commit ec7434e9a17cf6435ad56816aa3c83debca5ee3b
Author: Matthias Neeracher <neeri@Tuyo.fritz.box>
Date:   Sun Jan 1 22:36:27 2017 +0100

    Added programming exercise 1 (Week 3)

diff --git a/NNML1/dataset1.mat b/NNML1/dataset1.mat
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diff --git a/NNML1/dataset2.mat b/NNML1/dataset2.mat
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diff --git a/NNML1/dataset3.mat b/NNML1/dataset3.mat
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diff --git a/NNML1/dataset4.mat b/NNML1/dataset4.mat
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diff --git a/NNML1/learn_perceptron.m b/NNML1/learn_perceptron.m
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+++ b/NNML1/learn_perceptron.m
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+%% Learns the weights of a perceptron and displays the results.
+function [w] = learn_perceptron(neg_examples_nobias,pos_examples_nobias,w_init,w_gen_feas)
+%% 
+% Learns the weights of a perceptron for a 2-dimensional dataset and plots
+% the perceptron at each iteration where an iteration is defined as one
+% full pass through the data. If a generously feasible weight vector
+% is provided then the visualization will also show the distance
+% of the learned weight vectors to the generously feasible weight vector.
+% Required Inputs:
+%   neg_examples_nobias - The num_neg_examples x 2 matrix for the examples with target 0.
+%       num_neg_examples is the number of examples for the negative class.
+%   pos_examples_nobias - The num_pos_examples x 2 matrix for the examples with target 1.
+%       num_pos_examples is the number of examples for the positive class.
+%   w_init - A 3-dimensional initial weight vector. The last element is the bias.
+%   w_gen_feas - A generously feasible weight vector.
+% Returns:
+%   w - The learned weight vector.
+%%
+
+%Bookkeeping
+num_neg_examples = size(neg_examples_nobias,1);
+num_pos_examples = size(pos_examples_nobias,1);
+num_err_history = [];
+w_dist_history = [];
+
+%Here we add a column of ones to the examples in order to allow us to learn
+%bias parameters.
+neg_examples = [neg_examples_nobias,ones(num_neg_examples,1)];
+pos_examples = [pos_examples_nobias,ones(num_pos_examples,1)];
+
+%If weight vectors have not been provided, initialize them appropriately.
+if (~exist('w_init','var') || isempty(w_init))
+    w = randn(3,1);
+else
+    w = w_init;
+end
+
+if (~exist('w_gen_feas','var'))
+    w_gen_feas = [];
+end
+
+%Find the data points that the perceptron has incorrectly classified
+%and record the number of errors it makes.
+iter = 0;
+[mistakes0, mistakes1] = eval_perceptron(neg_examples,pos_examples,w);
+num_errs = size(mistakes0,1) + size(mistakes1,1);
+num_err_history(end+1) = num_errs;
+fprintf('Number of errors in iteration %d:\t%d\n',iter,num_errs);
+fprintf(['weights:\t', mat2str(w), '\n']);
+plot_perceptron(neg_examples, pos_examples, mistakes0, mistakes1, num_err_history, w, w_dist_history);
+key = input('<Press enter to continue, q to quit.>', 's');
+if (key == 'q')
+    return;
+end
+
+%If a generously feasible weight vector exists, record the distance
+%to it from the initial weight vector.
+if (length(w_gen_feas) ~= 0)
+    w_dist_history(end+1) = norm(w - w_gen_feas);
+end
+
+%Iterate until the perceptron has correctly classified all points.
+while (num_errs > 0)
+    iter = iter + 1;
+
+    %Update the weights of the perceptron.
+    w = update_weights(neg_examples,pos_examples,w);
+
+    %If a generously feasible weight vector exists, record the distance
+    %to it from the current weight vector.
+    if (length(w_gen_feas) ~= 0)
+        w_dist_history(end+1) = norm(w - w_gen_feas);
+    end
+
+    %Find the data points that the perceptron has incorrectly classified.
+    %and record the number of errors it makes.
+    [mistakes0, mistakes1] = eval_perceptron(neg_examples,pos_examples,w);
+    num_errs = size(mistakes0,1) + size(mistakes1,1);
+    num_err_history(end+1) = num_errs;
+
+    fprintf('Number of errors in iteration %d:\t%d\n',iter,num_errs);
+    fprintf(['weights:\t', mat2str(w), '\n']);
+    plot_perceptron(neg_examples, pos_examples, mistakes0, mistakes1, num_err_history, w, w_dist_history);
+    key = input('<Press enter to continue, q to quit.>', 's');
+    if (key == 'q')
+        break;
+    end
+end
+
+%WRITE THE CODE TO COMPLETE THIS FUNCTION
+function [w] = update_weights(neg_examples, pos_examples, w_current)
+%% 
+% Updates the weights of the perceptron for incorrectly classified points
+% using the perceptron update algorithm. This function makes one sweep
+% over the dataset.
+% Inputs:
+%   neg_examples - The num_neg_examples x 3 matrix for the examples with target 0.
+%       num_neg_examples is the number of examples for the negative class.
+%   pos_examples- The num_pos_examples x 3 matrix for the examples with target 1.
+%       num_pos_examples is the number of examples for the positive class.
+%   w_current - A 3-dimensional weight vector, the last element is the bias.
+% Returns:
+%   w - The weight vector after one pass through the dataset using the perceptron
+%       learning rule.
+%%
+w = w_current;
+num_neg_examples = size(neg_examples,1);
+num_pos_examples = size(pos_examples,1);
+for i=1:num_neg_examples
+    this_case = neg_examples(i,:);
+    x = this_case'; %Hint
+    activation = this_case*w;
+    if (activation >= 0)
+        w = w-x
+    end
+end
+for i=1:num_pos_examples
+    this_case = pos_examples(i,:);
+    x = this_case';
+    activation = this_case*w;
+    if (activation < 0)
+        w = w+x
+    end
+end
+
+function [mistakes0, mistakes1] =  eval_perceptron(neg_examples, pos_examples, w)
+%% 
+% Evaluates the perceptron using a given weight vector. Here, evaluation
+% refers to finding the data points that the perceptron incorrectly classifies.
+% Inputs:
+%   neg_examples - The num_neg_examples x 3 matrix for the examples with target 0.
+%       num_neg_examples is the number of examples for the negative class.
+%   pos_examples- The num_pos_examples x 3 matrix for the examples with target 1.
+%       num_pos_examples is the number of examples for the positive class.
+%   w - A 3-dimensional weight vector, the last element is the bias.
+% Returns:
+%   mistakes0 - A vector containing the indices of the negative examples that have been
+%       incorrectly classified as positive.
+%   mistakes0 - A vector containing the indices of the positive examples that have been
+%       incorrectly classified as negative.
+%%
+num_neg_examples = size(neg_examples,1);
+num_pos_examples = size(pos_examples,1);
+mistakes0 = [];
+mistakes1 = [];
+for i=1:num_neg_examples
+    x = neg_examples(i,:)';
+    activation = x'*w;
+    if (activation >= 0)
+        mistakes0 = [mistakes0;i];
+    end
+end
+for i=1:num_pos_examples
+    x = pos_examples(i,:)';
+    activation = x'*w;
+    if (activation < 0)
+        mistakes1 = [mistakes1;i];
+    end
+end
+
diff --git a/NNML1/plot_perceptron.m b/NNML1/plot_perceptron.m
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+++ b/NNML1/plot_perceptron.m
@@ -0,0 +1,75 @@
+%% Plots information about a perceptron classifier on a 2-dimensional dataset.
+function plot_perceptron(neg_examples, pos_examples, mistakes0, mistakes1, num_err_history, w, w_dist_history)
+%%
+% The top-left plot shows the dataset and the classification boundary given by
+% the weights of the perceptron. The negative examples are shown as circles
+% while the positive examples are shown as squares. If an example is colored
+% green then it means that the example has been correctly classified by the
+% provided weights. If it is colored red then it has been incorrectly classified.
+% The top-right plot shows the number of mistakes the perceptron algorithm has
+% made in each iteration so far.
+% The bottom-left plot shows the distance to some generously feasible weight
+% vector if one has been provided (note, there can be an infinite number of these).
+% Points that the classifier has made a mistake on are shown in red,
+% while points that are correctly classified are shown in green.
+% The goal is for all of the points to be green (if it is possible to do so).
+% Inputs:
+%   neg_examples - The num_neg_examples x 3 matrix for the examples with target 0.
+%       num_neg_examples is the number of examples for the negative class.
+%   pos_examples- The num_pos_examples x 3 matrix for the examples with target 1.
+%       num_pos_examples is the number of examples for the positive class.
+%   mistakes0 - A vector containing the indices of the datapoints from class 0 incorrectly
+%       classified by the perceptron. This is a subset of neg_examples.
+%   mistakes1 - A vector containing the indices of the datapoints from class 1 incorrectly
+%       classified by the perceptron. This is a subset of pos_examples.
+%   num_err_history - A vector containing the number of mistakes for each
+%       iteration of learning so far.
+%   w - A 3-dimensional vector corresponding to the current weights of the
+%       perceptron. The last element is the bias.
+%   w_dist_history - A vector containing the L2-distance to a generously
+%       feasible weight vector for each iteration of learning so far.
+%       Empty if one has not been provided.
+%%
+f = figure(1);
+clf(f);
+
+neg_correct_ind = setdiff(1:size(neg_examples,1),mistakes0);
+pos_correct_ind = setdiff(1:size(pos_examples,1),mistakes1);
+
+subplot(2,2,1);
+hold on;
+if (~isempty(neg_examples))
+	plot(neg_examples(neg_correct_ind,1),neg_examples(neg_correct_ind,2),'og','markersize',20);
+end
+if (~isempty(pos_examples))
+	plot(pos_examples(pos_correct_ind,1),pos_examples(pos_correct_ind,2),'sg','markersize',20);
+end
+if (size(mistakes0,1) > 0)
+	plot(neg_examples(mistakes0,1),neg_examples(mistakes0,2),'or','markersize',20);
+end
+if (size(mistakes1,1) > 0)
+	plot(pos_examples(mistakes1,1),pos_examples(mistakes1,2),'sr','markersize',20);
+end
+title('Classifier');
+
+%In order to plot the decision line, we just need to get two points.
+plot([-5,5],[(-w(end)+5*w(1))/w(2),(-w(end)-5*w(1))/w(2)],'k')
+xlim([-1,1]);
+ylim([-1,1]);
+hold off;
+
+subplot(2,2,2);
+plot(0:length(num_err_history)-1,num_err_history);
+xlim([-1,max(15,length(num_err_history))]);
+ylim([0,size(neg_examples,1)+size(pos_examples,1)+1]);
+title('Number of errors');
+xlabel('Iteration');
+ylabel('Number of errors');
+
+subplot(2,2,3);
+plot(0:length(w_dist_history)-1,w_dist_history);
+xlim([-1,max(15,length(num_err_history))]);
+ylim([0,15]);
+title('Distance')
+xlabel('Iteration');
+ylabel('Distance');
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