Find Twin Primes between 1 and 1000

[hey77]

how can i calculate all the twin primes, and then add up how many there are?
a prime is any number than can only be divised by 1 and itself, and a twin prime is two primes seperated by 2 ( for example 3 and 5)
here is code for finding a prime if it helps

 #include <iostream> 
using namespace std;
int main ()
{
    int prime, range=1000;
    int n;
    for ( prime = 2; prime<=range; prime++ )
    {
        for ( n=2; n < prime; n++ )
 
 
            if (prime%n==0 )
                break;
            if ( n==prime )
                cout << prime << endl;
            }
    return 0;
}
 

[sicarie]

how can i calculate all the twin primes, and then add up how many there are?
a prime is any number than can only be divised by 1 and itself, and a twin prime is two primes seperated by 2 ( for example 3 and 5)
here is code for finding a prime if it helps

#include <iostream>
using namespace std;
int main ()
{
    int prime, range=1000;
    int n;
    for ( prime = 2; prime<=range; prime++ )
    {
        for ( n=2; n < prime; n++ )
 
 
            if (prime%n==0 )
                break;
            if ( n==prime )
                cout << prime << endl;
            }
    return 0;
}
 

I would put the answers of that in an array, and then iterate through the array comparing each element with its next largest to see if it is part of a twin prime set, and outputting or doing whatever you needed with those.

[hey77]

how can you get it to take one prime and then look at its next in the list?

[sicarie]

how can you get it to take one prime and then look at its next in the list?

Instead of cout'ing the number, assign it to an array (I would recommend reading this first to get a general idea, and then this for a little more info into how memeory is handled - if you are interested), and then create two tmp variables.

Assign the tmp vars (possibly: int tmp_var1, tmp_var2;) then you can use a loop to go through and point one to one place in the index, and another to the place after. So you have two primes, and all you need to do is check if they are twin primes.

Does that make sense (ie, can you construct an algorithm (basic list of steps) from that)?

[hey77]

makes sense! thanks....only problem is i dont think i am allowed to use arrays yet

[RRick]

You can also speed up the inner loop by checking for n <= sqrt(prime). I turns out that if a number has any factors (i.e. not prime), then at least one of those factors is <= sqrt( prime).

For n in the for loop, try:

#include "math.h"
.......
for ( n=2,  n<= int(sqrt(prime))+1, n++)
......
 
 

[sicarie]

makes sense! thanks....only problem is i dont think i am allowed to use arrays yet

Then you can create the tmp, and store it, and then calculate the next one. At that point, you check the tmp to the current (in the 'if (n==prime)' loop), and then output or whatever.

The tricky part will be the initialization - you start from two, so you need to set the tmp_prime1 (or whatever you call it), initially, to something that won't be seen as a twin prime to 2 (I believe 0 would qualify), so you would want to set it to -1 or something, and then set tmp_prime1 to 2 after the first iteration. Or you could set it to 2, and start the count from 3... It all depends how you want to implement it.

[hey77]

how could i set the tmp to the previous prime before it becomes the next?
for example set the tmp to 2, change prime to 3, check if they are a twin, and then repeat the process? would this work? ( maybe i should check how many twin primes there are :)

#include <iostream>
using namespace std;
int main ()
{
    int prime, range=1000;
    int n,tmp=0,count=0;
    for ( prime = 2; prime<=range; prime++ )
    {
        tmp=prime;
        for ( n=2; n < prime; n++ )
 
 
            if (prime%n==0 )
                break;
            if ( prime=tmp+2 )
                count=count+1;
            }
    cout << "Twin Primes = " << count << endl;
    return 0;
}

[sicarie]

how could i set the tmp to the previous prime before it becomes the next?
for example set the tmp to 2, change prime to 3, check if they are a twin, and then repeat the process? would this work? ( maybe i should check how many twin primes there are :)

#include <iostream>
using namespace std;
int main ()
{
    int prime, range=1000;
    int n,tmp=0,count=0;
    for ( prime = 2; prime<=range; prime++ )
    {
        tmp=prime;
        for ( n=2; n < prime; n++ )
 
 
            if (prime%n==0 )
                break;
            if ( prime=tmp+2 )
                count=count+1;
            }
    cout << "Twin Primes = " << count << endl;
    return 0;
}

Personally, I would change the initial value of whatever tmp to be something odd so that it is never considered for the twin prime, that way you wouldn't really have to modify much of your code. Then, I would add, where you had the cout<< prime the if (prime=tmp+2); cout << tmp<<prime; tmp=prime;

Something like that (sorry it's kinda squished together, I'm at work - I'll try to clarify later, if that's too confusing).

[sicarie]

#include <iostream>
using namespace std;
int main ()
{
    int prime, range=1000;
    int n,tmp=0,count=0;
    for ( prime = 2; prime<=range; prime++ )
    {
        tmp=prime;
        for ( n=2; n < prime; n++ ) {
            if (prime%n==0 ) {
                 if ( prime=tmp+2 ) {
                // I believe this is where you know one is a twin prime
                                // so cout prime and tmp (or whatever)
                                // and set tmp equal to prime for the next pass
                            }
            }
               }
        }
    cout << "Twin Primes = " << count << endl;
    return 0;
}
 

I believe that filling those lines in will give you what you need.

[hey77]

thanks for your help!

[Manjiri]

how can i calculate all the twin primes, and then add up how many there are?
a prime is any number than can only be divised by 1 and itself, and a twin prime is two primes seperated by 2 ( for example 3 and 5)
here is code for finding a prime if it helps

 #include <iostream> 
using namespace std;
int main ()
{
    int prime, range=1000;
    int n;
    for ( prime = 2; prime<=range; prime++ )
    {
        for ( n=2; n < prime; n++ )
 
 
            if (prime%n==0 )
                break;
            if ( n==prime )
                cout << prime << endl;
            }
    return 0;
}
 

Hello friend... here i have the solution... you can see...

/* Find Twin Primes between 1 and 1000 */
 
#include<iostream.h>
 
int main()
{
int i,j,n,flag=0,prime_number=0,temp;
cout<<"Enter the number up to which you want to search the twin prime numbers"<<endl;
cin>>n;
for(i=3; i<=n; i++)
{
for(j=2; j<i; j++)
{
if((i%j)==0)
{
flag=1;
}
}
if(flag==0)
{
if(prime_number==0)
{
prime_number=i;
}
else if(i-prime_number==2)
{
cout<<"Twin prime number : "<<"("<< prime_number<<"," << i<<")"<<endl<<endl;
prime_number=i;
}
else
{
prime_number=i;
}
}
flag=0;
 
}
return 0;
}

[Manjiri]

thanks for your help!

Ohhh........ Got it.. it's ok...

[hey77]

no man your code helped too........much more concise then my jumbled mess

[Manjiri]

no man your code helped too........much more concise then my jumbled mess

Ok... it's good then....