Python/project_euler/problem_07/sol1.py

# -*- coding: utf-8 -*-
"""
By listing the first six prime numbers:

    2, 3, 5, 7, 11, and 13

We can see that the 6th prime is 13. What is the Nth prime number?
"""
from __future__ import print_function
from math import sqrt

try:
    raw_input  # Python 2
except NameError:
    raw_input = input  # Python 3


def isprime(n):
    if n == 2:
        return True
    elif n % 2 == 0:
        return False
    else:
        sq = int(sqrt(n)) + 1
        for i in range(3, sq, 2):
            if n % i == 0:
                return False
    return True


def solution(n):
    """Returns the n-th prime number.
 
    >>> solution(6)
    13
    >>> solution(1)
    2
    >>> solution(3)
    5
    >>> solution(20)
    71
    >>> solution(50)
    229
    >>> solution(100)
    541
    """
    i = 0
    j = 1
    while i != n and j < 3:
        j += 1
        if isprime(j):
            i += 1
    while i != n:
        j += 2
        if isprime(j):
            i += 1
    return j


if __name__ == "__main__":
    print(solution(int(raw_input().strip())))