Need help for simple prime number question

allenfanwenyuan · #1 04-Apr-2007, 07:59

The question is to Write a C++ program that takes an integer n and displays the first prime number after n.
Your program should output n itself in case n is prime.
Sample 1:
Enter an integer: 17
17
Press any key to continue . . .

Sample 2:
Enter an integer: 63
67
Press any key to continue . . .

Sample 3:
Enter an integer: 12345678
12345701
Press any key to continue . . .

my code is

CPP / C++ / C Code:

void primeNo(int a)
{
    int o=0;
    for (int i=2;i<=a;i++)
    {
        if(a%i==0)
        {
                  ++o;
        }
                  }
                  if(o==1)
                  cout<<a<<endl;
                  

                  }

int main()
{
    int n;
    cout << "Enter an integer: " << flush;
    cin >> n;
    primeNo(n);
    
    system("pause");
    return 0;
}

I can test and generate if input is a prime number ,but get stuck with how to generate the first prime number after input which is not a prime number.
plz for help!!!

davekw7x · #2 04-Apr-2007, 09:45

Quote:

Originally Posted by allenfanwenyuan

The question is to Write a C++ program that takes an integer n and displays the first prime number after n.

I can test and generate if input is a prime number ,but get stuck with how to generate the first prime number after input which is not a prime number.
plz for help!!!

You could try something like this:

Make a boolean function that tests whether its argument is a prime number. The function doesn't print anything out; it returns "true" if the argument is a prime number, and "false" if not:

CPP / C++ / C Code:

bool IsPrimeNo(int a)
{
 // test to see whether "a" is prime
 // if it is: then return "true"
 // else:  return "false"
}

Then your main program can look something like this:

Code:

BEGIN MAIN PROGRAM
    Get value of <n> from user.

    IF (IsPrimeNo(n))
    THEN
        print message: <n> " is a prime number"
    ELSE
        print message: <n> " is not a prime number"

        Make a loop that increments <n> and tests to see
        whether it is a prime number.
        Exit the loop when it is.

        print message: <n> " is the next larger prime number"
    END IF
END MAIN PROGRAM

As a matter of style, I made my output statements a little more verbose than your assignment. I suggest that you make your output exactly as the assignment requires.

A few runs for mine might look like:

Code:

Enter an integer: 37
37 is a prime number


Enter an integer: 99
99 is not a prime number
101 is the next larger prime number


Enter an integer: 129
129 is not a prime number
131 is the next larger prime number

Regards,

Dave

#3 04-Apr-2007, 11:40

You may want to rethink your algorithm somewhat, but here is a very simple example of what I think you're trying to accomplish:

CPP / C++ / C Code:

#include <iostream>
#include <cmath>

using namespace std;

bool IsPrimeNumber( int number )
{
    bool bIsNotPrime = false;
    if( number > 2 )
    {
        for( int i = 2; i <= sqrt( number ); i++ )
        {
            if( number % i == 0 )
            {
                bIsNotPrime = true;
                break;
            }
        }
    }
    else
    {
        switch( number )
        {
        case 2:
            bIsNotPrime = false;
            break;
        default:
            bIsNotPrime = true;
            break;
        }
    }
    return !bIsNotPrime;
}

int main()
{
    cout << "Enter an integer: ";
    int number;
    cin >> number;

    bool bIsPrime = false;
    while( !bIsPrime )
    {
        if( (bIsPrime = IsPrimeNumber( number )) )
        {
            cout << number << endl;
        }
        else
        {
            ++number;
        }
    }
    return 0;
}

Output:

Code:

Enter an integer: 7
7

Enter an integer: 8
11

Enter an integer: 13
13

Enter an integer: 27
29

:davis:

allenfanwenyuan · #4 04-Apr-2007, 18:10

cheers ,guys .....i am fully understand that ....!many thanks

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
i have a question about the number of players you can have when hosting games.	brandon1482	Computer Software Forum - Games	1	24-Feb-2007 13:08
Converting a number amount to text	Godzilla	CPP / C++ Forum	5	31-Mar-2006 11:38
Program Needed: Perfect Number Program in C language	koool_kid	C Programming Language	12	02-Dec-2005 14:02
Anyone can write a program code for this???	chriskan76	C Programming Language	1	19-Oct-2004 20:25
Help w/ prime # determination	crystalattice	C Programming Language	17	18-Apr-2004 21:43

Need help for simple prime number question

Re: need help for simple prime number question

Re: need help for simple prime number question

Re: need help for simple prime number question