Messene Primes

This is a discussion on Messene Primes within the C++ Programming forums, part of the General Programming Boards category; is this output correct? i am checking for messene primes. Code: #include <iostream> #include <iomanip> using namespace std; bool IsPrime ...

  1. #1
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    Messene Primes

    is this output correct?
    i am checking for messene primes.
    Code:
    #include <iostream>
    #include <iomanip>
    
    using namespace std;
    
    
    bool IsPrime (long n );
    long power2 ( long n );
    
    int  main()
    {
    
    char reply;
    
    cout << "Mersenne Primes by corey" << endl;
    cout << endl;
    cout << "n" << setw(30) << setfill(' ') << "Mersene Prime" << endl;
    cout << "==" << setw(29) << setfill(' ')<< "=============" << endl;
    
    for ( int i = 0; i < 100; i++ ) {
      if ( IsPrime ( i ) )
      power2(i);
    
    }
    
    cin >> reply;
    return 0;
      }
    
    
    bool IsPrime(long n)
      {
         int i = 0; 
    	 int counter = 0;
         	
         for ( i = 1; i <= n; i++ )
         {
            if ( n % i == 0 )
         {
     		counter++;
         }
         }
         	if (counter == 2)
    
         	return true;
    
         	else
         		
    	return false;
    
         	
         }
    
    long power2 ( long n )
    {
    
    cout << n << setw(30) << setfill(' ') << (1L << n) - 1 << endl;
    
    
    return 0;
    }
    output:
    Code:
    Mersenne Primes by corey
    
    n                 Mersene Prime
    ==                =============
    2                             3
    3                             7
    5                            31
    7                           127
    11                          2047
    13                          8191
    17                        131071
    19                        524287
    23                       8388607
    29                     536870911
    31                    2147483647
    37                            31
    41                           511
    43                          2047
    47                         32767
    53                       2097151
    59                     134217727
    61                     536870911
    67                             7
    71                           127
    73                           511
    79                         32767
    83                        524287
    89                      33554431
    97                             1
    thank you

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Of course it's not correct. 2^37 is most certainly not less than 2^31.

  3. #3
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    thats what i thought. why does it fall apart?
    do i need to check if it's a mesenne prime ?

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    You can't fit 2^37 inside a long int. And I doubt you can fit 2^67 in a long long, either. You'll have to develop a way to handle longer numbers than the system provides (or work with someone else's).

  5. #5
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    Quote Originally Posted by tabstop View Post
    You can't fit 2^37 inside a long int. And I doubt you can fit 2^67 in a long long, either. You'll have to develop a way to handle longer numbers than the system provides (or work with someone else's).
    well i guess this is as far as i go, because i am not sure how to do that..
    thank you

  6. #6
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    well, this was an assignment.
    it's only CS1, but it seems like a bigger task than what we have learned...

    Assignment:

    Write a C++ program to calculate and display all the Mersenne primes from 2 through 1,000,000. Your results should be as presented below, under testing.

    Testing:

    Mersenne Primes
    n Mersenne Prime
    == ==============

    2 3
    3 7
    5 31
    7 127
    13 8191
    17 131071
    19 524287

    Press q (or any other key) followed by 'Enter' to quit:

  7. #7
    and the Hat of Guessing tabstop's Avatar
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    Well, yeah. As you can see, or if you don't trust your teacher get out your calculator and compute, 2^20 = 1048576 so n can't (shouldn't) go any higher than that.

  8. #8
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    using this formula to make/find a mersenne prime
    (1L << n) - 1

    how is that the same as 2^n-1 ?

    dont really understand whats going on with it.
    can someone explain?
    thank you

  9. #9
    and the Hat of Guessing tabstop's Avatar
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    How do you get from 1 to 10? From 47 to 470? From 12342 to 123420?

    Now think about how that will change when you use binary.

  10. #10
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    thank you i got it now.
    in binary 1L shifts it to 2 and 2L would shift it to 4
    Last edited by mrsirpoopsalot; 01-31-2009 at 11:39 PM.

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