Maps and Hashing completed

Author: linkel

4-maps-hashing/Hashing-notes.md

@@ -46,3 +46,12 @@ What number would you recommend his function to divide by to speed it up?
 87 creates collisions. 
 125 is divisible by 5 and hence also creates collisions. 
 1001 is wasted space.
+
+### Hash Maps and String Keys
+
+Starting with a key value pair, one can run the keys through a hash function and then store the value in the bucket belonging to the hashed result of the key. 
+
+This can be done with strings too, most typically by using the ASCII value of the string. Python's ord() function converts a char into its ASCII value, and the chr() function converts it back.
+
+Apparently the standard in Java prefers big hash tables over collisions, so they use an equation like:
+s[0]*31^(n-1) + s[1]*31^(n-2) + ... s[n-1]

4-maps-hashing/StringKeys-mine.py

@@ -0,0 +1,63 @@
+"""Write a HashTable class that stores strings
+in a hash table, where keys are calculated
+using the first two letters of the string."""
+
+class HashTable(object):
+    def __init__(self):
+        self.table = [None]*10000
+
+    def store(self, string):
+        """Input a string that's stored in 
+        the table."""
+        # Hash Value = (ASCII Value of First Letter * 100) + ASCII Value of Second Letter 
+        hash_val = ord(string[0])*100 + ord(string[1])
+        if self.table[hash_val] == None:
+            self.table[hash_val] = [string]
+        else:
+            self.table[hash_val].append(string)
+        
+    def lookup(self, string):
+        """Return the hash value if the
+        string is already in the table.
+        Return -1 otherwise."""
+        hash_val = ord(string[0])*100 + ord(string[1])
+        if self.table[hash_val] != None:
+            for i in self.table[hash_val]:
+                if i == string:
+                    return hash_val
+        return -1
+
+    def calculate_hash_value(self, string):
+        """Helper function to calulate a
+        hash value from a string."""
+        hash_val = ord(string[0])*100 + ord(string[1])
+        return hash_val
+        
+    # for myself
+    def show_bucket(self, string):
+        hash_val = ord(string[0])*100 + ord(string[1])
+        if self.table[hash_val] != None:
+            for i in self.table[hash_val]:
+                print(i)
+    
+# Setup
+hash_table = HashTable()
+
+# Test calculate_hash_value
+# Should be 8568
+print hash_table.calculate_hash_value('UDACITY')
+
+# Test lookup edge case
+# Should be -1
+print hash_table.lookup('UDACITY')
+
+# Test store
+hash_table.store('UDACITY')
+# Should be 8568
+print hash_table.lookup('UDACITY')
+
+# Test store edge case
+hash_table.store('UDACIOUS')
+# Should be 8568
+print hash_table.lookup('UDACIOUS')
+hash_table.show_bucket('UDACIOUS')