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Learning Objectives:
One way to reduce complexity is to use a regularization function that encourages weights to be exactly zero. For linear models such as regression, a zero weight is equivalent to not using the corresponding feature at all. In addition to avoiding overfitting, the resulting model will be more efficient.
L1 regularization is a good way to increase sparsity.
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from __future__ import print_function
import math
from IPython import display
from matplotlib import cm
from matplotlib import gridspec
from matplotlib import pyplot as plt
import numpy as np
import pandas as pd
from sklearn import metrics
import tensorflow as tf
from tensorflow.python.data import Dataset
tf.logging.set_verbosity(tf.logging.ERROR)
pd.options.display.max_rows = 10
pd.options.display.float_format = '{:.1f}'.format
california_housing_dataframe = pd.read_csv("https://download.mlcc.google.com/mledu-datasets/california_housing_train.csv", sep=",")
california_housing_dataframe = california_housing_dataframe.reindex(
np.random.permutation(california_housing_dataframe.index))
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def preprocess_features(california_housing_dataframe):
"""Prepares input features from California housing data set.
Args:
california_housing_dataframe: A Pandas DataFrame expected to contain data
from the California housing data set.
Returns:
A DataFrame that contains the features to be used for the model, including
synthetic features.
"""
selected_features = california_housing_dataframe[
["latitude",
"longitude",
"housing_median_age",
"total_rooms",
"total_bedrooms",
"population",
"households",
"median_income"]]
processed_features = selected_features.copy()
# Create a synthetic feature.
processed_features["rooms_per_person"] = (
california_housing_dataframe["total_rooms"] /
california_housing_dataframe["population"])
return processed_features
def preprocess_targets(california_housing_dataframe):
"""Prepares target features (i.e., labels) from California housing data set.
Args:
california_housing_dataframe: A Pandas DataFrame expected to contain data
from the California housing data set.
Returns:
A DataFrame that contains the target feature.
"""
output_targets = pd.DataFrame()
# Create a boolean categorical feature representing whether the
# median_house_value is above a set threshold.
output_targets["median_house_value_is_high"] = (
california_housing_dataframe["median_house_value"] > 265000).astype(float)
return output_targets
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# Choose the first 12000 (out of 17000) examples for training.
training_examples = preprocess_features(california_housing_dataframe.head(12000))
training_targets = preprocess_targets(california_housing_dataframe.head(12000))
# Choose the last 5000 (out of 17000) examples for validation.
validation_examples = preprocess_features(california_housing_dataframe.tail(5000))
validation_targets = preprocess_targets(california_housing_dataframe.tail(5000))
# Double-check that we've done the right thing.
print("Training examples summary:")
display.display(training_examples.describe())
print("Validation examples summary:")
display.display(validation_examples.describe())
print("Training targets summary:")
display.display(training_targets.describe())
print("Validation targets summary:")
display.display(validation_targets.describe())
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def my_input_fn(features, targets, batch_size=1, shuffle=True, num_epochs=None):
"""Trains a linear regression model.
Args:
features: pandas DataFrame of features
targets: pandas DataFrame of targets
batch_size: Size of batches to be passed to the model
shuffle: True or False. Whether to shuffle the data.
num_epochs: Number of epochs for which data should be repeated. None = repeat indefinitely
Returns:
Tuple of (features, labels) for next data batch
"""
# Convert pandas data into a dict of np arrays.
features = {key:np.array(value) for key,value in dict(features).items()}
# Construct a dataset, and configure batching/repeating.
ds = Dataset.from_tensor_slices((features,targets)) # warning: 2GB limit
ds = ds.batch(batch_size).repeat(num_epochs)
# Shuffle the data, if specified.
if shuffle:
ds = ds.shuffle(10000)
# Return the next batch of data.
features, labels = ds.make_one_shot_iterator().get_next()
return features, labels
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def get_quantile_based_buckets(feature_values, num_buckets):
quantiles = feature_values.quantile(
[(i+1.)/(num_buckets + 1.) for i in range(num_buckets)])
return [quantiles[q] for q in quantiles.keys()]
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def construct_feature_columns():
"""Construct the TensorFlow Feature Columns.
Returns:
A set of feature columns
"""
bucketized_households = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("households"),
boundaries=get_quantile_based_buckets(training_examples["households"], 10))
bucketized_longitude = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("longitude"),
boundaries=get_quantile_based_buckets(training_examples["longitude"], 50))
bucketized_latitude = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("latitude"),
boundaries=get_quantile_based_buckets(training_examples["latitude"], 50))
bucketized_housing_median_age = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("housing_median_age"),
boundaries=get_quantile_based_buckets(
training_examples["housing_median_age"], 10))
bucketized_total_rooms = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("total_rooms"),
boundaries=get_quantile_based_buckets(training_examples["total_rooms"], 10))
bucketized_total_bedrooms = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("total_bedrooms"),
boundaries=get_quantile_based_buckets(training_examples["total_bedrooms"], 10))
bucketized_population = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("population"),
boundaries=get_quantile_based_buckets(training_examples["population"], 10))
bucketized_median_income = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("median_income"),
boundaries=get_quantile_based_buckets(training_examples["median_income"], 10))
bucketized_rooms_per_person = tf.feature_column.bucketized_column(
tf.feature_column.numeric_column("rooms_per_person"),
boundaries=get_quantile_based_buckets(
training_examples["rooms_per_person"], 10))
long_x_lat = tf.feature_column.crossed_column(
set([bucketized_longitude, bucketized_latitude]), hash_bucket_size=1000)
feature_columns = set([
long_x_lat,
bucketized_longitude,
bucketized_latitude,
bucketized_housing_median_age,
bucketized_total_rooms,
bucketized_total_bedrooms,
bucketized_population,
bucketized_households,
bucketized_median_income,
bucketized_rooms_per_person])
return feature_columns
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def model_size(estimator):
variables = estimator.get_variable_names()
size = 0
for variable in variables:
if not any(x in variable
for x in ['global_step',
'centered_bias_weight',
'bias_weight',
'Ftrl']
):
size += np.count_nonzero(estimator.get_variable_value(variable))
return size
Your team needs to build a highly accurate Logistic Regression model on the SmartRing, a ring that is so smart it can sense the demographics of a city block ('median_income', 'avg_rooms', 'households', ..., etc.) and tell you whether the given city block is high cost city block or not.
Since the SmartRing is small, the engineering team has determined that it can only handle a model that has no more than 600 parameters. On the other hand, the product management team has determined that the model is not launchable unless the LogLoss is less than 0.35 on the holdout test set.
Can you use your secret weapon—L1 regularization—to tune the model to satisfy both the size and accuracy constraints?
Find an L1 regularization strength parameter which satisfies both constraints — model size is less than 600 and log-loss is less than 0.35 on validation set.
The following code will help you get started. There are many ways to apply regularization to your model. Here, we chose to do it using FtrlOptimizer, which is designed to give better results with L1 regularization than standard gradient descent.
Again, the model will train on the entire data set, so expect it to run slower than normal.
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def train_linear_classifier_model(
learning_rate,
regularization_strength,
steps,
batch_size,
feature_columns,
training_examples,
training_targets,
validation_examples,
validation_targets):
"""Trains a linear regression model.
In addition to training, this function also prints training progress information,
as well as a plot of the training and validation loss over time.
Args:
learning_rate: A `float`, the learning rate.
regularization_strength: A `float` that indicates the strength of the L1
regularization. A value of `0.0` means no regularization.
steps: A non-zero `int`, the total number of training steps. A training step
consists of a forward and backward pass using a single batch.
feature_columns: A `set` specifying the input feature columns to use.
training_examples: A `DataFrame` containing one or more columns from
`california_housing_dataframe` to use as input features for training.
training_targets: A `DataFrame` containing exactly one column from
`california_housing_dataframe` to use as target for training.
validation_examples: A `DataFrame` containing one or more columns from
`california_housing_dataframe` to use as input features for validation.
validation_targets: A `DataFrame` containing exactly one column from
`california_housing_dataframe` to use as target for validation.
Returns:
A `LinearClassifier` object trained on the training data.
"""
periods = 7
steps_per_period = steps / periods
# Create a linear classifier object.
my_optimizer = tf.train.FtrlOptimizer(learning_rate=learning_rate, l1_regularization_strength=regularization_strength)
my_optimizer = tf.contrib.estimator.clip_gradients_by_norm(my_optimizer, 5.0)
linear_classifier = tf.estimator.LinearClassifier(
feature_columns=feature_columns,
optimizer=my_optimizer
)
# Create input functions.
training_input_fn = lambda: my_input_fn(training_examples,
training_targets["median_house_value_is_high"],
batch_size=batch_size)
predict_training_input_fn = lambda: my_input_fn(training_examples,
training_targets["median_house_value_is_high"],
num_epochs=1,
shuffle=False)
predict_validation_input_fn = lambda: my_input_fn(validation_examples,
validation_targets["median_house_value_is_high"],
num_epochs=1,
shuffle=False)
# Train the model, but do so inside a loop so that we can periodically assess
# loss metrics.
print("Training model...")
print("LogLoss (on validation data):")
training_log_losses = []
validation_log_losses = []
for period in range (0, periods):
# Train the model, starting from the prior state.
linear_classifier.train(
input_fn=training_input_fn,
steps=steps_per_period
)
# Take a break and compute predictions.
training_probabilities = linear_classifier.predict(input_fn=predict_training_input_fn)
training_probabilities = np.array([item['probabilities'] for item in training_probabilities])
validation_probabilities = linear_classifier.predict(input_fn=predict_validation_input_fn)
validation_probabilities = np.array([item['probabilities'] for item in validation_probabilities])
# Compute training and validation loss.
training_log_loss = metrics.log_loss(training_targets, training_probabilities)
validation_log_loss = metrics.log_loss(validation_targets, validation_probabilities)
# Occasionally print the current loss.
print(" period %02d : %0.2f" % (period, validation_log_loss))
# Add the loss metrics from this period to our list.
training_log_losses.append(training_log_loss)
validation_log_losses.append(validation_log_loss)
print("Model training finished.")
# Output a graph of loss metrics over periods.
plt.ylabel("LogLoss")
plt.xlabel("Periods")
plt.title("LogLoss vs. Periods")
plt.tight_layout()
plt.plot(training_log_losses, label="training")
plt.plot(validation_log_losses, label="validation")
plt.legend()
return linear_classifier
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linear_classifier = train_linear_classifier_model(
learning_rate=0.1,
# TWEAK THE REGULARIZATION VALUE BELOW
regularization_strength=0.0,
steps=300,
batch_size=100,
feature_columns=construct_feature_columns(),
training_examples=training_examples,
training_targets=training_targets,
validation_examples=validation_examples,
validation_targets=validation_targets)
print("Model size:", model_size(linear_classifier))
Because you're reducing model size while training on a toy dataset, you're forced to raise regularization strength to 0.8, which is anomalously high. When training on real-world datasets, regularization strength is much smaller than 0.8. Note that you're trading off model size versus loss. As you decrease model size by increasing regularization, your loss can increase.
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linear_classifier = train_linear_classifier_model(
learning_rate=0.1,
regularization_strength=0.8,
steps=300,
batch_size=100,
feature_columns=construct_feature_columns(),
training_examples=training_examples,
training_targets=training_targets,
validation_examples=validation_examples,
validation_targets=validation_targets)
print("Model size:", model_size(linear_classifier))