Deep learning：十五(Self-Taught Learning练习)

本文转自http://www.cnblogs.com/tornadomeet/archive/2013/03/24/2979408.html

前言：

　　本次实验主要是练习soft- taught learning的实现。参考的资料为网页：http://deeplearning.stanford.edu/wiki/index.php/Exercise:Self-Taught_Learning。Soft-taught leaning是用的无监督学习来学习到特征提取的参数，然后用有监督学习来训练分类器。这里分别是用的sparse autoencoder和softmax regression。实验的数据依旧是手写数字数据库MNIST Dataset.

 

　　实验基础：

　　从前面的知识可以知道，sparse autoencoder的输出应该是和输入数据尺寸大小一样的，且很相近，那么我们训练出的sparse autoencoder模型该怎样提取出特征向量呢？其实输入样本经过sparse code提取出特征的表达式就是隐含层的输出了，首先来看看前面的经典sparse code模型，如下图所示：

 　　

　　拿掉那个后面的输出层后，隐含层的值就是我们所需要的特征值了，如下图所示：

 　　

　　从教程中可知，在unsupervised learning中有两个观点需要特别注意，一个是self-taught learning，一个是semi-supervised learning。Self-taught learning是完全无监督的。教程中有举了个例子，很好的说明了这个问题，比如说我们需要设计一个系统来分类出轿车和摩托车。如果我们给出的训练样本图片是自然界中随便下载的（也就是说这些图片中可能有轿车和摩托车，有可能都没有，且大多数情况下是没有的），然后使用的是这些样本来特征模型的话，那么此时的方法就叫做self-taught learning。如果我们训练的样本图片都是轿车和摩托车的图片，只是我们不知道哪张图对应哪种车，也就是说没有标注，此时的方法不能叫做是严格的unsupervised feature，只能叫做是semi-supervised learning。

　　一些matlab函数：

　　numel:

　　比如说n = numel(A)表示返回矩阵A中元素的个数。

　　unique:

　　unique为找出向量中的非重复元素并进行排序后输出。

　　

　　实验结果：

　　采用数字5~9的样本来进行无监督训练，采用的方法是sparse autoencoder，可以提取出这些数据的权值，权值转换成图片显示如下：

 　　

　　但是本次实验主要是进行0~4这5个数字的分类，虽然进行无监督训练用的是数字5~9的训练样本，这依然不会影响后面的结果。只是后面的分类器设计是用的softmax regression，所以是有监督的。最后据官网网页上的结果精度是98%，而直接用原始的像素点进行分类器的设计不仅效果要差（才96%），而且训练的速度也会变慢不少。

 

　　实验主要部分代码：

　　stlExercise.m:

%% CS294A/CS294W Self-taught Learning Exercise%  Instructions%  ------------% %  This file contains code that helps you get started on the%  self-taught learning. You will need to complete code in feedForwardAutoencoder.m%  You will also need to have implemented sparseAutoencoderCost.m and %  softmaxCost.m from previous exercises.%%% ======================================================================%  STEP 0: Here we provide the relevant parameters values that will%  allow your sparse autoencoder to get good filters; you do not need to %  change the parameters below.inputSize  = 28 * 28;numLabels  = 5;hiddenSize = 200;sparsityParam = 0.1; % desired average activation of the hidden units.                     % (This was denoted by the Greek alphabet rho, which looks like a lower-case "p",                     %  in the lecture notes). lambda = 3e-3;       % weight decay parameter       beta = 3;            % weight of sparsity penalty term   maxIter = 400;%% ======================================================================%  STEP 1: Load data from the MNIST database%%  This loads our training and test data from the MNIST database files.%  We have sorted the data for you in this so that you will not have to%  change it.% Load MNIST database filesmnistData   = loadMNISTImages('train-images.idx3-ubyte');mnistLabels = loadMNISTLabels('train-labels.idx1-ubyte');% Set Unlabeled Set (All Images)% Simulate a Labeled and Unlabeled setlabeledSet   = find(mnistLabels >= 0 & mnistLabels <= 4);unlabeledSet = find(mnistLabels >= 5);%%增加的一行代码unlabeledSet = unlabeledSet(1:end/3);numTest = round(numel(labeledSet)/2);%拿一半的样本来训练%numTrain = round(numel(labeledSet)/3);trainSet = labeledSet(1:numTrain);testSet  = labeledSet(numTrain+1:2*numTrain);unlabeledData = mnistData(:, unlabeledSet);%%为什么这两句连在一起都要出错呢？% pack;trainData   = mnistData(:, trainSet);trainLabels = mnistLabels(trainSet)' + 1; % Shift Labels to the Range 1-5% mnistData2 = mnistData;testData   = mnistData(:, testSet);testLabels = mnistLabels(testSet)' + 1;   % Shift Labels to the Range 1-5% Output Some Statisticsfprintf('# examples in unlabeled set: %d\n', size(unlabeledData, 2));fprintf('# examples in supervised training set: %d\n\n', size(trainData, 2));fprintf('# examples in supervised testing set: %d\n\n', size(testData, 2));%% ======================================================================%  STEP 2: Train the sparse autoencoder%  This trains the sparse autoencoder on the unlabeled training%  images. %  Randomly initialize the parameterstheta = initializeParameters(hiddenSize, inputSize);%% ----------------- YOUR CODE HERE ----------------------%  Find opttheta by running the sparse autoencoder on%  unlabeledTrainingImagesopttheta = theta; addpath minFunc/options.Method = 'lbfgs';options.maxIter = 400;options.display = 'on';[opttheta, loss] = minFunc( @(p) sparseAutoencoderLoss(p, ...      inputSize, hiddenSize, ...      lambda, sparsityParam, ...      beta, unlabeledData), ...      theta, options);%% -----------------------------------------------------                          % Visualize weightsW1 = reshape(opttheta(1:hiddenSize * inputSize), hiddenSize, inputSize);display_network(W1');%%======================================================================%% STEP 3: Extract Features from the Supervised Dataset%  %  You need to complete the code in feedForwardAutoencoder.m so that the %  following command will extract features from the data.trainFeatures = feedForwardAutoencoder(opttheta, hiddenSize, inputSize, ...                                       trainData);testFeatures = feedForwardAutoencoder(opttheta, hiddenSize, inputSize, ...                                       testData);%%======================================================================%% STEP 4: Train the softmax classifiersoftmaxModel = struct;  %% ----------------- YOUR CODE HERE ----------------------%  Use softmaxTrain.m from the previous exercise to train a multi-class%  classifier. %  Use lambda = 1e-4 for the weight regularization for softmaxlambda = 1e-4;inputSize = hiddenSize;numClasses = numel(unique(trainLabels));%unique为找出向量中的非重复元素并进行排序% You need to compute softmaxModel using softmaxTrain on trainFeatures and% trainLabels% You need to compute softmaxModel using softmaxTrain on trainFeatures and% trainLabelsoptions.maxIter = 100;softmaxModel = softmaxTrain(inputSize, numClasses, lambda, ...                            trainFeatures, trainLabels, options);%% -----------------------------------------------------%%======================================================================%% STEP 5: Testing %% ----------------- YOUR CODE HERE ----------------------% Compute Predictions on the test set (testFeatures) using softmaxPredict% and softmaxModel[pred] = softmaxPredict(softmaxModel, testFeatures);%% -----------------------------------------------------% Classification Scorefprintf('Test Accuracy: %f%%\n', 100*mean(pred(:) == testLabels(:)));% (note that we shift the labels by 1, so that digit 0 now corresponds to%  label 1)%% Accuracy is the proportion of correctly classified images% The results for our implementation was:%% Accuracy: 98.3%%% 

 

　　feedForwardAutoencoder.m:

function [activation] = feedForwardAutoencoder(theta, hiddenSize, visibleSize, data)% theta: trained weights from the autoencoder% visibleSize: the number of input units (probably 64) % hiddenSize: the number of hidden units (probably 25) % data: Our matrix containing the training data as columns.  So, data(:,i) is the i-th training example.   % We first convert theta to the (W1, W2, b1, b2) matrix/vector format, so that this % follows the notation convention of the lecture notes. W1 = reshape(theta(1:hiddenSize*visibleSize), hiddenSize, visibleSize);b1 = theta(2*hiddenSize*visibleSize+1:2*hiddenSize*visibleSize+hiddenSize);%% ---------- YOUR CODE HERE --------------------------------------%  Instructions: Compute the activation of the hidden layer for the Sparse Autoencoder.activation  = sigmoid(W1*data+repmat(b1,[1,size(data,2)]));%-------------------------------------------------------------------end%-------------------------------------------------------------------% Here's an implementation of the sigmoid function, which you may find useful% in your computation of the costs and the gradients.  This inputs a (row or% column) vector (say (z1, z2, z3)) and returns (f(z1), f(z2), f(z3)). function sigm = sigmoid(x)    sigm = 1 ./ (1 + exp(-x));end

 

 

　　参考资料:

     http://deeplearning.stanford.edu/wiki/index.php/Exercise:Self-Taught_Learning

     MNIST Dataset