0

```
# Split a dataset based on an attribute and an attribute value
def t_split(x, y, z):
l, r = list(), list()
for row in z:
if row[x] < y:
l.append(row)
else:
r.append(row)
return l, r
# Calculate the Gini index for a split dataset
def gini_index(grp, class_values):
gini = 0.0
for class_value in class_values:
for group in grp:
size = len(group)
if size == 0:
continue
proportion = [row[-1] for row in group].count(class_value) / float(size)
gini += (proportion * (1.0 - proportion))
return gini
# Select the best split point for a dataset
def get_split(dataset):
class_values = list(set(row[-1] for row in dataset))
b_index, b_value, b_score, b_groups = 999, 999, 999, None
for index in range(len(dataset[0])-1):
for row in dataset:
groups = t_split(index, row[index], dataset)
gini = gini_index(groups, class_values)
if gini < b_score:
b_index, b_value, b_score, b_groups = index, row[index], gini, groups
return {'index':b_index, 'value':b_value, 'groups':b_groups}
# Create a terminal node value
def to_terminal(group):
outcomes = [row[-1] for row in group]
return max(set(outcomes), key=outcomes.count)
# Create child splits for a node or make terminal
def split(node, max_depth, min_size, depth):
left, right = node['groups']
del(node['groups'])
# check for a no split
if not left or not right:
node['left'] = node['right'] = to_terminal(left + right)
return
# check for max depth
if depth >= max_depth:
node['left'], node['right'] = to_terminal(left), to_terminal(right)
return
# process left child
if len(left) <= min_size:
node['left'] = to_terminal(left)
else:
node['left'] = get_split(left)
split(node['left'], max_depth, min_size, depth+1)
# process right child
if len(right) <= min_size:
node['right'] = to_terminal(right)
else:
node['right'] = get_split(right)
split(node['right'], max_depth, min_size, depth+1)
# Build a decision tree
def build_tree(train, max_depth, min_size):
root = get_split(train)
split(root, max_depth, min_size, 1)
return root
# Print a decision tree
def print_tree(node, depth=0):
if isinstance(node, dict):
print('%s[X%d < %.3f]' % ((depth*' ', (node['index']+1), node['value'])))
print_tree(node['left'], depth+1)
print_tree(node['right'], depth+1)
else:
print('%s[%s]' % ((depth*' ', node)))
dataset = [[2.771244718,1.784783929,0],
[1.728571309,1.169761413,0],
[3.678319846,2.81281357,0],
[3.961043357,2.61995032,0],
[2.999208922,2.209014212,0],
[7.497545867,3.162953546,1],
[9.00220326,3.339047188,1],
[7.444542326,0.476683375,1],
[10.12493903,3.234550982,1],
[6.642287351,3.319983761,1]]
tree = build_tree(dataset, 1, 1)
print_tree(tree)
```