added updated sparse array

Author: PythonCHB

Solutions/Session08/sparse_array.py

@@ -1,37 +1,62 @@
-#!/usr/bin/env python
 
+"""
+example of emulating a sequence
+"""
 
 class SparseArray(object):
-    def __init__(self, iterable):
-        self.values = {}
-        self.length = len(iterable)
-        for i, val in enumerate(iterable):
-            if val:
-                self.values[i] = val
-        print self.values
-
-    def __getitem__(self, index):
-        if index >= self.length:
-            raise IndexError("sparse array index out of range")
-        else:
-            return self.values.get(index, 0)
 
-    def __setitem__(self, index, value):
-        if index >= self.length:
-            raise IndexError("sparse array index out of range")
-        else:
-            if value == 0:
-                self.values.pop(index, None)
-            else:
-                self.values[index] = value
-
-    def __delitem__(self, index):
-        if index >= self.length:
-            raise IndexError("sparse array index out of range")
-        else:
-            self.values.pop(index, None)
-            self.length -= 1
+    def __init__(self, my_array=()):
+        self.length = len(my_array)
+        self.sparse_array = self.convert_to_sparse(my_array)
 
+    def convert_to_sparse(self, my_array):
+        sparse_array = {}
+        for index, number in enumerate(my_array):
+            if number:
+                sparse_array[index] = number
+        return sparse_array
 
     def __len__(self):
         return self.length
+
+    def __getitem__(self, key):
+        try:
+            return self.sparse_array[key]
+        except KeyError:
+            if key >= self.length:
+                raise IndexError('array index out of range')
+            return 0
+
+    def __setitem__(self, key, value):
+        if key > self.length:
+            raise IndexError('array assignment index out of range')
+        if value != 0:
+            self.sparse_array[key] = value
+        else:
+            # if the value is being set to zero, we probably need to
+            # remove a key from our dictionary.
+            self.sparse_array.pop(key, None)
+
+    def __delitem__(self, key):
+        # we probably need to move the keys if we are not deleting the last
+        # number, use pop in case it was a zero
+        if key == self.length - 1:
+            self.sparse_array.pop(key, None)
+        else:
+            # since we need to adjust all of the keys after the one we are
+            # deleting, probably most efficient to create a new dictionary
+            new_dict = {}
+            for k, v in self.sparse_array.iteritems():
+                if k >= key:
+                    new_dict[k - 1] = v
+                else:
+                    new_dict[k] = v
+            # note we don't want to do update, since we need to make sure we are
+            # getting rid of the old keys, when we moved the value to a new key
+            self.sparse_array = new_dict
+        # length is now one shorter
+        self.length -= 1
+
+
+
+

Solutions/Session08/test_sparse_array.py

@@ -1,6 +1,7 @@
 import pytest
 from sparse_array import SparseArray
 
+
 def set_up():
     my_array = [2, 0, 0, 0, 3, 0, 0, 0, 4, 5, 6, 0, 2, 9]
     my_sparse = SparseArray(my_array)