CS224d Assignment Sentiment Analysis

import numpy as np
import random

from data_utils import *

from q1_softmax import softmax
from q2_gradcheck import gradcheck_naive
from q3_sgd import load_saved_params

def getSentenceFeature(tokens, wordVectors, sentence):
    """ Obtain the sentence feature for sentiment analysis by averaging its word vectors """
    # Implement computation for the sentence features given a sentence.                                                       
    
    # Inputs:                                                         
    # - tokens: a dictionary that maps words to their indices in    
    #          the word vector list                                
    # - wordVectors: word vectors (each row) for all tokens                
    # - sentence: a list of words in the sentence of interest 

    # Output:                                                         
    # - sentVector: feature vector for the sentence    
    
    sentVector = np.zeros((wordVectors.shape[1],))
    #将句子的特征向量设定为句子的所有词向量的平均
    ### YOUR CODE HERE
    for word in sentence:
        word_vec = wordVectors[tokens[word]]
        sentVector += word_vec
    sentVector = sentVector / len(sentence)
    ### END YOUR CODE
    
    return sentVector

def softmaxRegression(features, labels, weights, regularization = 0.0, nopredictions = False):
    """ Softmax Regression """
    # Implement softmax regression with weight regularization.        
    
    # Inputs:                                                         
    # - features: feature vectors, each row is a feature vector     
    # - labels: labels corresponding to the feature vectors         
    # - weights: weights of the regressor                           
    # - regularization: L2 regularization constant                  
    
    # Output:                                                         
    # - cost: cost of the regressor                                 
    # - grad: gradient of the regressor cost with respect to its    
    #        weights                                               
    # - pred: label predictions of the regressor (you might find    
    #        np.argmax helpful)  
    
    prob = softmax(features.dot(weights))
    if len(features.shape) > 1:
        N = features.shape[0]
    else:
        N = 1
    # A vectorized implementation of    1/N * sum(cross_entropy(x_i, y_i)) + 1/2*|w|^2
    cost = np.sum(-np.log(prob[range(N), labels])) / N 
    cost += 0.5 * regularization * np.sum(weights ** 2)
    
    ### YOUR CODE HERE: compute the gradients and predictions
    grad = np.array(prob)
    grad[range(N), labels] -= 1.0
    grad = features.T.dot(grad) / N
    grad += regularization * weights

    if N > 1:
        pred = np.argmax(prob, axis=1)
    else:
        pred = np.argmax(prob)

    ### END YOUR CODE
    
    if nopredictions:
        return cost, grad
    else:
        return cost, grad, pred

def accuracy(y, yhat):
    """ Precision for classifier """
    assert(y.shape == yhat.shape)
    return np.sum(y == yhat) * 100.0 / y.size

def softmax_wrapper(features, labels, weights, regularization = 0.0):
    cost, grad, _ = softmaxRegression(features, labels, weights, 
        regularization)
    return cost, grad

def sanity_check():
    """
    Run python q4_softmaxreg.py.
    """
    random.seed(314159)
    np.random.seed(265)

    dataset = StanfordSentiment()
    tokens = dataset.tokens()
    nWords = len(tokens)

    _, wordVectors0, _ = load_saved_params()
    wordVectors = (wordVectors0[:nWords, :] + wordVectors0[nWords:, :])
    dimVectors = wordVectors.shape[1]

    dummy_weights = 0.1 * np.random.randn(dimVectors, 5)
    dummy_features = np.zeros((10, dimVectors))
    dummy_labels = np.zeros((10,), dtype=np.int32)    
    for i in xrange(10):
        words, dummy_labels[i] = dataset.getRandomTrainSentence()
        dummy_features[i, :] = getSentenceFeature(tokens, wordVectors, words)
    print("==== Gradient check for softmax regression ====")
    gradcheck_naive(lambda weights: softmaxRegression(dummy_features,
        dummy_labels, weights, 1.0, nopredictions = True), dummy_weights)

    print("\n=== Results ===")
    print(softmaxRegression(dummy_features, dummy_labels, dummy_weights, 1.0))

if __name__ == "__main__":
    sanity_check()

加入正则化项能够有效防止过拟合。