么它不起作用。

Answer 1

行 factorial = factorial * n 引用名为 factorial 的局部变量 - 您从未创建过。所以你使用的是未绑定变量。

您似乎正试图在函数外部的全局范围内引用 factorial。在这种情况下，您需要声明使用 global 关键字

def factorial_recursive(n):
    global factorial
    while n > 1:
        factorial = factorial * n
        factorial_recursive(n - 1)
    return factorial

但是请记住，使用 global 变量几乎总是一个糟糕的主意。

为什么不干脆把它干掉，然后累积成倍数呢？

def factorial_recursive(n):
    if n == 1:
        return 1
    return n * factorial_recursive(n - 1)

这是最常见和朴素的递归阶乘实现。但它肯定比使用全局变量要好得多。

使用递归函数的阶乘。跟踪代码但无法弄清楚为什么会出现错误

Factorial using recursive functions. Traced the code but cannot figure out why the error is coming

python

recursion

factorial