MATLAB: Plotting in 3D many sets of x and y values against a third axis time

3dcsvdatalarge setsMATLABplot

Hi,

So through an experiment, I have hundreds of sets of 2048 x and 2048 y values which I want to plot against eachother over time. I think what I might be looking for is a surface plot, however when I looked at the documentation and tried it with some dummy numbers, it didn't play nicely.

Just to be clear, I have all the data in CSV files, so I have the x values which are wavelength which are always the same. I then have the y values which are absorption which are the things changing and finally I have time which I would like to plot on the z axis. The time corresponds to one set of 2048

How would I go about doing this? I've been sat here thinking for a couple of hours now and can't think of a good way to do it.

Thank you!

P.S I attached a badly drawn image of how I'm hoping it'll look

Best Answer

Something like this?

t = linspace(0, 10, 1000);
w = linspace(1, 3, 10)';
a = linspace(0.2, 1, 10)';
x = a.*sin(w.*t);
f = figure();
ax = axes();
view(ax,[30 30]);
grid(ax);
hold(ax);
for i=1:10
    plot3(t, w(i)*ones(size(t)), x(i,:), 'LineWidth', 1)
end

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MATLAB: Make code faster for import and array creation

Hi Jenna, I bet this is a pre-allocation issue.

When you build a big matrix one column at a time (such as is being done for mydata_X and mydata_Y), MATLAB needs to do some juggling of memory to make the variable a little bit bigger on each iteration. If you know how big the end result will be before you start building your matrix, you can initialise the matrix to this size just once, and MATLAB no longer has the overhead of juggling memory space inside the loop.

That said, there might even be a sneakier way of avoiding the loops you're using. Let's say that after you read all the files:

%bring the data in from CSV format into a cell-array
csvFiles = dir('*.csv');         %works for the current directory only
numfiles = length(csvFiles);     %try to keep only the data you want in the dir
mydata = cell(1,numfiles);       %finds the total number of files in the dir
%save each XY data file into a cell of a cell-array called 'mydata'
for k = 1:numfiles
    mydata{k} = importdata(csvFiles(k).name); 
end

At this point, is the matrix of every cell in mydata the same size? It seems that it should be, based on the rest of your code... If it is, you can make a 3D matrix with one "sheet" for each cell in mydata like this:

myData3d = cat(3, mydata{:});

And then you can get mydata_X like this:

mydata_X = myData3d(:,1,:);

Now, mydata_X here will be an N-by-1-by-NFILES matrix, whereas I think from your code that your original mydata_X ended up as a N-by-NFILES matrix. If you want to get the same as your original, you can just use reshape or permute:

mydata_X = permute(myData3d(:,1,:), [1 3 2]);

The same can be done for mydata_Y.

Did this get you going? If it works, it will be much faster that the loop that builds matrices one column at a time.

And also, the last loop you have:

for i=1:numfiles
    arrayOfZ = 1:numfiles
    mydata_Z(i,:) = arrayOfZ(1,:) % here I assign 1 through 'numfiles' ...
end

Can be replaced by:

mydata_Z = repmat(1:numfiles, numfiles, 1);

Although I'm not quite sure what purpose this variable serves... it will be of a different size to mydata_X and mydata_Y ...

MATLAB: Spectrum graph from sinusoidal signal in wavelength

This is a modified version of the example on MATLAB fft() documentation page. For this example, I assume wavelength = 1/f. And I restrict the wavelength in range 0.003 to 1.

Fs = 1000;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;        % Time vector
S = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t);
X = S + 2*randn(size(t));
Y = fft(X);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
lambda = 1./f;
idx = (lambda > 0.003) & (lambda < 0.1);
lambda = lambda(idx);
P1 = P1(idx);
plot(lambda, P1);
xlabel('wavelength');
ylabel('amplitude')

Best Answer

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MATLAB: Make code faster for import and array creation

MATLAB: Spectrum graph from sinusoidal signal in wavelength

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