Find prime number

[Mazd]

Hi,i need to write program to print prime number between 1-20,please give me outline :)

[CodyShort]

Hi.. to be able to do this you are going to have to do something like this I believe...

#include <iostream>

int main(){
    for(int i = 1; i <= 20; i++)
    {
        if(i % 2 != 0){
            std::cout<< "\n" << i << "\n";
        }
    }
}

[Mazd]

Thanks.I will check it.

[Mazd]

CodyShort!---it work but what about 2..is not prime number?

[jankidudel]

#include <iostream>
using namespace std;
int main() {
	cout << 2 <<endl;
	for (int i=3; i <= 19; i+=2)
	{
	    if(((i % 2)!=0) && ((i % 3)!=0) && ((i % 5)!=0) && ((i % 7)!=0)) // from 1 to 20 that's enough
			cout << i <<endl;	// but there are more fancier methods
								// of computing prime numbers, try to search.
	}
	return 0;
}

[R0mai]

@jankidudel

#include <iostream>

int main() {
    std::cout << "2\n3\n5\n7\n11\n13\n17\n19" << std::endl; // from 1 to 20 that's enough
}

[Mazd]

Hah..hah.Thanks Guys I got this solution

#include <iostream>

using namespace std;
//using namespace system;


int isPrime(long num) // Returns 1 (true) if its prime, or 0 (false) if not
{
if (num < 2) // 1 is not a prime number
return 0;

// if it is > 2 and an even number, then it definitely is not a prime
if (num > 2 && (num % 2) == 0)
return 0;

//considering the fact all can be divided by 1 and itself,
//start checking if there is any other divisor, if found one then no need to continue, it is not a prime
for(int i = 2; i < num; i++ )
{
cout << " divisor: " << i << endl;
if ( (num % i) == 0) // if it is divisible by i
{
// a divisor other than 1 and the number itself
return 0; // no need for further checking
}
}


return 1; // if all hurdles/checks are crossed, heyyyy, its a prime
}

int main()
{
int num;
do {
cout << " enter a number (0 to stop) " << endl;
cin >> num;
if (num) {
if (isPrime(num))
cout << num << " is a prime numebr " << endl;
else
cout << num << " is NOT a prime numebr " << endl;
}
} while (num > 0);

return 0;
}

[CodyShort]

@mazd, all that commenting not proper indentaton

And I'm sorry Imust have mis read this the first time, I though you wanted to print odd numbers my bad... Here is the correct and dynamic method to use for this...

#include <iostream>
#include <math.h>

using namespace std;

bool prime(int);

int main()
{
    for(int number = 1; number <= 20; number++){
    bool isPrime = prime(number);
        if (isPrime)
        {
            cout<< endl << number << endl;
        }
    }
}

bool prime(int number)
{

    for(int i = 2; i <= sqrt(number); i++)
    {
        if (number % i == 0){
            return false;
        }
    }
    return true;
}

[ne555]

@CodyShort that is not dynamic.
The dynamic method uses only the primes that were already found to compute the modulus operation.

[R0mai]

The dynamic method uses only the primes that were already found to compute the modulus operation.

Isn't that called Sieve of Eratosthenes?

[mcleano]

Ah I was just about to suggest the Sieve of Eratosthenes!

[CodyShort]

yeah it depends on what you consider like "dynamic" what I meant by dynamic is that what ever number you plug into it, it willl return all of the prime numbers up to that number...

[Mazd]

@CodyShort

I got this error msg :(

Error 1 error C2668: 'sqrt' : ambiguous call to overloaded function

[CodyShort]

odd... what compiler are you using?

[Mazd]

Visual C++ 2008

[Mazd]

It run in xwDev C++ but it say..
press any key to continue.. <----after press it print 1, next press 2, next press 3...up to 19

ref.QTN

between 1-20

[hamsterman]

Error 1 error C2668: 'sqrt' : ambiguous call to overloaded function

sqrt has overloads for floats and doubles. Since you're passing an int, VC++ doesn't know which function to call. Use sqrtf.

[Mazd]

Thanks hamsterman

[geko]

I have a similar problem with my program but i need it to list the divisor its sapose to look like this but for some reason it wont compile :|


Input a positive integer : 8
Testing 1 ->
The divisors: 1
The number 1 is not a prime number.
Testing 2 ->
The divisors: 1 2
The number 2 is a prime number.
Testing 3 ->
The divisors: 1 3
The number 3 is a prime number.
Testing 4 ->
The divisors: 1 2 4
The number 4 is not a prime number.
Testing 5 ->
The divisors: 1 5
The number 5 is a prime number.
Testing 6 ->
The divisors: 1 2 3 6
The number 6 is not a prime number.
Testing 7 ->
The divisors: 1 7
The number 7 is a prime number.
Testing 8 ->
The divisors: 1 2 4 8
The number 8 is not a prime number.