Question

我编写了这个python代码来查找任何素数，第1个，第2个，第1000个等等。我运行了代码并返回了它，无论我输入的是什么整数：

2 is the 1 prime
3 is the 2 prime
5 is the 3 prime
7 is the 4 prime

这是代码（用python 2.7.8编写）：

 #code to find the nth prime
def isprime(n):
    '''check if integer n is a prime'''
    # make sure n is a positive integer
    n = abs(int(n))
    # 0 and 1 are not primes
    if n < 2:
        return False
        # 2 is the only even prime number
    if n == 2:
        return True
        # all other even numbers are not primes
    if not n:
        return False
        # range starts with 3 and only needs to go up the squareroot of n for all odd numbers
    for x in range(3, int(n**0.5)+1, 2):
        if n % x == 0:
            return False
    return True
num_ofprimes = 0
candidate_prime = 2
final_primes = raw_input("What prime would you like to find?")
while num_ofprimes <= final_primes:
    if isprime(candidate_prime) == True:
        if candidate_prime == 2:
            num_ofprimes = num_ofprimes + 1
            print (str(candidate_prime) + " is the " + str(num_ofprimes) + " prime")
            candidate_prime = candidate_prime + 1
            #2 is prime
        elif candidate_prime % 2 == 0:
            candidate_prime = candidate_prime + 1
            #if a number is even it is not prime
        else:
            num_ofprimes = num_ofprimes + 1
            print (str(candidate_prime) + " is the " + str(num_ofprimes) + " prime")
            candidate_prime = candidate_prime + 1
            # checks all odd numbers to see if prime then prints out if true
print ("All done!")

Answer 1

你的程序在找到前几个素数之后不会停止，但它会进入一个无限循环，不再产生输出。这样做的原因是，如果您的isprime检查失败，则永远不会增加candidate_prime变量！

此外，正如评论中所述，您应该将num_ofprimes与int(final_primes)进行比较;否则，您要将int与str进行比较，这非常类似于将苹果与橙子进行比较。

最后，您应该检查数字是否在isprime函数内。这不仅可以使您的isprime函数实际返回偶数数字的正确结果，而且还可以使您的代码更加紧凑，因为您不再需要if/elif/else块以下的所有if isprime块1}}检查。

Answer 2

除了tobias的重要评论之外，您似乎无缘无故地在脚本中做了很多额外的工作。 while循环可以简化为：

while num_ofprimes < final_primes:
    if isprime(candidate_prime):
            num_ofprimes = num_ofprimes + 1
            print (str(candidate_prime) + " is the " + str(num_ofprimes) + " prime")
    candidate_prime = candidate_prime + 1

Answer 3

for num in range(3, 1000):
    if num > 1:
        for i in range(2, num):
            if (num % i) == 0:
                break
        else:
            print(num)

用于查找未按预期工作的任何素数的代码

3 个答案: