Python-使用除數檢查數字是否為質數

Question

我必須編寫一個程序，通過檢查range（1，num）中的除數來檢查數字是否為質數。 （例如，如果輸入為5，則程序將檢查2是除數，然后是3，然后是4）。 有人可以解釋一下如何將最后一個表達式放在程序中（“這是質數”打印函數），以便它僅出現在“ for”循環中的“ else”函數之后，而不出現在“ if div = = 0：“語句？ 謝謝！

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這一點：

設置標志！

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表達式具有可選的else子句 ：

循環語句可以包含else子句； 當循環通過用盡列表而終止時（帶有for）或條件變為假（具有while時），則執行此命令，但當循環由break語句終止時，則不執行該命令。

因此，您的代碼應如下所示：

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

假設您已將除數除以1，那么您所要做的就是計算除數以外的除數（數字本身）。 因此，為了保持您的代碼結構，我添加了一個計數器。

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!")