Python While 循环查找素数未终止

Question

flag = 0
n = int(input('\nEnter whole number to check : '))
i = 2
while i <= (n/2):
    if (n%i) == 0:
        flag = 1
        break
if n == 1:
    print('1 is neither prime nor composite')
elif flag == 0:
    print(n,' is a prime number.')
elif flag == 1:
    print(n,' is not a prime number.')

Upon Entering a number >= 3, the program stalls and the cursor keeps blinking endlessly. I initially tries 277, then 13, then 5, then 3 - none of which gave a result even after a minute.

Entering 2 worked.

There must be something wrong with the code.

Answer 1

Your loop is not changing n or i, which are the conditions on which it stops.

I think the correct code should be:

flag = 0
n = int(input('\nEnter whole number to check : '))
i = 2
while i <= (n/2):
    if (n%i) == 0:
        flag = 1
        break
    i += 1
if n == 1:
    print('1 is neither prime nor composite')
elif flag == 0:
    print(n,' is a prime number.')
elif flag == 1:
    print(n,' is not a prime number.')

Answer 2

If I understand correctly, you're trying to check if a number entered is a prime number.

This code works:

# prime numbers are greater than 1, num is the entered number
if num > 1:
   for i in range(2,num):
       if (num % i) == 0:
           print(num,"is not a prime number")
           break
else:
   print(num,"is a prime number")

At the end, you can also check for number 1 if you want.

Answer 3

The code below works:

flag = 0
n = int(input('\nEnter whole number to check : '))
i = 2
while i <= (n/2):
    if (n%i) == 0:
        flag = 1
        break
    else:
        i += 1
    
if n == 1:
    print('1 is neither prime nor composite')
elif flag == 0:
    print(n,' is a prime number.')
elif flag == 1:
    print(n,' is not a prime number.')

This is because you need to increment i by 1 after every iteration, or your while loop will run endlessly.