I used a simple recursive function. The solution, implemented in C, is here: sprime.c

The logic is the following:
1. The function receives “n”.
2. If n is greater than 1, we do function(n-1)
3. If n=1, we just return the four only primes with one digit: 2, 3, 5 and 7.
4. We have an array called cont, that keep the value of numbers with each number of digits (its index).
5. After do function(n-1), when n is greater than 1, I do the following loop:

for i=0 until cont[n-1]; do
for j=1 until 9, increasing 2; do
num=prime[n-1][i]*10+j;
if (is_prime(num)) {
prime[n][cont[n]++]=num;
}
}
}

This function, only make num=prime[n-1][i]*10+j, where prime[n-1][i] is a prime with less one algarism and we do it times 10 because we are on decimal system. We do plus “j” because “j” is a odd value that can be the unit of a prime.

So, we formed all the primes with each n.

Understood? Sorry my bad english, i’m trying to improve it… lol

Any problem, answer me!