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Jul 16th, 2005

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C++ prime number program help

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Below is my code for printing prime numbers up to a given input by user. However, my calculations are not working correctly and my output is printing twice. Any idea?

#include <iostream>
#include <cmath>

using namespace std;
void prime_num(int);

int main()
{

cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;

prime_num(num);
}

void prime_num( int num)
{
int check_prime = 0;

for ( int i = 0; i <= num; i++)
{
check_prime = 1;

for ( int j = 2; j <= i/2; j++)
{
if ( i % j == 0 )

check_prime = 0;

if ( check_prime != 0 )
{
cout << i << endl;
}
}
}
}

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Prime Number program.. with a twist in C

djbsabkcb

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Jul 16th, 2005

Re: C++ prime number program help

Here's what I went with. Note you want to do a break if you find out it's divisible as there is no point in checking all the numbers beyond it for divisibility..

C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 0; i <= num; i++)
		{
				for ( int j = 2; j <= num; j++)
				{
						if ( i!=j && i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 0; i <= num; i++)
		{
				for ( int j = 2; j <= num; j++)
				{
						if ( i!=j && i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}

Enter a number and I will generate the prime numbers up to that number: 100
Prime:1
Prime:2
Prime:3
Prime:5
Prime:7
Prime:11
Prime:13
Prime:17
Prime:19
Prime:23
Prime:29
Prime:31
Prime:37
Prime:41
Prime:43
Prime:47
Prime:53
Prime:59
Prime:61
Prime:67
Prime:71
Prime:73
Prime:79
Prime:83
Prime:89
Prime:97

Note you can check your results here:
http://www.utm.edu/research/primes/lists/small/1000.txt

winbatch

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Jul 16th, 2005

Re: C++ prime number program help

It is printing the number multiple times, becuse of:

C++ Syntax (Toggle Plain Text)
        for ( int j = 2; j <= i/2; j++)
        {
            if ( i % j == 0 )
            {
                check_prime = 0;
            }
 
            if ( check_prime != 0 )
            {
                cout << i << endl;
            }
        }
        for ( int j = 2; j <= i/2; j++)
        {
            if ( i % j == 0 )
            {
                check_prime = 0;
            }
            
            if ( check_prime != 0 )
            {
                cout << i << endl;
            }
        }

here you are printing out the number when check prime != 0.
but untill i%j == 0 the program are going to ouput that the number is prime, even when it aint.
this is bad design, and I think you need to rewrite your function.

check out: http://www.daniweb.com/techtalkforum...064-prime.html

zyruz

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Jul 16th, 2005

Re: C++ prime number program help

I see what you are saying but that still isn't right because 1 is not a prime number

djbsabkcb

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Jul 16th, 2005

Re: C++ prime number program help

Quote originally posted by winbatch ...

Here's what I went with. Note you want to do a break if you find out it's divisible as there is no point in checking all the numbers beyond it for divisibility..
C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 0; i <= num; i++)
		{
				for ( int j = 2; j <= num; j++)
				{
						if ( i!=j && i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}
#include <iostream> using namespace std; void prime_num(int); int main() { cout << " Enter a number and I will generate the prime numbers up to that number: "; int num = 0; cin >> num; prime_num(num); } void prime_num( int num) { bool isPrime=true; for ( int i = 0; i <= num; i++) { for ( int j = 2; j <= num; j++) { if ( i!=j && i % j == 0 ) { isPrime=false; break; } } if (isPrime) { cout <<"Prime:"<< i << endl; } isPrime=true; } }
Enter a number and I will generate the prime numbers up to that number: 100
Prime:1
Prime:2
Prime:3
Prime:5
Prime:7
Prime:11
Prime:13
Prime:17
Prime:19
Prime:23
Prime:29
Prime:31
Prime:37
Prime:41
Prime:43
Prime:47
Prime:53
Prime:59
Prime:61
Prime:67
Prime:71
Prime:73
Prime:79
Prime:83
Prime:89
Prime:97

Note you can check your results here:
http://www.utm.edu/research/primes/lists/small/1000.txt

I see what you are saying but that still isn't right because 1 is not a prime number

djbsabkcb

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Jul 16th, 2005

Re: C++ prime number program help

So Picky

. As you wish:

C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 2; i <= num; i++)
		{
				for ( int j = 2; j <i; j++)
				{
						if ( i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 2; i <= num; i++)
		{
				for ( int j = 2; j <i; j++)
				{
						if ( i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}

Enter a number and I will generate the prime numbers up to that number: 150
Prime:2
Prime:3
Prime:5
Prime:7
Prime:11
Prime:13
Prime:17
Prime:19
Prime:23
Prime:29
Prime:31
Prime:37
Prime:41
Prime:43
Prime:47
Prime:53
Prime:59
Prime:61
Prime:67
Prime:71
Prime:73
Prime:79
Prime:83
Prime:89
Prime:97
Prime:101
Prime:103
Prime:107
Prime:109
Prime:113
Prime:127
Prime:131
Prime:137
Prime:139
Prime:149

winbatch

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Jul 16th, 2005

Re: C++ prime number program help

Quote originally posted by winbatch ...

So Picky . As you wish:
C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
void prime_num(int);
int main()
{
cout << " Enter a number and I will generate the prime numbers up to that number: ";
int num = 0;
cin >> num;
prime_num(num);
}
 
void prime_num( int num)
{
		bool isPrime=true;
		for ( int i = 2; i <= num; i++)
		{
				for ( int j = 2; j <i; j++)
				{
						if ( i % j == 0 )
						{
						  isPrime=false;
						  break;
						}
				}
				if (isPrime)
				{
				  cout <<"Prime:"<< i << endl;
				}
				isPrime=true;
		}
}
#include <iostream> using namespace std; void prime_num(int); int main() { cout << " Enter a number and I will generate the prime numbers up to that number: "; int num = 0; cin >> num; prime_num(num); } void prime_num( int num) { bool isPrime=true; for ( int i = 2; i <= num; i++) { for ( int j = 2; j <i; j++) { if ( i % j == 0 ) { isPrime=false; break; } } if (isPrime) { cout <<"Prime:"<< i << endl; } isPrime=true; } }
Enter a number and I will generate the prime numbers up to that number: 150
Prime:2
Prime:3
Prime:5
Prime:7
Prime:11
Prime:13
Prime:17
Prime:19
Prime:23
Prime:29
Prime:31
Prime:37
Prime:41
Prime:43
Prime:47
Prime:53
Prime:59
Prime:61
Prime:67
Prime:71
Prime:73
Prime:79
Prime:83
Prime:89
Prime:97
Prime:101
Prime:103
Prime:107
Prime:109
Prime:113
Prime:127
Prime:131
Prime:137
Prime:139
Prime:149

thanks, that is correct.

its funny you can sit in front of this screen and try to debug for hours and once you figure it out it was something small to begin with. Very frustrating.

djbsabkcb

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Jul 17th, 2005

Re: C++ prime number program help

I know what you mean...

By the way, no need to quote the entire previous posting when putting a reply (only the relevant piece)..

winbatch

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Sep 4th, 2005

Re: C++ prime number program help

Hello My friend, I do not know if my response will be in time for you but here is a master code for finding all prime numbers up to any number 'x' that you input...

C++ Syntax (Toggle Plain Text)
# include <cmath>          // This library enable the use of sqrt.
# include <iostream>
using namespace std;
 
void primenum(long double);     // Prototype...
int c = 0;
 
int main()
{
long double x = 0;
cout<<"\n This program will generate all prime numbers up to the"
    <<"\n number you have entered below...\n";
cout<<"\n Please enter a number: ";
cin>> x;
 
cout<<"\n Here are all the prime numbers up to "<<x<<".\n";
primenum(x);            //function invocation...
cout<<endl<<"\nThere are "<<c
    <<" prime numbers less than or equal to "<<x<<".\n\n";
 
return 0;
}
 
// This function will determine the primenumbers up to num.
void primenum(long double x)
{
    bool prime = true;                  // Calculates the square-root of 'x'
    int number2;
     number2 =(int) floor (sqrt (x));
 
    for (int i = 1; i <= x; i++){       
        for ( int j = 2; j <= number2; j++){
            if ( i!=j && i % j == 0 ){      
                prime = false;
                break;
            }
        }
        if (prime){
            cout <<"  "<<i<<" ";
            c += 1;
        }
        prime = true;
    }
}
# include <cmath>          // This library enable the use of sqrt.
# include <iostream>
using namespace std;

void primenum(long double);     // Prototype...
int c = 0;

int main()
{
long double x = 0;
cout<<"\n This program will generate all prime numbers up to the"
    <<"\n number you have entered below...\n";
cout<<"\n Please enter a number: ";
cin>> x;

cout<<"\n Here are all the prime numbers up to "<<x<<".\n";
primenum(x);            //function invocation...
cout<<endl<<"\nThere are "<<c
    <<" prime numbers less than or equal to "<<x<<".\n\n";

return 0;
}

// This function will determine the primenumbers up to num.
void primenum(long double x)
{
    bool prime = true;                  // Calculates the square-root of 'x'
    int number2;
     number2 =(int) floor (sqrt (x));

    for (int i = 1; i <= x; i++){       
        for ( int j = 2; j <= number2; j++){
            if ( i!=j && i % j == 0 ){      
                prime = false;
                break;
            }
        }
        if (prime){
            cout <<"  "<<i<<" ";
            c += 1;
        }
        prime = true;
    }
}

// In you need any explanation for any parts of this program send me a message and i will reply as soon as i can...

Last edited by Dave Sinkula; Sep 4th, 2005 at 12:07 pm. Reason: Added [code][/code] tags.

Pcyther

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Mar 26th, 2007

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Re: C++ prime number program help

Click to Expand / Collapse Quote originally posted by Pcyther ...

Hello My friend, I do not know if my response will be in time for you but here is a master code for finding all prime numbers up to any number 'x' that you input...
C++ Syntax (Toggle Plain Text)
# include <cmath>          // This library enable the use of sqrt.
# include <iostream>
using namespace std;
 
void primenum(long double);     // Prototype...
int c = 0;
 
int main()
{
long double x = 0;
cout<<"\n This program will generate all prime numbers up to the"
    <<"\n number you have entered below...\n";
cout<<"\n Please enter a number: ";
cin>> x;
 
cout<<"\n Here are all the prime numbers up to "<<x<<".\n";
primenum(x);            //function invocation...
cout<<endl<<"\nThere are "<<c
    <<" prime numbers less than or equal to "<<x<<".\n\n";
 
return 0;
}
 
// This function will determine the primenumbers up to num.
void primenum(long double x)
{
    bool prime = true;                  // Calculates the square-root of 'x'
    int number2;
     number2 =(int) floor (sqrt (x));
 
    for (int i = 1; i <= x; i++){       
        for ( int j = 2; j <= number2; j++){
            if ( i!=j && i % j == 0 ){      
                prime = false;
                break;
            }
        }
        if (prime){
            cout <<"  "<<i<<" ";
            c += 1;
        }
        prime = true;
    }
}
# include <cmath> // This library enable the use of sqrt. # include <iostream> using namespace std; void primenum(long double); // Prototype... int c = 0; int main() { long double x = 0; cout<<"\n This program will generate all prime numbers up to the" <<"\n number you have entered below...\n"; cout<<"\n Please enter a number: "; cin>> x; cout<<"\n Here are all the prime numbers up to "<<x<<".\n"; primenum(x); //function invocation... cout<<endl<<"\nThere are "<<c <<" prime numbers less than or equal to "<<x<<".\n\n"; return 0; } // This function will determine the primenumbers up to num. void primenum(long double x) { bool prime = true; // Calculates the square-root of 'x' int number2; number2 =(int) floor (sqrt (x)); for (int i = 1; i <= x; i++){ for ( int j = 2; j <= number2; j++){ if ( i!=j && i % j == 0 ){ prime = false; break; } } if (prime){ cout <<" "<<i<<" "; c += 1; } prime = true; } }
// In you need any explanation for any parts of this program send me a message and i will reply as soon as i can...

very very very nice programmmm......
and thanks for this .....

inoxmum

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