Merge pull request #202 from domix80/Problem_007

Feat: Project Euler Problem 7

Author: StepfenShawn

project_euler/problem_007/sol1.rb

@@ -0,0 +1,40 @@
+#Project Euler Problem 7:  https://projecteuler.net/problem=7
+#10001st prime
+#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 10001st prime number?
+#References: https://en.wikipedia.org/wiki/Prime_number
+
+def is_prime?(number)   
+    value = true
+    if number > 1 and number < 4
+        # 2 and 3 are primes
+        value = true
+    elsif number < 2 or number % 2 == 0 or number % 3 == 0
+    # Negatives, 0, 1, all even numbers, all multiples of 3 are not primes
+        value = false
+    end
+    end_range = (Math.sqrt(number) + 1).to_i
+    # All primes number are in format of 6k +/- 1
+    for i in (5..end_range).step(6)
+        if number % i == 0 or number % (i + 2) == 0
+            value = false
+        end
+    end
+    result = value
+end
+
+def solution(nth = 10001)
+  primes = Array.new()
+  num = 2
+  while primes.length < nth
+    if is_prime?(num)
+      primes.append(num)
+    end
+    num += 1
+  end
+  primes[primes.length - 1]
+end
+
+answer = solution()
+p answer