Python: How to get Prime Numbers from string using Permutations and Combinations

Question

Complete the following function to return a list of strings lesser than 999 which form prime numbers from a given string of numbers. For example if the given string is “23167” . All the prime numbers less than or equal to 999 in the string are 2,3,23,31,7,167 . So, your function must return [2,3,23,31,7,167] . Although 23167 is a prime number we can ignore it because it is greater than 999. Output: should be list of integers

def primenumber_from_string(string1):
    #your code goes here
    return []

if _name=='main_':
    #you can run any test cases here
    print(primenumber_from_string("8487934"))

Answer 1

Approach the problem systematically. Start with all the possible one-digit numbers, and check if they are prime. Then try all the possible two-digit numbers and see if they are prime. Then try all the possible three-digit numbers and check them all. Lastly show your accumulated results in the expected format.

Answer 2

So it's a basic algorithmic problem.
I'll give you some pseudo - code.
At this point, I'll only give you Brut-force solution, but I'll think about "proper" approach.
So:
Take first digit -> check
Second -> check
Third -> check
{ ... }
compose two digit numbers -> check.
When this is completed, put them into your list.
For checking if number is prime fast ( O(sqrt(N)) ) you can use this function:

## Made by kam021m on: 12.12.2019  ##
## Resources used:
## * GeeksForGeeks article how to check primes fast
## *
def is_prime(n):
    if n == 1:
        return False
    i = 2
    while i*i <= n:
        if n % i == 0:
            return False
        i += 1
    return True

This basically checks if number is prime by % (i) when i is lower than sqrt from n.

If I'll find faster solution, I will post it here.
EDIT 1: also wanted to ask, is it your homework or sth? Do you have time limits on it?