prime number function

[bluesoleli]

Hey Everyone,

What's the function for a program to find a prime number? I basically want to make something that tells me if a number entered is a prime number.

I know I'll need a cmath function i think?
a cin for num1
a cout for a true of false value

I can't figure out how to write the single line of code which is for a prime number though... If I had that, it's all smooth sailing lol. I know the definition of a prime is a number that can only be divided by one and itself...

so maybe like...

if (num1/1) = num1 <--- divide by one
not sure about divided by itself?

[sasanet]

why don't u USE GOOGLE??
it took me 5 seconds to find out what prime number mean:)

are the numbers than just can be divided between themselves and one (1).
example:
2,3,5,7,11,13,17,19,23
4 is not a prime number because is the result of 2*2
6 is not a prime number bacause is the result of 3*2
8 is not a prime number bacause is the result of 2*2*2
9 is not a prime number bacause is the result of 3*3
10 is not a prime number bacause is the result of 5*2
12 is not a prime number bacause is the result of 2*3*2
14 is not a prime number bacause is the result of 7*2

now when we know what prime number is we can make some code :D

#include <iostream>
using namespace std;

void prime_num(int);

int main() {
	cout << " Enter a number and I will generate the prime numbers up to that number: ";
	int num = 0;
	cin >> num;
	prime_num(num);
        return 0;
}

void prime_num( int num){
	bool isPrime=true;
	for ( int i = 0; i <= num; i++) {
		for ( int j = 2; j <= num; j++) {
			if ( i!=j && i % j == 0 ) {
				isPrime=false;
				break;
			}
		}
	if (isPrime)
	cout <<"Prime:"<< i << endl;
	isPrime=true;
	}
}

[coder777]

The % operator tells you the remainder of the division. If you find a number (below the number in question) that has no remainder then it's not a prime number.

[bluesoleli]

oh ok, got it... kinda.

um. i'm curious to know something... using the code above, i want to eliminate the cin values and only come up with a return value of prime numbers say... from 1 to 20.

so i know i'll have to put initial as 1 and then a stop at 20... how would i go about doing that?

[z05DSL3A]

Sieve of Eratosthenes
http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

[bluesoleli]

Ok got it... managed to make it go from 1 to 20 automatically. Thanks!