## Python, , decision_tree.py

``````"""
Implementation of a basic regression decision tree.
Input data set: The input data set must be 1-dimensional with continuous labels.
Output: The decision tree maps a real number input to a real number output.
"""
import numpy as np

class Decision_Tree:
def __init__(self, depth=5, min_leaf_size=5):
self.depth = depth
self.decision_boundary = 0
self.left = None
self.right = None
self.min_leaf_size = min_leaf_size
self.prediction = None

def mean_squared_error(self, labels, prediction):
"""
mean_squared_error:
@param labels: a one dimensional numpy array
@param prediction: a floating point value
return value: mean_squared_error calculates the error if prediction is used to
estimate the labels
>>> tester = Decision_Tree()
>>> test_labels = np.array([1,2,3,4,5,6,7,8,9,10])
>>> test_prediction = np.float(6)
>>> tester.mean_squared_error(test_labels, test_prediction) == (
...     Test_Decision_Tree.helper_mean_squared_error_test(test_labels,
...         test_prediction))
True
>>> test_labels = np.array([1,2,3])
>>> test_prediction = np.float(2)
>>> tester.mean_squared_error(test_labels, test_prediction) == (
...     Test_Decision_Tree.helper_mean_squared_error_test(test_labels,
...         test_prediction))
True
"""
if labels.ndim != 1:
print("Error: Input labels must be one dimensional")

return np.mean((labels - prediction) ** 2)

def train(self, X, y):
"""
train:
@param X: a one dimensional numpy array
@param y: a one dimensional numpy array.
The contents of y are the labels for the corresponding X values

train does not have a return value
"""

"""
this section is to check that the inputs conform to our dimensionality
constraints
"""
if X.ndim != 1:
print("Error: Input data set must be one dimensional")
return
if len(X) != len(y):
print("Error: X and y have different lengths")
return
if y.ndim != 1:
print("Error: Data set labels must be one dimensional")
return

if len(X) < 2 * self.min_leaf_size:
self.prediction = np.mean(y)
return

if self.depth == 1:
self.prediction = np.mean(y)
return

best_split = 0
min_error = self.mean_squared_error(X, np.mean(y)) * 2

"""
loop over all possible splits for the decision tree. find the best split.
if no split exists that is less than 2 * error for the entire array
then the data set is not split and the average for the entire array is used as
the predictor
"""
for i in range(len(X)):
if len(X[:i]) < self.min_leaf_size:
continue
elif len(X[i:]) < self.min_leaf_size:
continue
else:
error_left = self.mean_squared_error(X[:i], np.mean(y[:i]))
error_right = self.mean_squared_error(X[i:], np.mean(y[i:]))
error = error_left + error_right
if error < min_error:
best_split = i
min_error = error

if best_split != 0:
left_X = X[:best_split]
left_y = y[:best_split]
right_X = X[best_split:]
right_y = y[best_split:]

self.decision_boundary = X[best_split]
self.left = Decision_Tree(
depth=self.depth - 1, min_leaf_size=self.min_leaf_size
)
self.right = Decision_Tree(
depth=self.depth - 1, min_leaf_size=self.min_leaf_size
)
self.left.train(left_X, left_y)
self.right.train(right_X, right_y)
else:
self.prediction = np.mean(y)

return

def predict(self, x):
"""
predict:
@param x: a floating point value to predict the label of
the prediction function works by recursively calling the predict function
of the appropriate subtrees based on the tree's decision boundary
"""
if self.prediction is not None:
return self.prediction
elif self.left or self.right is not None:
if x >= self.decision_boundary:
return self.right.predict(x)
else:
return self.left.predict(x)
else:
print("Error: Decision tree not yet trained")
return None

class Test_Decision_Tree:
"""Decision Tres test class"""

@staticmethod
def helper_mean_squared_error_test(labels, prediction):
"""
helper_mean_squared_error_test:
@param labels: a one dimensional numpy array
@param prediction: a floating point value
return value: helper_mean_squared_error_test calculates the mean squared error
"""
squared_error_sum = np.float(0)
for label in labels:
squared_error_sum += (label - prediction) ** 2

return np.float(squared_error_sum / labels.size)

def main():
"""
In this demonstration we're generating a sample data set from the sin function in
numpy.  We then train a decision tree on the data set and use the decision tree to
predict the label of 10 different test values. Then the mean squared error over
this test is displayed.
"""
X = np.arange(-1.0, 1.0, 0.005)
y = np.sin(X)

tree = Decision_Tree(depth=10, min_leaf_size=10)
tree.train(X, y)

test_cases = (np.random.rand(10) * 2) - 1
predictions = np.array([tree.predict(x) for x in test_cases])
avg_error = np.mean((predictions - test_cases) ** 2)

print("Test values: " + str(test_cases))
print("Predictions: " + str(predictions))
print("Average error: " + str(avg_error))

if __name__ == "__main__":
main()
import doctest

doctest.testmod(name="mean_squarred_error", verbose=True)
``````