Question

我无法弄清楚为什么这个python函数在递归调用自身时会返回None。

这是我解决项目欧拉问题的一部分。我无论如何都以更好的方式解决了这个问题，但这仍然让我感到烦恼，因为函数似乎工作正常 - 它似乎知道我想要返回的变量的值。

def next_prime(previous):
    if previous % 2 == 0:
        candidate = previous + 1
    else:
    candidate = previous + 2
    print "trying", candidate
    prime = True
    for div in range(2,candidate//2,1):
        if candidate % div == 0:
            prime = False
            print candidate, "is not prime - divisible by", div
            next_prime(candidate)
            break
    if prime is True:
        print candidate, "is prime"
        #return candidate

last = 896576
print "After", last, ", the next prime is..."
next_prime(last)

这给出了：

After 896576 , the next prime is...
trying 896577
896577 is not prime - divisible by 3
trying 896579
896579 is not prime - divisible by 701
trying 896581
896581 is not prime - divisible by 7
trying 896583
896583 is not prime - divisible by 3
trying 896585
896585 is not prime - divisible by 5
trying 896587
896587 is prime

但是如果我取消注释return语句，它只返回一个值，如果第一次尝试是素数，否则返回None。

Answer 1

如果找不到素数，你忘了返回一个值：

for div in range(2,candidate//2,1):
    if candidate % div == 0:
        prime = False
        print candidate, "is not prime - divisible by", div
        return next_prime(candidate)

虽然递归并不适合这里。它比简单的迭代方法更优雅。此外，如果您遇到两个连续素数之间存在大量非素数的区域，您可能会溢出堆栈。

Answer 2

正如其他人所说，这不是递归的真正地方。以下是使用迭代的示例。我还定义了另一个测试整数的素数的函数 - 我认为这会使代码更简单。

def is_prime(n):
    """Return True if n is prime."""
    for i in xrange(2, n//2):
        if n%i == 0:
            return False
    return True

def next_prime(n):
    """Returns the next prime number after n."""
    if n % 2 == 0:
        candidate = n + 1
    else:
        candidate = n + 2
    while not is_prime(candidate):
        candidate += 2
    return candidate

if __name__ == '__main__':
    n = 896576
    print next_prime(n)

Answer 3

请注意，您正在对next_prime函数进行递归调用，但不会从调用函数返回它的值。

替换行：

print candidate, "is not prime - divisible by", div
next_prime(candidate)

与

print candidate, "is not prime - divisible by", div
return next_prime(candidate)

Python函数在递归后返回None

3 个答案: