plotData does not plot anything

I'm trying to plot a set of points in Matlab using plotData function, but the figure window is empty. Moreover, the plot function works correctly, I only have issues with the plotData function. Here is my code:

x = [1;2;3;4;5]
y = x.^2
plotData(x,y)

MATLAB Version: 9.1.0.441655 (R2016b) Operating System: Mac OS X Version: 10.10.5 Build: 14F27

[Stephen23]

The online documentation search cannot find any reference to "plotData":

https://www.mathworks.com/help/search.html?qdoc=plotData

Nor can "plotData" be found in the list of MATLAB functions:

https://www.mathworks.com/help/matlab/functionlist.html

Can you please show a documentation reference, or anything that shows that this is a MATLAB function.

[Ivan Panshin]

Hm, interesting. I found this function in the files for my homework assignment from machine learning class. It said that I can use either matlab of octave. And yeah, I can't find this function in the documentation as well.

[Ujval Madhu]

@Ivan Panshin it seems the function to use is plot(x,y) for plotting rather than plotData - which is a function we create in the homework which makes use of 'plot' method to get the 2d-plot of the data.

Answer 1

[Geoff Hayes]

Ivan - if you are using the plotData function from https://github.com/zhouxc/Stanford-Machine-Learning-Course/blob/master/Logistic%20Regression/mlclass-ex2/plotData.m, then I observe the following errors when running your code

Index exceeds matrix dimensions.

Error in plotData (line 14)
plot(X(pos , 1) , X(pos , 2) , 'k+' , 'LineWidth' , 2 , 'MarkerSize' , 7);

The function seems to assume that the second input parameter y consists of ones and zeros for positive and negative elements of x. Also, the code assumes that x is a Mx2 matrix.

[Ivan Panshin]

No, unfortunately, it doesn't seem that way. I have attached the files that they provided me + one example of how they use this function (you can find it in ex1.m file)

%%======================= Part 2: Plotting =======================
fprintf('Plotting Data ...\n')
data = load('ex1data1.txt');
X = data(:, 1); y = data(:, 2);
m = length(y); % number of training examples

% Plot Data
% Note: You have to complete the code in plotData.m
plotData(X, y);

fprintf('Program paused. Press enter to continue.\n');
pause;

[Geoff Hayes]

Ivan - please attach the relevant code. I'm reluctant to open a zip file. :)

[haoran wan]

Use plot replace plotData, Because this is matlab rather than Octave

Answer 2

[Anthony Garber]

Just in case anyone stumbles across this in the future... plotData is a function created in a .m file included in the classes homework download. It simply opens a figure:

function plotData(x, y)
%PLOTDATA Plots the data points x and y into a new figure 
%   PLOTDATA(x,y) plots the data points and gives the figure axes labels of
%   population and profit.

figure; % open a new figure window

% ====================== YOUR CODE HERE ======================
% Instructions: Plot the training data into a figure using the 
%               "figure" and "plot" commands. Set the axes labels using
%               the "xlabel" and "ylabel" commands. Assume the 
%               population and revenue data have been passed in
%               as the x and y arguments of this function.
%
% Hint: You can use the 'rx' option with plot to have the markers
%       appear as red crosses. Furthermore, you can make the
%       markers larger by using plot(..., 'rx', 'MarkerSize', 10);

% ============================================================

end

[Andrea Sbaragli]

same problem... X unrecognized..

[Image Analyst]

His code said

X = data(:, 1); y = data(:, 2);
m = length(y); % number of training examples
% Plot Data
% Note: You have to complete the code in plotData.m
plotData(X, y);

So it couldn't say X was unrecognized. If data is there, then it would have X. If data were not available then it would complain about data, not X.

Answer 3

[Image Analyst]

Try plot() instead of plotData:

plot(x, y, 'k+-' , 'LineWidth' , 2 , 'MarkerSize' , 7);
grid on;
xlabel('X', 'FontSize', 20);
ylabel('Y', 'FontSize', 20);

Answer 4

[N/A]

From the comments, I understand that 'plotData.m' intends to plot the classification data where X indicates the location of a point and y corresponds to the classification of the said point (either 0 or 1). One can then use a simple code (attached herein) to separate the two classes and then plot them individually.

Hope it helps...

X_pos=X(y==1,:);
X_neg=X(y==0,:);
plot(X_pos(:,1),X_pos(:,2),'k+');
plot(X_neg(:,1),X_neg(:,2),'ko');