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.

Code:

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std::cout << (IsPrime(Input) ? "prime" : "not prime") << std::endl;

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try this instead of that code you got there.

Code:

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inline bool IsPrime(long& n)
{
  return((n & (n - 1)) == 0);
}

---

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.

>>> and it was not thread bumping...

From the board rules. You did read the board rules didn't you? I added the bold font to help you see.

Quote:

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5. Don't bump threads. (Bumping: posting messages on threads to move them up the list or to post on a thread that has been inactive for two weeks or longer).

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In what way was replying to a long dead thread from August 2004 not thread bumping? I'm intruiged.

Reporting mods for picking you up on your rule breaking is not going to help you.

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..

The common sense is that a questioner asking a question here 18 months ago is not likely to be sitting waiting for an answer still. He will have found an answer somewhere else or solved it himself.

If you posted a very basic question 18 months ago, would you still be looking for an answer?

you seriously have no common sense. wow. cant you think off that some poeple might still be looking at that thread? someone even redicted to that thread. someone might have find it usefull what i posted. do you really think the forums are gonna get any better by that fked up rule . i dont think so. im gonna enjoy my time at codeguru alot more than here.

Quote:

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Originally Posted by Mitsukai

you seriously have no common sense. wow. cant you think off that some poeple might still be looking at that thread?

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Those are the rules...

>try this instead of that code you got there.
Apparently you didn't. :rolleyes:

Quote:

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im gonna enjoy my time at codeguru alot more than here.

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I think we're going to enjoy your time there a lot more than here, too.

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

There are reasons for the rules here. They have been developed over many years, by a lot of very experienced people, and for the most, they work. We will not be changing them anytime soon.

>>> im gonna enjoy my time at codeguru alot more than here.

If you approach codeguru, (where many of us are also regulars of course), the way you approached here, I somehow doubt it.

Feel free to ignore this advice, but the net is a big bad ugly place, where people are quick to flame, and far more mercilessly then you have ever been shown here.

If you have been upset by the way you were treated here, you really need to consider an alternative strategy for your learning process.

Good luck.

*** No more off topic in this thread now please. Adrian...

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?

Which rule restricted answering to this thread? 5?
We should see the goal of the rule. It wants to prevent dead topics to come on first page. Answering to an unanswered thread is different. For example maybe I have had asked a very hard question that nobody could answered, but 5 months later somebody came and answered it. You think it is illegal? Of course it is not. It is the goal of the forum.

Quote:

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Originally Posted by mltngpot

all prime numbers (except 2) are prime, once again wasting precious processes.

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shouldn't that read, all prime numbers, other than 2, are odd. ?

so testing for if it is odd, then seeing if it has ANY divisors other than 1 and itself is all you need to do. [ test for division by 2, 3, 5 ]
iand non prime number with divide by one or more of these three.
not all odd number have a square root that is an integer. ie: 15 is not prime, yet does not square root either to an integer. so testing for a square root is not going to result in a prime number.

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++)

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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.