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Practise Problem, Pair of Prime Numbers

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Practise Problem, Pair of Prime Numbers

 
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  #1
Sep 4th, 2008
Q.Write a program which will print all the pairs of prime numbers whose sum equals the number entered by the user. ( suggested by Aniseed ) (Intermediate)

C++ Syntax (Toggle Plain Text) 
  1. #include<iostream.h>
  2. #include<conio.h>
  3. #include<math.h>
  4.  
  5. int prime(int);
  6. void main()
  7. {
  8.  int i,j,c1,c2,num,f1,f2;
  9.  clrscr();
  10.  cout<<"Enter the number-";
  11.  cin>>num;
  12.  for(i=2,j=num-2;i<=num/2;i++,j--)
  13.  {
  14.   f1=prime(i);
  15.   if(f1) 
  16.    { 
  17.     f2=prime(j);
  18. 	if(f2)
  19.     cout<<"Pair:"<<i<<" + "<<j<<endl;	
  20.    }
  21.  }  
  22.  getch();
  23. }	  
  24. int prime(int a)
  25. {
  26.  int i;
  27.  for(i=2;i<=sqrt(a);i++) 
  28.   if(a%i==0)
  29.    return 0;
  30.   else 
  31.    continue;	
  32.  return 1;
  33. }
Plz verify it. Any suggestions,comments or recommendations are welcomed.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #2
Sep 4th, 2008
> #include<iostream.h>
This is old. The correct form is #include <iostream>
If you (or your compiler) don't know about namespaces, then it's time you looked into it, or upgraded your compiler. namespaces have been standard issue since 1998, so there's really no excuse any more.

> #include<conio.h>
This is non-standard. If you can write your code without it, do so.

> #include<math.h>
In a namespace enabled compiler, this would be #include <cmath>

> void main()
This is just wrong. main returns an int.

Your prime() function could be more efficient, especially for large numbers.
How many even primes are there?
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Re: Practise Problem, Pair of Prime Numbers

 
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  #3
Sep 4th, 2008
Salem's post is just OK. Things that I more wanna say that if you write #include<iostream> then you must write "using namespace std;" next line & you can use #include<cmath> in place of #include<math.h>. That's all.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #4
Sep 4th, 2008
I know about namespaces and the new standards i.e #include<iostream> , but ya my compiler "Turbo C++ v3.0" is old and dont understands the new formats like namespaces and the new style header files. An upgraded, according to the new standards, code will be

C++ Syntax (Toggle Plain Text) 
  1. #include<iostream>
  2. #include<cmath>
  3. using namespace std;
  4. bool isprime(int);
  5. int main()
  6. {
  7.  int i,j,c1,c2,num;
  8.  bool f1,f2;
  9.  cout<<"Enter the number-";
  10.  cin>>num;
  11.  for(i=2,j=num-2;i<=num/2;i++,j--)
  12.  {
  13.   f1=isprime(i);
  14.   if(f1) 
  15.    { 
  16.     f2=isprime(j);
  17. 	if(f2)
  18.     cout<<"Pair:"<<i<<" + "<<j<<endl;	
  19.    }
  20.  }  
  21.  return 0;
  22. }	  
  23. bool isprime(int a)
  24. {
  25.  int i;
  26.  for(i=2;i<=sqrt(a);i++) 
  27.   if(a%i==0)
  28.    return false;
  29.   else 
  30.    continue;	
  31.  return true;
  32. }
And thanks Salem and hiraksarkardg for your concern. Is conio.h not ISO C++ standard. Which funtion to be used in place of clrscr() and getch()?
Last edited by vidit_X; Sep 4th, 2008 at 1:53 pm.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #5
Sep 4th, 2008
> Is conio.h not ISO C++ standard.
Correct.

> Which funtion to be used in place of clrscr() and getch()?
Such things only matter (to some extent) when you're running from the IDE.
Most programs have neither.
Consider 'dir', which neither clears the screen, nor waits for a key press.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #6
Sep 4th, 2008
Why void main is deprecated in ISO standards? Where can i get a brief overview of the ISO C++ standards?
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Re: Practise Problem, Pair of Prime Numbers

 
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  #7
Sep 4th, 2008
You can't deprecate what was never standard to begin with.
void main has always been wrong, despite what many popular books and sloppy compilers say or do
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Re: Practise Problem, Pair of Prime Numbers

 
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  #8
Sep 4th, 2008
Thanks Salem for help, I think you really hate those who use "void main()" and you love "int main()" as your avtar and line under name says
Last edited by vidit_X; Sep 4th, 2008 at 3:19 pm.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #9
Sep 5th, 2008
Any other tips for efficiency?
There is only 1 even prime number i.e 2.
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Re: Practise Problem, Pair of Prime Numbers

 
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  #10
Sep 5th, 2008
>>There is only 1 even prime number i.e 2

So, is there a simple way to determine if a number is even or odd? And, if any factor above 2 is even can the number be prime?

This might be a little more optimized:
C++ Syntax (Toggle Plain Text) 
  1. bool isprime(int a)
  2. { 
  3.   bool result = true;
  4.   int i; 
  5.  
  6.   //assume a > 0
  7.   if(a % 2 == 0)
  8.     result  = false;
  9.   else  
  10.   {
  11.      //check for factors, i, that are odd and above 1
  12.     for(i = 3; i <= sqrt(a); i += 2) 
  13.     {  
  14.        if(a % i == 0) 
  15.        {
  16.           result = false;  
  17.           break;
  18.         }
  19.     }
  20.     return result;
  21. }
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