how to print first 30 twin primes?

[confusedcmsc]

hey!

there's something i would like to ask you guys...

i am having a problem on how to print the first 30 twin primes...

for those who don't know what twin primes are...those are pair of prime numbers that have a difference of two...

i am aiming for the result of:
(3,5)
(5,7)
(11,13)
(17,19) and so on...

but i dont have any idea on how to make a program of it...

there was someone who helped me make it but we only managed to print pairs of odd numbers upto 97...

i would really appreciate your help...
thanks

[Dheeraj Joshi]

Calculate 1st prime
Calculate 2nd prime

See the difference

Calculate 3rd prime

See the difference between 2nd prime and 3rd prime. And so on.

Regards
Dheeraj Joshi

[jkmyoung]

I suggest first writing an isPrime(i) function, that tests whether the number i is prime.

===
After 3 and 5, all (twin) primes have the form 6n-1, 6n+1. (Use the sieve of erasothenes to figure out why.)

Pseudo:

Print out (3, 5)
for (n = 1; there are less than 30 twin primes so far; n++)
  Is 6n-1 prime and is 6n+1 prime?
    Print out 6n-1 and 6n+1

Your isPrime function need not check for 2 and 3 as divisors.

====
Aside for better speed: (IGNORE IF THIS CONFUSES YOU).
If you want to improve the searching further after (5,7), multiply by 5:
30n-1, 30n+1
30n+5, 30n+7 X
30n+11, 30n+13
30n+17, 30n+19
30n+23, 30n+25 X

So all twin primes after (5,7) are in one of the 3 forms:
30n-1, 30n+1
30n+11, 30n+13
30n+17, 30n+19

Then you isPrime function need not check for 2,3, or 5 as divisors.