Python-使用除数检查数字是否为质数

Question

我必须编写一个程序，通过检查range（1，num）中的除数来检查数字是否为质数。 （例如，如果输入为5，则程序将检查2是除数，然后是3，然后是4）。 有人可以解释一下如何将最后一个表达式放在程序中（“这是质数”打印函数），以便它仅出现在“ for”循环中的“ else”函数之后，而不出现在“ if div = = 0：“语句？ 谢谢！

while True:
    num = int(input("Enter a positive number to see if it's a prime number: "))
    if num > 1:
        break
    elif num == 1:
        print("1 is technically not a prime number.")
    else:
        print("Number cannot be negative- try again.")

for num1 in range(2,num):
    div = num%num1
    if div == 0:
        print(num1,"is a divisor of",num,"... stopping.")
        print(num,"is not a prime number.")
        break
    else:
        print(num1,"is NOT a divisor of",num,"... continuing")

print(num,"is a prime number!")

Answer 1

有很多方法可以做到这一点：

设置标志！

is_prime = True # a boolean flag
for num1 in range(2,num):
    div = num%num1
    if div == 0:
        print(num1,"is a divisor of",num,"... stopping.")
        print(num,"is not a prime number.")
        is_prime = False
        break
    else:
        print(num1,"is NOT a divisor of",num,"... continuing")

 if is_prime:
     print(num,"is a prime number!")

for-else使用！

for num1 in range(2,num):
    div = num%num1
    if div == 0:
        print(num1,"is a divisor of",num,"... stopping.")
        print(num,"is not a prime number.")
        is_prime = False
        break
    else:
        print(num1,"is NOT a divisor of",num,"... continuing")
 else: 
      print(num,"is a prime number!") # will execute only when for-loop ends

Answer 2

Python的for表达式具有可选的else子句 ：

循环语句可以包含else子句； 当循环通过用尽列表而终止时（带有for）或条件变为假（具有while时），则执行此命令，但当循环由break语句终止时，则不执行该命令。

因此，您的代码应如下所示：

while True:
    num = int(input("Enter a positive number to see if it's a prime number: "))
    if num > 1:
        break
    elif num == 1:
        print("1 is technically not a prime number.")
    else:
        print("Number cannot be negative- try again.")

for num1 in range(2,num):
    div = num%num1
    if div == 0:
        print(num1,"is a divisor of",num,"... stopping.")
        print(num,"is not a prime number.")
        break
    else:
        print(num1,"is NOT a divisor of",num,"... continuing")
else:
    print(num,"is a prime number!")

Answer 3

假设您已将除数除以1，那么您所要做的就是计算除数以外的除数（数字本身）。 因此，为了保持您的代码结构，我添加了一个计数器。

while True:
    num = int(input("Enter a positive number to see if it's a prime number: "))
    if num > 1:
        break
    elif num == 1:
        print("1 is technically not a prime number.")
    else:
        print("Number cannot be negative- try again.")

for num1 in range(2,num):
    div = num%num1
    count = 0
    if div == 0:
        print(num1,"is a divisor of",num,"... stopping.")
        print(num,"is not a prime number.")
        break
    else:
        count += 1
        print(num1,"is NOT a divisor of",num,"... continuing")

if count == 1:
    print(num,"is a prime number!")

Answer 4

我添加了一行sys.exit（）

while True:
num = int(input("Enter a positive number to see if it's a prime number: "))
if num > 1:
    break
elif num == 1:
    print("1 is technically not a prime number.")
else:
    print("Number cannot be negative- try again.")

for num1 in range(2,num):
div = num%num1
if div == 0:
    print(num1,"is a divisor of",num,"... stopping.")
    print(num,"is not a prime number.")
    sys.exit()
else:
    print(num1,"is NOT a divisor of",num,"... continuing")

print(num,"is a prime number!")