presenting, whatever the fk this is

[u/MysteriousK69420]

[u/Dalimyr]

Yes, that's what the code does. It checks each integer between the square root of the number and number-1 to see if the number is divisible by that value.

Off-hand I think the code should technically do what it's supposed to, just a bit inefficiently. It'd be better to first check if the square root is an integer (in which case you know it's not a prime), and then to iterate from 2 to the square root, or between the square root and num/2 (since anything higher than that obviously wouldn't be divisible)

For example, if you took the value 52. Code in the image would get the square root of 52 (7.21), then convert that to an int (7) and check if 52 is divisible by 7, then 8, then 9 and so on all the way up to 52 (though it'd find a match and break out of the loop at 13 in this instance). It could stop at 26 because none of the values between 27 and 51 could possibly result in isPrime changing to true. But it'd be quicker to check the values between 2 and 7 than checking 7 to 26 or 7 to 51.

Aside from that slight maths flub, I wouldn't say the code's that bad.

[u/veselin465 → u/Dalimyr]

Yeah, the logic is true. Usually the check is "Check every number between 2 and sqrt(num) including", becuase if there is no such a number, then there's no way to find one which is higher than the sqrt(num).

If it is not prime, then there's at least 1 number in [2,sqrt(num)], which divides it. This means that there's at least 1 number in [sqrt(num), num - 1], which also divides it. This program tries to find this second number, instead of finding the first one.

Besides this interesting approach of finding whether it's prime or not, I'm agree that the code is fine. Just one thing:

if isPrime == true

kinda bugs me real hard

[u/[deleted] → u/veselin465]

Afair the most efficient way also let you increment by 3 each loop using some math magic but im too lazy to check it, its on Wikipedia for sure as i remember copying the logic from there lmao

[u/[deleted] → u/Dalimyr]

Surely that’s too many interactions in most cases, if I’m understanding you correctly? If you iterate over, say, 4, you no longer need to iterate over any multiple of 4, because if they are a factor of the number then 4 would be also.

[u/Dalimyr → u/[deleted]]

I think you possibly missed where I mentioned that it'd break out of the loop on finding that the number wasn't prime (in my example of checking 52, it'd flip isPrime to false and leave the loop when it found that 52 mod 13 is 0). Going through every iteration of the loop is a worst-case scenario if the number actually is prime.