plotData does not plot anything

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Ivan Panshin
Ivan Panshin 2017-1-6
评论： Ujval Madhu 2021-11-4
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
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Ivan Panshin
Ivan Panshin 2017-1-6
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
Ujval Madhu 2021-11-4
@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.

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回答（4 个）

Geoff Hayes
Geoff Hayes 2017-1-6
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.
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Geoff Hayes
Geoff Hayes 2017-1-6
Ivan - please attach the relevant code. I'm reluctant to open a zip file. :)
haoran wan
haoran wan 2020-9-29
Use plot replace plotData, Because this is matlab rather than Octave

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Anthony Garber
Anthony Garber 2018-2-2
编辑：Anthony Garber 2018-2-2
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
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Andrea Sbaragli
Andrea Sbaragli 2020-9-21
same problem... X unrecognized..
Image Analyst
Image Analyst 2020-9-21
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.

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Image Analyst
Image Analyst 2017-1-6
Try plot() instead of plotData:
plot(x, y, 'k+-' , 'LineWidth' , 2 , 'MarkerSize' , 7);
grid on;
xlabel('X', 'FontSize', 20);
ylabel('Y', 'FontSize', 20);
N/A
N/A 2019-10-10
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');

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