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Prime Number

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Prime Number

 
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  #1
Oct 20th, 2008
C++ Syntax (Toggle Plain Text) 
  1.  
  2. #include <iostream>
  3. using namespace std;
  4.  
  5. int main ()
  6.  
  7. {
  8.  
  9.    int j;
  10.    int t;
  11.    int p;
  12.    int num;
  13.    cout << " Please enter an even integer greater than 2: ";
  14.    cin >> num;
  15.  
  16.    for ( int i = 2 ; i < num; i++ )
  17.  
  18.  
  19.       if   ( ( i == 2 ) or  ( i % 2  != 0  ) )
  20.  
  21.   {
  22.  
  23.          p = i;
  24.       }
  25.  
  26.    {
  27.          for ( int j = 2; j > p; j++ )
  28.  
  29.             if ( p % j != 0 )
  30.      t = p;
  31.  
  32.          cout << t << endl;
  33.  
  34.       }
  35.  
  36.    return 0;
  37.  
  38.  
  39.   }


I'm using this program to give me the prime numbers that are less than the number entered, but it's not working?
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Re: Prime Number

 
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  #2
Oct 20th, 2008
The spacing in your code makes it very hard to read:

C++ Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. using namespace std;
  3.  
  4. int main ()
  5.  
  6. {
  7.    int j;
  8.    int t;
  9.    int p;
  10.    int num;
  11.    cout << " Please enter an even integer greater than 2: ";
  12.    cin >> num;
  13.  
  14.    for ( int i = 2 ; i < num; i++ )
  15.       if   ( ( i == 2 ) or  ( i % 2  != 0  ) )
  16.       {
  17.          p = i;
  18.       }
  19.  
  20.       {
  21.          for ( int j = 2; j > p; j++ )
  22.             if ( p % j != 0 )
  23.                 t = p;
  24.  
  25.          cout << t << endl;
  26.       }
  27.  
  28.    return 0;
  29.   }

You do realize that the top for-loop starts on line 14 and goes to line 18? Lines 20 - 26 are not part of the for-loop that starts on line 14.
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Re: Prime Number

 
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  #3
Oct 21st, 2008
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Originally Posted by VernonDozier View Post
The spacing in your code makes it very hard to read:

C++ Syntax (Toggle Plain Text) 
  1. #include <iostream>
  2. using namespace std;
  3.  
  4. int main ()
  5.  
  6. {
  7.    int j;
  8.    int t;
  9.    int p;
  10.    int num;
  11.    cout << " Please enter an even integer greater than 2: ";
  12.    cin >> num;
  13.  
  14.    for ( int i = 2 ; i < num; i++ )
  15.       if   ( ( i == 2 ) or  ( i % 2  != 0  ) )
  16.       {
  17.          p = i;
  18.       }
  19.  
  20.       {
  21.          for ( int j = 2; j > p; j++ )
  22.             if ( p % j != 0 )
  23.                 t = p;
  24.  
  25.          cout << t << endl;
  26.       }
  27.  
  28.    return 0;
  29.   }

You do realize that the top for-loop starts on line 14 and goes to line 18? Lines 20 - 26 are not part of the for-loop that starts on line 14.
ok how do I fix that?
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Re: Prime Number

 
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  #4
Oct 21st, 2008
wrap it in braces
{

blah

}



Also, in your if statement, I think you're missing an else?

      if   ( ( i == 2 ) or  ( i % 2  != 0  ) )
      {
         p = i;
      }
      else  // ????
      {
           for ( int j = 2; j > p; j++ )
           {
                if ( p % j != 0 )
                    t = p;
           }
            
            cout << t << endl;
        }
Last edited by chococrack; Oct 21st, 2008 at 1:29 am.
I would love to change the world, but they won't give me the source code
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Re: Prime Number

 
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  #5
Oct 21st, 2008
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Originally Posted by koman View Post
ok how do I fix that?

You need brackets around what you want in the for-loop. No brackets means that only the next instruction is part of the loop. Put brackets around what needs to be repeated:

C++ Syntax (Toggle Plain Text) 
  1. for (/* for-loop code */)
  2. {
  3.      // code to be repeated.  Inside brackets.
  4. }
  5. // code that is not repeated.  Outside the brackets.
Last edited by VernonDozier; Oct 21st, 2008 at 1:24 am.
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Re: Prime Number

 
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  #6
Oct 21st, 2008
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Originally Posted by VernonDozier View Post
You need brackets around what you want in the for-loop. No brackets means that only the next instruction is part of the loop. Put brackets around what needs to be repeated:

C++ Syntax (Toggle Plain Text) 
  1. for (/* for-loop code */)
  2. {
  3.      // code to be repeated.  Inside brackets.
  4. }
  5. // code that is not repeated.  Outside the brackets.
I've tried chaning the brackets and it still doesn't work.
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Re: Prime Number

 
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  #7
Oct 21st, 2008
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Originally Posted by koman View Post
I've tried chaning the brackets and it still doesn't work.
Post what you've tried.
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Re: Prime Number

 
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  #8
Oct 21st, 2008
I am curious ..

how did you get this to compile ?

if ( ( i == 2 ) or ( i % 2 != 0 ) )
There is no keyword called "or" in C++ its denoted by "||"
Last edited by stilllearning; Oct 21st, 2008 at 2:13 am.
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Re: Prime Number

 
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  #9
Oct 21st, 2008
Also in addition to the braces, your logic is messed up. See the comments in your code.

C++ Syntax (Toggle Plain Text) 
  1. for ( int i = 2 ; i < num; i++ )
  2.       /* you can add a special case for 2, you only need to check for the factors of
  3.           odd numbers which are greater than 2 */
  4.       if   ( ( i == 2 ) or  ( i % 2  != 0  ) ) 
  5.       {
  6.         /* you don't really need this placeholder. you can just use "i" */
  7.          p = i;
  8.       }
  9.  
  10.       {
  11.          /* this doesn't make sense... you should be checking from 2 upto the odd number.
  12.              j will always be less than p */ 
  13.          for ( int j = 2; j > p; j++ )
  14.            /* this doesn't help either, if your odd number is not divisible by the last 
  15.                number then it will assume it to be prime ? */ 
  16.             if ( p % j != 0 )
  17.                 t = p;
  18.  
  19.          cout << t << endl;
  20.       }

Here is some pseudocode to help

if number is odd
  for i = 2 to number
    if number divisible by i
      haveFactors = true;
      break;
    end if
  end for
  if haveFactors is false, we have a prime number. 
end if
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Re: Prime Number

 
0
  #10
Oct 21st, 2008
C++ Syntax (Toggle Plain Text) 
  1.  
  2. #include <iostream>
  3. using namespace std;
  4.  
  5. int main ()
  6.  
  7. {
  8.    int q;
  9.    int j;
  10.    int sum = 0;
  11.    int p;
  12.    int num;
  13.    cout << " Please enter an even integer greater than 2: ";
  14.    cin >> num;
  15.  
  16.    for ( int i = 2; i < num; i++ )
  17.  
  18.    {
  19.       if  ( ( i == 2 ) or ( i % 2 != 0 ) )
  20.  
  21.          p  = i;
  22.  
  23.       for ( int j = 1; j <= p; j++ )
  24.  
  25.       {
  26.          if ( p % j == 0 )
  27.  
  28.          {
  29.             sum += j;
  30.  
  31.          }
  32.  
  33.       }
  34.  
  35.       {
  36.          if ( sum == p + 1 )
  37.  
  38.  q = p;
  39.  
  40.       }
  41.       cout << q << endl;
  42.  
  43.    }
  44.  
  45.    return 0;
  46.  }


i'm trying to get the ODD numbers but its gives even numbers
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