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Jul 6th, 2007
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isPrime()

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I need to write a main() function, that loops over a bunch of
numbers from 0 to 100, calls another function called isPrime() for each of them, and print out the numbers that are prime.
I'm new to C++ and any help would be appreciated.
Last edited by forumposters; Jul 6th, 2007 at 9:08 pm.
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forumposters is offline Offline
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Jul 6th, 2007
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Re: isPrime()

>I'm new to C++ and any help would be appreciated.
At least start trying to solve your problem before asking for help. It makes us like you more and ignore you less.
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Narue is offline Offline
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Jul 6th, 2007
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Re: isPrime()

What narue said applies. But I'll help you a little: isPrime(int x)
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sillyboy is offline Offline
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Jul 7th, 2007
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Re: isPrime()

Here's what I've come up with so far:
C++ Syntax (Toggle Plain Text)
  1. #include <iostream>
  2.  
  3.   bool IsPrime(int num)
  4.   {
  5.  if(num == 0)
  6.    return true;
  7.  
  8.  num = abs(num);
  9.  
  10.   for(int i = 2; i <= sqrt(num); i++)
  11.    if(num % i == 0)
  12.   return false;
  13.  
  14.   return true;
  15.   }
Last edited by ~s.o.s~; Jul 7th, 2007 at 3:38 am. Reason: Added code tags, learn to use them.
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Re: isPrime()

You've been around over a year and haven't read the Rules yet? You also need to read this. It will explain that you need to tell us what problem you are having so we don't have to guess -- among other things
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WaltP is offline Offline
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Re: isPrime()

>Here's what I've come up with so far
Somehow I find it hard to believe that you can write IsPrime but not the main driver to print the primes in the range [0..100). I'm guessing you were given IsPrime and told to write main, which means you've come up with nothing so far. But, the hard part is done already. All you need to do is loop from 0 to 100 and call IsPrime on the current number. If it returns true, print the number:
C++ Syntax (Toggle Plain Text)
  1. for ( int i = 0; i < 100; i++ ) {
  2.   if ( IsPrime ( i ) )
  3.     cout<< i <<" is prime\n";
  4. }
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Re: isPrime()

Click to Expand / Collapse  Quote originally posted by Narue ...
>Here's what I've come up with so far
Somehow I find it hard to believe that you can write IsPrime but not the main driver to print the primes in the range [0..100). I'm guessing you were given IsPrime and told to write main, which means you've come up with nothing so far.
[/code]
LOL! If some people would put half of the effort that they put into "avoiding work" into something productive, they would have had the task acomplished by now!

If laziness is truely the mother of invention, we have quite a group of potential inventors here!
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Aug 28th, 2010
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Re: isPrime()
This is more efficient.

#include <iostream>

bool IsPrime(int num)
{
if(num == 0)
return true;

num = abs(num);

if(num % 2 == 0) return true;

for(int i = 3; i <= sqrt(num); i+=2)
if(num % i == 0)
return false;

return true;
}
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soeja is offline Offline
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Aug 28th, 2010
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Re: isPrime()
C++ Syntax (Toggle Plain Text)
  1. #include <iostream>
  2.  
  3. bool IsPrime(int num)
  4. {
  5. if(num == 0)
  6. return true;
  7.  
  8. num = abs(num);
  9.  
  10. if(num % 2 == 0) return true;
  11.  
  12. for(int i = 3; i <= sqrt(num); i+=2)
  13. if(num % i == 0)
  14. return false;
  15.  
  16. return true;
  17. }
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soeja is offline Offline
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Apr 11th, 2011
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IsPrime - optimized

Hi Soeja,
This is more efficient.. and for 4 , 6 , 8(i.e., even no) .. ur code snippet will give wrong ans.. 
  1. #include <iostream>
  2.  
  3. bool IsPrime (int num)
  4. {
  5. if ( 2 == num ) return true ;
  6. int l = sqrt(num) ;
  7. for(int i = 3; i <= l; i+=2)
  8. if ( 0 == ( num % i ) )
  9. return false;
  10.  
  11. return true;
  12. }
Last edited by Ezzaral; Jun 2nd, 2011 at 1:39 pm. Reason: Snipped blog link. Please confine personal links to your user signature.
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parameshyss 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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