prime number?

[lvlichael72]

What is the code for finding which a number is prime or not?

[Stewbond]

bool isPrime(int n)
{
    for (int i = 0; i < sqrt(n); ++i)
        if (n%i)
            return false;
    return true;
}

[lvlichael72]

I still have problem and this code has many errors on debuging....

[Stewbond]

show us code. What are the errors? The problem is in your use of this code, not with the code itself.

I'm not writing the whole thing for you.

[Chervil]

@Stewbond the code you posted is flawed, in more ways than one. If you test it you will see.

[Catfish2]

Looks like somebody forgot to drink their coffee. :)

[Chervil]

Let's look at the requirements.

If the number is 0 or 1 it isn't prime.

After that, we divide the number by every integer greater than 1 and less than the number itself.

If the remainder in every case is non-zero, the number is prime.

(we could reword the above, if the remainder in any case is equal to zero, the number is not prime)

That's all.

As a refinement, it is possible to improve the efficiency by testing the remainder when dividing by integers up to and including the square root. But care needs to be taken to avoid calling the square root function repeatedly for the same number - that could reduce or negate any efficiency gains.

[code2004]

Implementation of Chervil's specification with usage example: on run, program takes positive integer (0<=n<=4294967295) input from user and checks if prime, then outputs appropriate statement.

#include <iostream>
#include <cmath>
using namespace std;

bool isPrime(unsigned int n) {
    if(n==0|n==1) return false;
    double s=sqrt(n);
    for(int i=2;i<s;i++)
        if(n%i==0) return false;
    return true;
}

int main() {
    unsigned int i;
    cout << "Enter an integer >= 0:\n";
    cin >> i;
    if(isPrime(i))
        cout << i << " is prime.\n";
    else
        cout << i << " is not prime.\n";
    return 0;
}

[Chervil]

@code2004
Pretty good. I thought it was spot on, but unfortunately reports that 4, 9 and 25 are prime.

You missed one part, which was in bold text in my previous post.

[alexandermir]

#include<iostream.h>
#include<conio.h>

void main()

int a,b;

cout<<"enter the number";
cin>>a;


if(a%a==0 && a%1==a)

cout<<"prime number";

else
cout<<"not prime";

getch();
}



please help me this gives a problem, any one can solve it??:(

[Chervil]

@alexandermir
You need an open brace after main() {

[Felicia123]

i do a simple program. not really know it's working or not. hope it's help

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

using namespace std;

int main(){

	float square;
	int a = 0;
	bool correct = 1;

	while( a != -1 ){
		cout << "Enter number [press -1 to exit] : ";
		cin  >> a;
		for( int i = 2 ; i < a ; i++ ){
			if ( a % i == 0 ){
				correct = false;
				break;
			}
			if( correct != false )
			correct = true;
		}
		

		if( correct == true )
			cout << a << " is a prime number ! " << endl;
		else if ( a == 1 || a == 0 || correct == false)
			cout << a << " is not a prime number ! " << endl;
		else
			cout << a << " is not a prime number ! " << endl;
		cin.clear();
		cin.ignore(100,'\n');
		correct = 1;
	}

	system( "pause" );
	return 0;
}

[lvlichael72]

Hey guys thanks for all of your guidance , i do this source code that working well but has a little problem ... when it print the result ,print it forever without stop ... so how can i stop it?

#include <iostream>
using namespace std;
int main()
{[output][/output]
int x , i = 2;
cout << " Enter a number : " ;
cin >> x;

  while (x !=0)
  {
		  if (x <= i)
			  cout << " The number u entered is Prime " ;
		  else if (x > i && x % i==0)
		  cout << " The number u entered is Composite ";
		  cout << endl;
		   if (x > i && x % i!=0)
		  i++;
		 
		   if (x==1)
			   cout << " the number is neither prime or composite" ;
		   
  }
  cin.get();
  cin.get();
}

[Felicia123]

your code totally wrong
u should do a looping to check the number and not only divide by 2 and check the remainder.

[Chervil]

@lvlichael72
You created an infinite loop. the condition while (x !=0) never changes once the loop has been entered, since x doesn't change.

There are at least three ways to fix this. One would be to change the loop condition itself, to depend on something which does change, such as the integer i, which is the only thing that changes. Another would be to change a value such as this x=0; within the loop to make the condition false when you want to exit. Lastly you could use the break; statement which is used to exit from various types of control structures.

But still, your code is quite complex and messy. In effect you are implementing a version of this:

bool isPrime(int x)
{
    if (x==1)
        return false;
    int i=2;
    while (i < x)
    {
        if (x%i == 0)
            return false;
        i++;
    }
    return true;
}