Prime Numbers - Python

Question

I am a newbie in Python programming and I need a feedback. I wrote a program that prints prime numbers up to the given integer. But the output is different than expected. Could you please give me a feedback about this code. What am I doing wrong and why?

Thank you so much.

user_integer = int(input("Please enter an integer : "))
for num in range(0, user_integer + 1):
    for i in range(2, num):
        if (num % i) == 0:
            break
        else:
            print(num)

Welcome :) It's good practice to say what the output was and what you were expecting — 7koFnMiP, Oct 29 '20 at 11:29
Does this answer your question? [Prime Numbers python](https://stackoverflow.com/questions/35391063/prime-numbers-python) — Ale, Oct 29 '20 at 11:31
You should also look at better algorithms, such as [Sieve of Eratosthenes - Finding Primes Python](https://stackoverflow.com/questions/3939660/sieve-of-eratosthenes-finding-primes-python) — Booboo, Oct 29 '20 at 11:41
There is an example code of finding prime numbers in [Python Tutorial](https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops) — adamkwm, Oct 29 '20 at 11:49

score 1 · Answer 1 · answered Oct 29 '20 at 11:42

This code:

user_integer = int(input("Please enter an integer : "))
for num in range(0, user_integer + 1):
    for i in range(2, num):
        if (num % i) == 0:
            break
        else:
            print(num)

will print every odd number, including non-primes. Let num be equal 9 then 9%2==0 is False and else body is executed. You should firstly check all potential divisors and then print if appriopate that is:

user_integer = int(input("Please enter an integer : "))
for num in range(2, user_integer + 1):
    for i in range(2, num):
        if (num % i) == 0:
            break
    else:
        print(num)

Output (for input 25):

Note identation of else - here it is part of for-else not if-else, so print is done if and only if there was not break issued.

score 1 · Answer 2 · answered Oct 29 '20 at 11:44

Your error is caused by the placement of your else statement.

Essentially, the else statement executes every time the number cannot be divided. So for instance with the number nine:

9 % 2 isn't equal to 0, so it will print 9, even though nine isn't prime.

You could do something like this.

user_integer = int(input("Please enter an integer : "))

for num in range(0, user_integer + 1):
    for i in range(2, num):
        if (num % i) == 0:
            break
        
    else:
        print(num)

score -2 · Answer 3 · edited Oct 29 '20 at 14:48

-2

If you are new at python, better start looking at writing optimized codes. you can use list comprehension to achieve like [i for i in range(0,user,integer)]

edited Oct 29 '20 at 14:48

Don

3,876
10
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answered Oct 29 '20 at 11:48

Akshay

31
3

Prime Numbers - Python

3 Answers3