Iteration of the output in simple prime number

Question

I'm trying to find out whether a given number is prime or not. For example, when I enter the input as 5, it should output "5 is a prime".

However, I get multiple, conflicting outputs.

This is my code:

num = int(input("Enter the number: "))
if num > 1:
  for i in range(2,num):
    if (num % i) == 0:
      print(num,"is not a prime number")
      #print(i,"times",num//i,"is",num)
      break
    else:
        print(num,"is a prime number")
        # if input number is less than
        # or equal to 1, it is not prime
  else:
          print(num,"is not a prime number")

Output is:

Enter the number: 5

5 is a prime number
5 is a prime number
5 is a prime number
5 is not a prime number

The above output shows that 5 is prime number thrice and that 5 is not a prime number once. What is my mistake?

5 % 2 = 1 - prints "is a prime number". 5 % 3 = 2 - same. 5 % 4 = 1 - same. Then you finish the loop, so final else runs, printing "is not a prime number" — matszwecja
– matszwecja, Commented Jun 26, 2024 at 8:02
your line printing that 5 is a prime number is in your for loop, so everytime you check a number to see if 5 is divisible by it, it finds it's not and prints 5 is prime each time. You need to check all the numbers first, then print 5 is prime if its not divisible by any of the numbers — Emi OB
– Emi OB, Commented Jun 26, 2024 at 8:03
Your loop is going in the range (2,num). You might soon understand that the range (2,Roundup(sqrt(num))) is also fine :-) — Dominique
– Dominique, Commented Jun 26, 2024 at 8:12
Try writing a separate function that determines if a number is prime (or not). Then, call that based on the user input. You'll see that that approach simplifies your code considerably — jackal
– jackal, Commented Jun 26, 2024 at 9:34
You might also want to have a look at the answers to this question or at this article. Both deal with implementing the Sieve of Eratosthenes in Python, which is a traditional algorithm for finding prime numbers. — simon
– simon, Commented Jun 26, 2024 at 11:53

trincot · Accepted Answer · 2024-06-26 08:02:52Z

1

There are two problems:

You cannot know whether a number is prime before having done all iterations of the loop, so the inner else block is taking conclusions too soon. It might be there is still a value for i coming that is a divisor of num. So remove this inner else
The outer else prints a wrong message. This executes when no break was executed in the loop, i.e. when no divisor was found. So in that case you should print it is a prime number.

answered Jun 26, 2024 at 8:02

trincot

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