C++ question on prime numbers - c++

Question

I am trying to make a program that determines if the number is prime or composite. I have gotten thus far. Could you give me any ideas so that it will work? All primes will , however, because composites have values that are both r>0 and r==0, they will always be classified as prime. How can I fix this?
int main()
{
int pNumber, limit, x, r;
limit = 2;
x = 2;
cout << "Please enter any positive integer: " ;
cin >> pNumber;
if (pNumber < 0)
{
cout << "Invalid. Negative Number. " << endl;
return 0;
}
else if (pNumber == 0)
{
cout << "Invalid. Zero has an infinite number of divisors, and therefore neither composite nor prime." << endl;
return 0;
}
else if (pNumber == 1)
{
cout << "Valid. However, one is neither prime nor composite" << endl;
return 0;
}
else
{
while (limit < pNumber)
{
r = pNumber % x;
x++;
limit++;
if (r > 0)
cout << "Your number is prime" << endl;
else
{
cout << "Your number is composite" << endl;
return 0;
}
}
}
return 0;
}

Answer 1

Check out http://en.wikipedia.org/wiki/Prime_number and http://en.wikipedia.org/wiki/Primality_test
The simplest primality test is as
follows: Given an input number n,
check whether any integer m from 2 to
n − 1 divides n. If n is divisible by
any m then n is composite, otherwise
it is prime.

Answer 2

#include <iostream>
#include <math.h>
// Checks primality of a given integer
bool IsPrime(int n)
{
if (n == 2) return true;
bool result = true;
int i = 2;
double sq = ceil(sqrt(double(n)));
for (; i <= sq; ++i)
{
if (n % i == 0)
result = false;
}
return result;
}
int main()
{
std::cout << "NUMBER" << "\t" << "PRIME" << std::endl;
for (unsigned int i = 2; i <= 20; ++i)
std::cout << i << "\t" << (IsPrime(i)?"YES":"NO") << std::endl;
std::cin.get();
return 0;
}

Answer 3

bool check_prime(unsigned val) {
if (val == 2)
return true;
// otherwise, if it's even, it's not prime.
if ((val & 1) == 0)
return false;
// it's not even -- only check for odd divisors.
for (int i=3; i*i<=val; i+=2)
if (val % i == 0)
return false;
return true;
}

Answer 4

with respect ur code u didn't check if i enter 2 what will be happened,
and also u didn't return any thing if it is prime....thats why it is always returning prime in spite the number is composite .
here is the code below =>
#include<iostream>
using namespace std;
int main(){
int pNumber, limit, x, r;
limit = 2;
x = 2;
cout << "Please enter any positive integer: " ;
cin >> pNumber;
if (pNumber < 0){
cout << "Invalid. Negative Number. " << endl;
return 0;
}
else if (pNumber == 0){
cout << "Invalid. Zero has an infinite number of divisors, and therefore neither composite nor prime." << endl;
return 0;
}
else if (pNumber == 1){
cout << "Valid. However, one is neither prime nor composite" << endl;
return 0;
}
else if (pNumber == 2){
cout << " Your number is prime" << endl;
return 0;
}
else{
while (limit < pNumber){
r = pNumber % x;
x++;
limit++;
if (r > 0){
cout << "Your number is prime" << endl;
return 0;
}
else{
cout << "Your number is composite" << endl;
return 0;
}
}
}
return 0;
}

Answer 5

For one thing, you'll want to break out of your loop when you find some x where pNumber % x == 0. All you need to do is find one factor of pNumber greater than 1 and less than pNumber to prove it's not prime -- no point in searching further. If you get all the way to x = pNumber without finding one, then you know pNumber is prime. Actually, even if you get to the square root of pNumber without finding one, it's prime, since if it has a factor greater than that, it should have a factor less than that. Make sense?

Answer 6

I don't know what you have been taught thus far, but my discrete mathematics teacher was a fan of the Miller-Rabin test. It is a pretty accurate test that is very easy to code, within a few base tests you have a very negligible chance that you have a Carmichael Number. If you haven't gotten that far in your studies I would just stick to some basic division rules for numbers.

Answer 7

Simplest method is for a given number n , if it is perfectly divisible with any number between 2 to sqrt(n), its a composite, or else its prime

Answer 8

Hi i have done this that also without using math.h header file....Have used turboc compiler.
Following program checks whether the number is prime or composite.
#include<iostream.h>
#include<conio.h>
class prime
{
int a;
public:
void check();
};
void prime::check()
{
cout<<"Insert a number";
cin>>a;
int count=0;
for(int i=a;i>=1;i--)
{
if(a%i==0)
{
count++;
}
}
if(count==1)
{
cout<<"\nOne is neither prime nor composite";
}
if(count>2)
{
cout<<"\nThe number is composite " ;
}
if(count==2)
{
cout<<"\nThe numner is prime";
}
}
void main()
{
clrscr();
prime k;
k.check();
getch();
}