Thread: Prime Prime Prime Help Help Help

hi everyone, i have being working on this code for a long time.. but couldnt make it right

it is simple find the prime number from the user inputs

this is the code ....

Code:

bool IsPrime(int iNum)
{
// Check if i is less than square root of iNum (and cast iNum to type double)
    for (int i=2; i<=sqrt((double)iNum); i++) 
    {
        if (iNum == ((int)iNum/i)*i)// (inum%i == 0)		
        return false;
    }
    return true;
}

but this is not the exact code that i want ...

my aim is to find the prime number that the user will input the max number,,,

example here:

input: 4
output will be: not prime..

please help ...... i know there are lot of related topic but .... still couldnt help.

Wow - first thread bumping, and then posting something which plainly isn't an anwer.
Feel proud!.

im just pointing out something that is better. and it was not thread bumping... and i dont like you being rude to me...
im trieing to help him out by telling him something thats faster and easier and you bash me.
congrats.

how is posting something older than 2 weeks bumping a thread? where is the common sense in that. if you just reply to get it up k, or if the question was already answered and there is nothing to discuss ok. but clearly im forced to leave forums again bye..

Yes, answering the old not answered(but good) questions is a good idea.

1) your repeatedly checking the sqrt of iNum, do it once, before the loop, your wasting precious processing power.
2) your checking all numbers, even and odd, all prime numbers (except 2) are prime, once again wasting precious processes.
3) your comment is better written than your if statement itself
4) Isn't there a rule about not having other people do your homework?

sorry, not thinking, all prime numbers are odd except two.
and no prime number will have an integer sqrt, however, getting the sqrt of the number before hand will eliminate checking the sqrt multiple times. any program like this, is going to be run (most likely) using large numbers, since the small ones are readily available and the math is easy. 2, 3, 5, 7, 11.... so the amount of processes run cuts down signifigantly.

if you had say

Code:

  sqrtnum = sqrt(iNum);
  for (count = 0; count < sqrtnum; count++)

you would cut down on the execution of the sqrt. you could even make it faster, by checking to see if sqrtnum was an integer before the loop, so in the case of iNum being 100, the sqrt would be 10, thus skipping the loop. and making the function exponentially faster. in this case, it would skip running the loop 10 times, (or 4 times faster if you only check against prime numbers), and if you wanted to get really smart, you would do a check against 2 and 5 also before the loop, and only check numbers ending in 1, 3, 7,9 because if its even its composite as well as if it ends in 5.