Find all prime numbers from 1 to 1000. This is what I have done until now

[E607ko23]

#include<iostream>
#include <cstdlib>

using namespace std;

int main()
{
 int count=0;
 int number;
 cout<<"Enter an Integer:";
 cin>>number;

  for(int i=1;i<=number;i++)
  {
	   if(number%i==0)
	   {
			count++;
	   }
  }
     if(count==2)
  	 cout<<number << " is prime" << endl;
     else
     cout<<number <<" is not prime " << endl;
  system("PAUSE");
  return(0);
}

[Darkmaster]

whats the input for?
if you just have to find all the prim numbers in the range of 0 to 1000, all you need is std::cout

[E607ko23]

What I have done is to ask the user to input a particular number and check if it is prime or not. Now I want to display the prime numbers between 0 to 1000, but I can't make it work.

[Darkmaster]

i'd start with a for loop from 1 to 1000 fisrt

[E607ko23]

I've tried it, but still there is something wrong with my code

#include<iostream>
#include <cstdlib>

using namespace std;

int main()
{
 int count=0;
 for(int i=1;i<=1000;i++)
 {
  for(int j=1;j<=i;i++)
  {
	   if(i%j==0)
	   {
			count++;
	   }
  }
     if(count==2)
  	 cout<<i << " is prime" << endl;
     else
     cout<<i <<" is not prime " << endl;
 }
  system("PAUSE");
  return(0);
}

[coder777]

On line 11: you increase i instead of j

Btw: to make it faster you may want break; if count > 2

[z05DSL3A]

you also want to reset count before line 11.

[Darkmaster]

btw i%j == 0 is not the best way to check prim numbers.
e.g. if x % 2 != 0 and x % 3 != 0, x % 6 will also be != 0
but you can think about that once it's working

[ResidentBiscuit]

Could write up a quick sieve. Array of bool, set all to be true at first. Change each number divisible by 2 and greater than 2 to be false. Go to three, is it prime? If yes, leave true and set all multiples of 3 to be false. If no, set to false. Find next true element, this is prime. Mark all multiples of this to be false. Continue this until there's no more unmarked spots to move forward to.

[E607ko23]

Thank you for your help guys. I came up with the solution.

[aoeuaoeuaoeu]

Here's what I came up with:

#include <iostream>

using namespace std;

bool prime;

int main(int argc, char const *argv[])
{
	for(int i = 1; i <= 1000; i++){
		bool prime = true;

		for(int j = 2; j < i; j++){
			if(i % j == 0 || i == j){
				prime = false;
				break;
			}
		}

		if(prime == true){
			cout << i << " is prime." << endl;
		}
	}

	return 0;
}