175 lines
5.8 KiB
Python
175 lines
5.8 KiB
Python
"""
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==========================================================================
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Gaussian processes on discrete data structures
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==========================================================================
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This example illustrates the use of Gaussian processes for regression and
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classification tasks on data that are not in fixed-length feature vector form.
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This is achieved through the use of kernel functions that operates directly
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on discrete structures such as variable-length sequences, trees, and graphs.
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Specifically, here the input variables are some gene sequences stored as
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variable-length strings consisting of letters 'A', 'T', 'C', and 'G',
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while the output variables are floating point numbers and True/False labels
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in the regression and classification tasks, respectively.
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A kernel between the gene sequences is defined using R-convolution [1]_ by
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integrating a binary letter-wise kernel over all pairs of letters among a pair
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of strings.
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This example will generate three figures.
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In the first figure, we visualize the value of the kernel, i.e. the similarity
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of the sequences, using a colormap. Brighter color here indicates higher
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similarity.
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In the second figure, we show some regression result on a dataset of 6
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sequences. Here we use the 1st, 2nd, 4th, and 5th sequences as the training set
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to make predictions on the 3rd and 6th sequences.
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In the third figure, we demonstrate a classification model by training on 6
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sequences and make predictions on another 5 sequences. The ground truth here is
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simply whether there is at least one 'A' in the sequence. Here the model makes
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four correct classifications and fails on one.
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.. [1] Haussler, D. (1999). Convolution kernels on discrete structures
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(Vol. 646). Technical report, Department of Computer Science, University
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of California at Santa Cruz.
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"""
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print(__doc__)
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import numpy as np
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import matplotlib.pyplot as plt
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from sklearn.gaussian_process.kernels import Kernel, Hyperparameter
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from sklearn.gaussian_process.kernels import GenericKernelMixin
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from sklearn.gaussian_process import GaussianProcessRegressor
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from sklearn.gaussian_process import GaussianProcessClassifier
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from sklearn.base import clone
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class SequenceKernel(GenericKernelMixin, Kernel):
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'''
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A minimal (but valid) convolutional kernel for sequences of variable
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lengths.'''
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def __init__(self,
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baseline_similarity=0.5,
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baseline_similarity_bounds=(1e-5, 1)):
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self.baseline_similarity = baseline_similarity
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self.baseline_similarity_bounds = baseline_similarity_bounds
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@property
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def hyperparameter_baseline_similarity(self):
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return Hyperparameter("baseline_similarity",
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"numeric",
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self.baseline_similarity_bounds)
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def _f(self, s1, s2):
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'''
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kernel value between a pair of sequences
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'''
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return sum([1.0 if c1 == c2 else self.baseline_similarity
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for c1 in s1
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for c2 in s2])
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def _g(self, s1, s2):
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'''
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kernel derivative between a pair of sequences
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'''
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return sum([0.0 if c1 == c2 else 1.0
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for c1 in s1
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for c2 in s2])
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def __call__(self, X, Y=None, eval_gradient=False):
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if Y is None:
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Y = X
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if eval_gradient:
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return (np.array([[self._f(x, y) for y in Y] for x in X]),
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np.array([[[self._g(x, y)] for y in Y] for x in X]))
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else:
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return np.array([[self._f(x, y) for y in Y] for x in X])
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def diag(self, X):
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return np.array([self._f(x, x) for x in X])
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def is_stationary(self):
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return False
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def clone_with_theta(self, theta):
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cloned = clone(self)
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cloned.theta = theta
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return cloned
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kernel = SequenceKernel()
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'''
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Sequence similarity matrix under the kernel
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===========================================
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'''
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X = np.array(['AGCT', 'AGC', 'AACT', 'TAA', 'AAA', 'GAACA'])
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K = kernel(X)
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D = kernel.diag(X)
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plt.figure(figsize=(8, 5))
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plt.imshow(np.diag(D**-0.5).dot(K).dot(np.diag(D**-0.5)))
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plt.xticks(np.arange(len(X)), X)
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plt.yticks(np.arange(len(X)), X)
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plt.title('Sequence similarity under the kernel')
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'''
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Regression
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==========
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'''
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X = np.array(['AGCT', 'AGC', 'AACT', 'TAA', 'AAA', 'GAACA'])
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Y = np.array([1.0, 1.0, 2.0, 2.0, 3.0, 3.0])
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training_idx = [0, 1, 3, 4]
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gp = GaussianProcessRegressor(kernel=kernel)
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gp.fit(X[training_idx], Y[training_idx])
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plt.figure(figsize=(8, 5))
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plt.bar(np.arange(len(X)), gp.predict(X), color='b', label='prediction')
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plt.bar(training_idx, Y[training_idx], width=0.2, color='r',
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alpha=1, label='training')
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plt.xticks(np.arange(len(X)), X)
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plt.title('Regression on sequences')
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plt.legend()
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'''
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Classification
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==============
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'''
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X_train = np.array(['AGCT', 'CGA', 'TAAC', 'TCG', 'CTTT', 'TGCT'])
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# whether there are 'A's in the sequence
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Y_train = np.array([True, True, True, False, False, False])
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gp = GaussianProcessClassifier(kernel)
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gp.fit(X_train, Y_train)
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X_test = ['AAA', 'ATAG', 'CTC', 'CT', 'C']
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Y_test = [True, True, False, False, False]
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plt.figure(figsize=(8, 5))
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plt.scatter(np.arange(len(X_train)), [1.0 if c else -1.0 for c in Y_train],
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s=100, marker='o', edgecolor='none', facecolor=(1, 0.75, 0),
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label='training')
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plt.scatter(len(X_train) + np.arange(len(X_test)),
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[1.0 if c else -1.0 for c in Y_test],
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s=100, marker='o', edgecolor='none', facecolor='r', label='truth')
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plt.scatter(len(X_train) + np.arange(len(X_test)),
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[1.0 if c else -1.0 for c in gp.predict(X_test)],
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s=100, marker='x', edgecolor=(0, 1.0, 0.3), linewidth=2,
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label='prediction')
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plt.xticks(np.arange(len(X_train) + len(X_test)),
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np.concatenate((X_train, X_test)))
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plt.yticks([-1, 1], [False, True])
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plt.title('Classification on sequences')
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plt.legend()
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plt.show()
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