added sparse array example of emulating sequences

Author: codedragon

Solutions/Session08/sparse_array.py

@@ -0,0 +1,62 @@
+
+"""
+example of a callable class
+"""
+
+class SparseArray(object):
+
+    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

@@ -0,0 +1,79 @@
+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)
+    return (my_array, my_sparse)
+
+def test_object_exists():
+    my_array, my_sparse = set_up()
+    assert isinstance(my_sparse, SparseArray)
+
+def test_get_non_zero_number():
+    my_array, my_sparse = set_up()
+    assert my_sparse[4] == 3
+
+def test_get_zero():
+    my_array, my_sparse = set_up()
+    assert my_sparse[1] == 0
+
+def test_get_element_not_in_array():
+    my_array, my_sparse = set_up()
+    with pytest.raises(IndexError):
+        my_sparse[14]
+
+def test_get_lenght():
+    my_array, my_sparse = set_up()
+    assert len(my_sparse) == 14
+
+def test_change_number_in_array():
+    my_array, my_sparse = set_up()
+    my_sparse[0] = 3
+    assert my_sparse[0] == 3
+    # make sure others aren't changed
+    assert my_sparse[1] == 0
+    # make sure still same length
+    assert len(my_sparse) == 14
+
+def test_change_number_in_array_to_zero():
+    my_array, my_sparse = set_up()
+    my_sparse[4] = 0
+    assert my_sparse[4] == 0
+    # make sure still same length
+    assert len(my_sparse) == 14
+
+def test_change_number_in_array_from_zero():
+    my_array, my_sparse = set_up()
+    my_sparse[1] = 4
+    assert my_sparse[1] == 4
+    # make sure still same length
+    assert len(my_sparse) == 14
+
+def test_delete_number():
+    my_array, my_sparse = set_up()
+    del(my_sparse[4])
+    # if we delete the 4 position, should now be zero
+    assert my_sparse[4] == 0
+    # should have smaller length
+    assert len(my_sparse) == 13
+
+def test_delete_zero():
+    my_array, my_sparse = set_up()
+    del(my_sparse[5])
+    # should still be zero, but should have shorter length
+    assert my_sparse[5] == 0
+    assert len(my_sparse) == 13
+
+def test_delete_last_number():
+    my_array, my_sparse = set_up()
+    del(my_sparse[13])
+    # should get an error?
+    print 'print some stuf damnit'
+    with pytest.raises(IndexError):
+        my_sparse[13]
+    assert len(my_sparse) == 13
+
+
+