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Oct 20th, 2008
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Prime Number

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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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koman is offline Offline
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since Oct 2008
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Oct 20th, 2008
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Re: Prime Number

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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VernonDozier is offline Offline
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Oct 21st, 2008
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Re: Prime Number

Click to Expand / Collapse  Quote originally posted by VernonDozier ...
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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koman is offline Offline
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Oct 21st, 2008
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Re: Prime Number

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.
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chococrack is offline Offline
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Oct 21st, 2008
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Re: Prime Number

Click to Expand / Collapse  Quote originally posted by koman ...
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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Oct 21st, 2008
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Re: Prime Number

Click to Expand / Collapse  Quote originally posted by VernonDozier ...
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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Oct 21st, 2008
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Re: Prime Number

Click to Expand / Collapse  Quote originally posted by koman ...
I've tried chaning the brackets and it still doesn't work.
Post what you've tried.
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Oct 21st, 2008
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Re: Prime Number

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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stilllearning is offline Offline
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Oct 21st, 2008
1

Re: Prime Number

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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Oct 21st, 2008
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Re: Prime Number

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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koman is offline Offline
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This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.
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