Question

此python代码在代码的随机数生成器部分返回ValueError。我在函数中使用硬编码值运行它：def fermat_test(n): at the line a = random.randit(2,n-1)，似乎不然。我无法弄清楚为什么范围超出范围？

import random
def remainder(i,j):
    rem = i%j
    return rem
def is_even(n): return n % 2 == 0
def square(n): return n**2
def expmod(b, e, m):
    if e==0:
        return 1
    elif is_even(e):
        expm = expmod(b, e/2,m)
        return remainder(expm*expm,m)
    else:
     return remainder(b*expmod(b,e-1,m),m)
def fermat_test(n):
    a = random.randint(2,n-1)
    return expmod(a,n,n)==a
def is_fermat_prime(n, ntimes):
 if ntimes == 0:
     return True
 elif fermat_test(n):
    return is_fermat_prime(n,ntimes-1)
 else:
     return False
## this is the test you can use to test your code
def sum_of_fermat_primes(n):
 sum = 0
 for i in xrange(n+1):
     if is_fermat_prime(i, 70):
         sum += i
 return sum
print sum_of_fermat_primes(10)
print sum_of_fermat_primes(20)
print sum_of_fermat_primes(560)
print sum_of_fermat_primes(570)

Traceback (most recent call last):
  File "C:\Users\Terik\.atom\code.py", line 33, in <module>
    print sum_of_fermat_primes(10)
  File "C:\Users\Terik\.atom\code.py", line 30, in sum_of_fermat_primes
    if is_fermat_prime(i, 70):
  File "C:\Users\Terik\.atom\code.py", line 22, in is_fermat_prime
    elif fermat_test(n):
  File "C:\Users\Terik\.atom\code.py", line 17, in fermat_test
    a = random.randint(2,n-1)
  File "C:\Python27\lib\random.py", line 242, in randint
    return self.randrange(a, b+1)
  File "C:\Python27\lib\random.py", line 218, in randrange
    raise ValueError, "empty range for randrange() (%d,%d, %d)" % (istart, istop, width)
ValueError: empty range for randrange() (2,0, -2)

Answer 1

错误是因为

a = random.randint(2,n-1)

n - 1小于2。事实上，n的值来自i中的for i in xrange(n+1)，因此它从0，1，2等开始。最小值会导致random.randint(2,n-1)无效。

Python中的随机数范围，ValueError：范围为空（）

1 个答案: