Determine if a Number is Prime

[cyeb]

Yes, I know about the Sieve of Eratosthenes and Euler, I'm trying to make one on my own, and see how quick it can be. Here's my code:

int isprime(double isprime)
{
	double sqrtd= sqrt (isprime);
	sqrtd*=10;
	int sqrti= int(sqrtd);
	if (sqrti%10!=0)
		return true;
	else
		return false;
}

So basically, it square-roots the number, multiplies it by 10, and then determines if the last digit is a 0 or not.

The problem is, numbers like 26 have square roots of 5.099, and since 5.099 multiplied by 10 and converted to an int is 50, it is still counted as a prime because it ends in a 0.

I know I can just have a "sqrtd*=10;" and a "sqrtd*=100;", but who knows, maybe as numbers get higher, it will start to have more than just 1 zero after the decimal. Any ideas?

IS there any square roots of any number with more than one 0 after the decimal?

[m4ster r0shi]

So basically, it squares the number, multiplies it by 10, and then determines if the last digit is a 0 or not.

Why is this a condition for primality? With this logic 25 is a prime, as: sqrt(25)*10=5*10=50 = 0 (mod10)

[chrisname]

26 isn't prime anyway; 26 / 2 = 13...

[Albatross]

57 is a prime number.

-Grothendieck

P.S.- On a more serious note, you kinda need a new algorithm.

[cyeb]

Oh dang, I must have been half asleep while typing...Hold on, editing my post. (AHH! What was I thinking? Nevermind guys, I'm clicking "Topic is Solved." Stupidity can take over sometimes. Sorry for wasting your time, my code doesn't work at all. I'm seeing what's wrong.)

[QWERTYman]

Raise your hand if you noticed that this shouldn't compile because "isprime" is used as both the function name and variable name.

[R0mai]

this shouldn't compile because "isprime" is used as both the function name and variable name.

It compiles for me...

[chrisname]

It shouldn't.

[Duthomhas]

The local function variable name occludes the function name... so it compiles just fine. The trick is to properly reference the given objects:

#include <iostream>
using namespace std;

int foo( int foo )
  {
  cout << foo << endl;
  if (foo > 1) return ::foo( foo - 1 );
  else         return        foo - 1;
  }

int main()
  {
  cout << foo( 3 ) << endl;
  return 0;
  }

Enjoy!

[QWERTYman]

I didn't know that. ^_^

The more you knoooooooow...

[chrisname]

I still don't think you should do it. It's confusing. That it works isn't an excuse to use it -- index[array] (instead of array[index]) shouldn't work but it does. That doesn't mean it's good.

[quirkyusername]

you could use floats and fmod instead of ints and the % operator

[mloC]

I have a function which as far as I know is reliably on getting primes:

Please excuse me if it's not optimized as I don't have a formal education of C/C++

int f(int x)

{

int x;
static int divy,Mcounter;
    if (x <0) {x *= -1;}

	divy =1;

	Mcounter=0;

	for (int i =0;i <= x; i++)

	{

		divy++;

		1cout << "\n" << divy <<"\n" ;

	    rec = x % divy;

	    if( divy==x){divy += 1;continue;}

	    else if(rec == 0){Mcounter++;continue;}

	    else if (rec > 0){continue;}

	    

     }

     

     return Mcounter;
}

If it returns 0 it's prime. Otherwise it means that something divided evenly into it other than itself or one and thus not prime.