From 45f27c80b516f6d64bf50674226a4472bf7cfde6 Mon Sep 17 00:00:00 2001
From: duncanmichel <25329325+duncanmichel@users.noreply.github.com>
Date: Tue, 23 Apr 2019 13:56:27 -0400
Subject: [PATCH] Create euler3.py

---
 Euler/euler3.py | 50 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 Euler/euler3.py

diff --git a/Euler/euler3.py b/Euler/euler3.py
new file mode 100644
index 0000000..7cc3d19
--- /dev/null
+++ b/Euler/euler3.py
@@ -0,0 +1,50 @@
+"""
+
+"""
+
+#code
+def isPrime(x):
+    if x < 2: return False
+    for i in range(2,int(x**0.5)+1):
+        if x%i == 0:
+            return False
+    return True
+    
+def createSieve(maxNum):
+    if maxNum < 2: #no primes less than 2
+        return 0
+    nums = [1] * maxNum #identify 'prime'ness of nums up to maxNum
+    nums[0] = nums[1] = 0 #not primes
+    #print("sieve looks like",nums)
+    for index in range(2, int(maxNum ** 0.5) + 1):#no prime factors after square root
+        if nums[index] == 1:
+            nums[index*index:maxNum:index] = [0] * int((maxNum-index*index-1)/index + 1)  
+            nums[index] = index #put value at index
+        #print("at index",index,"sieve looks like",nums)
+    for index in range(int(maxNum ** 0.5) + 1,maxNum//2): #all numbers not marked '0' after square root are prime
+        if nums[index] == 1:
+            nums[index] = index #put value at index
+    #print("At end sieve looks like",nums)
+    return [x for x in nums if (x != 0 and x != 1)]
+
+def largestPrimeFactor(num:int,nums):
+    #if isPrime(num): # if num is prime, it will be the largest prime factor 
+    #    return num 
+    '''
+    #not working, because taking too long
+    for x in reversed(range(1,num//2+1,2)): #look down from half to 2
+        #print(x)
+        if num%x == 0 and isPrime(x):
+            return x
+    '''
+    #print(nums)
+    #not working, because memory error trying to allocate initial list for large values in createSieve() 
+    for x in reversed(nums): #check list of primes less than number to see if they are factors, largest to smallest
+        if num%x == 0: return x
+    return num #num must be prime
+
+if __name__ == "__main__":
+    #print("I am main")
+    n = 39
+    #n = 600851475143
+    print("The largest prime factor of ",n,"is",largestPrimeFactor(n,createSieve(n)))