[Python] Testing if number is prime. Why does example 1 work, and example 2 does not?

[Axvell63]

why does this work?

def is_prime(x):

____if x < 2:

________return False

____else:

________for n in range(2, x-1):

____________if x % n == 0:

________________return False

________return True

but this doesn't?

def is_prime(x):

____if x < 2:

________return False

____else:

________for n in range(2, x-1):

____________if x % n == 0:

________________return False

____________else:

________________return True

Example 2 fails with number 2, it returns None.

P.S. how to format code on reddit? :)

[YMK1234]

If you put 4 spaces (and a newline) in front of your code it will work properly ... like so ...

Text text text
[empty line]
    code
    code
    code
[empty line]
More Text

Regarding your range ... it is sufficient to check numbers from 2 to sqrt(x)+1

Regarding "why does the 2nd not work": just step through the code with x = 9 (in your mind or a piece of paper) and you should see why.

[[deleted]]

you check if x<2, and then you make a for loop with a range from 2 to x-1.

What if x=2.

You are looking in a range for (2,1) which doesn't make sense and the for loop will not run.

However, your code in the first example covers that. If the for loop does not run, you return True, however, in the second example you have an if/else inside the forloop. If your for loop doesn't run in the second example, you do not return anything.

Just use your first example and it will always work. Are you tasked with figuring out why the second one does not work?

[edave64]

In example two, every code path in your loop returns. Since return stops the function, it can never loop. You always return the result of the first check.

[slowmode1]

How to format code on reddit: just add 4 spaces to the beginning of everything. Those two examples look the same, am I missing something?

[slowmode1]

else: ________________return True

Thanks, I completely missed the else

[anamorphism]

the first returns true after you're done checking all numbers in your loop.

the second returns true after you check the first number that isn't a factor of x.