18、Python编程：函数、变量作用域与图像处理工具包

Python编程：函数、变量作用域与图像处理工具包

1. 素数判断程序

下面是一个改进后的素数判断程序，用于找出指定上限内的所有素数：

def main() :
    n = int(input("Please enter the upper limit: "))
    if n >= 2 :
        print(2)
    i = 3
    while i <= n :
        if isprime(i) :
            print(i)
        i = i + 2

#  Tests if an integer is a prime.
#  @param n any positive integer
#  @return True if n is a prime, False otherwise
def isprime(n) :
    if n == 1 :
        return False
    if n == 2 : 
        # 2 is a prime 
        return True 
    if n % 2 == 0 : 
        # No other even number is a prime 
        return False 
    # Try finding a number that divides n 
    k = 3 # No need to divide by 2 because n is odd 
    # Only need to try divisors up to sqrt(n)