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Prime number

 
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Prime number, C++
Kcbroncofan
post 8 Nov, 2005 - 09:01 PM
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I need to write a program that asks a user to input a positive integer. I then need to out put whether the number is prime. I also need to put in an error message if the user inputs a negative number.
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Dark_Nexus
post 9 Nov, 2005 - 12:21 AM
Post #2


or something bad...real bad.

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Could you please post what you have attempted so far, and we will be more than happy to guide you in the right direction.
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Kcbroncofan
post 9 Nov, 2005 - 04:51 AM
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This is what I have.

#include <iostream>

using namespace std;

int isprime(int Prime_Number);
int main()
{
int Number = 0;

int isprime(int Prime_Number)
{
for(int Temp = 2;Temp < Prime_Number;Temp ++)
if(!(Prime_Number % Temp))
return(0); return(1);
}

}
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Amadeus
post 9 Nov, 2005 - 06:04 AM
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g++ -o drink whiskey.cpp

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Well, you are declaring the function inisde the main function...that is a no no, I'm afraid...you should declare like so:
CODE

#include <iostream>

using namespace std;

int isprime(int Prime_Number);
int main()
{
int Number = 0;
cout<<"Enter a number"<<endl;
cin>>Number;
if(Number>0)
{
  if(isprime(Number)==0)
  {
     cout<<"Number is prime"<<endl;
  }
  else
  {
     cout<<"Number is not prime"<<endl;
  }
}
else
{
  cout<<"Positive numbers only"<<endl;
}
  return 0;
}


int isprime(int Prime_Number)
{
//prime number calculation here
}

As for finding a prime number, simply start at 1, loop to the number itslef and use the modulus...if the result of that operation (userNumber%loop number) equals 0, and the loop number is something other than 1 or the user number, the number is not prime.
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bullet proof penguin
post 15 Nov, 2005 - 12:43 PM
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just an idea, limited

have a do while trying to mod divide the input int by 2 and up (++) until you hit the input value and if anything else returns 0 than the number is not prime. about the error if neg just check if the input value is < 0.
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bullet proof penguin
post 16 Nov, 2005 - 12:19 PM
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so i also needed to do this for a class and this is my code, seems to work just fine.

QUOTE

/*
input positive integer and determine if it is prime
*/

#include <iostream.h>

void main()
{
long xaero;
int loop=42;
do
{
  long x=2;
  int p=0;
  cout << "\n\nPlease enter a positive non zero number (-13 to quit)\n";
  cin >> xaero;

  if (xaero == -13)
  {
  loop=13;
  }

  else
  {
  if (xaero < 1)
  {
    cout << "I said a POSITIVE NON ZERO";
    p=13;
  }
  else
  {
    for (x;x<=xaero;x++)
    {
    if ((xaero%x)==0 && (xaero != x))
      {
      x=xaero;
      cout << xaero << " is not a prime number";
      p=1;
      }
    }
  }
  if (p==0)
  {
    cout << xaero << " is a prime number";
  }
  }
}while (loop != 13);
}
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mikepetro
post 18 Nov, 2005 - 11:28 PM
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Hey man,

I know I'm going to get thrashed for this... but it happens. Here is my code written in JAVA for the same exact problem. I have it done in JAVA and the theory is the same, all you have to do is change the syntax. So here it is:

CODE
for (int i = 2; i <= (Math.sqrt(primeInput)); i++)
    {
 if ((primeInput % i) == 0)
     {
   isPrime = false;
   break;
     }
    }
    
    if (isPrime)
 JOptionPane.showMessageDialog(null, "The number you entered " + primeInput + " is PRIME!!", "We have found a prime!", JOptionPane.INFORMATION_MESSAGE);  
    else
 JOptionPane.showMessageDialog(null, "The number you entered " + primeInput + " is NOT PRIME!!", "We have FAILED!", JOptionPane.ERROR_MESSAGE);
    



Also, I forgot about your non-negative number input, once again here is my code in JAVA.

CODE
while (primeInput < 0)
    {
    JOptionPane.showMessageDialog(null,"That was not a positive integer!", "That was not a positive integer!", JOptionPane.ERROR_MESSAGE);
 
    inputStr = JOptionPane.showInputDialog("Please input a positive integer:  ");
    primeInput = Integer.parseInt(inputStr);
    }


This post has been edited by mikepetro: 18 Nov, 2005 - 11:32 PM
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sonal.12
post 14 Sep, 2008 - 08:40 PM
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QUOTE(Kcbroncofan @ 8 Nov, 2005 - 10:01 PM) *

I need to write a program that asks a user to input a positive integer. I then need to out put whether the number is prime. I also need to put in an error message if the user inputs a negative number.

ya i know but i hav 2 find out using functions
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homemade-jam
post 15 Sep, 2008 - 02:37 AM
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eeeAddict

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You only have to divide by a number upto the square root of the number, as after this you are just doing the sums the opposite way around
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numerical_jerome
post 15 Sep, 2008 - 07:20 PM
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also a hint for performance improvement, with the exception of 2 & 3, all primes are of the form (6n)+1 or (6n)-1, where n is a positive integer.* using this as a first test can greatly reduce the run time of your isprime(int) function, and only costs two statements:

CODE

int isprime(int input)
{
    if(input % 6 != 1 || input % 6 != 5)
    {
        return(0);
    }
}



* if you submit a proof of this with your assignment, your professor may give you extra points, or not, but still cool
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