Subscripted assignment dimension mismatch - Periodogram MATLAB

Question

I wrote the following code on MATLAB:

N = [64 128 256 512];
NumOfRuns = 50;   
PerLength = 2048;  
freq = 0:2/(PerLength-1):2;
for m=1:length(N)
    Px = zeros(NumOfRuns,PerLength);
    for i=1:NumOfRuns
        x = randn(1,N(m));
        Px(:,i) = periodogram(x);
    end
end

When I run this code on MATLAB, it gives the error:

Subscripted assignment dimension mismatch. Error in exampl13(line 10) Px(:,I)=periodogram(x).

Possible duplicate of Subscripted assignment dimension mismatch — Jones Joseph

Wolfie Wolfie · Accepted Answer · 2017-01-31T10:28:40

Code Notes:

N = [64 128 256 512];
NumOfRuns = 50;   
PerLength = 2048; 

% freq = 0:2/(PerLength-1):2; % you are not using freq

% You are looping from 1:4, defining Px each time, not storing Px outside of the loop. 
% After this loop, Px will just be in its 4th state. 
for m = 1:length(N)

    Px = zeros(NumOfRuns,PerLength);

    for i = 1:NumOfRuns 

        x = randn(1,N(m));
        Px(:,i) = periodogram(x);

    end

end

Periodogram:

You're assuming periodogram returns the same number of elements as are input to it. A simple test will tell you that is not true!

a = periodogram([1, 2]);
numel(a)
% >> ans = 129

Read the documentation here:

https://uk.mathworks.com/help/signal/ref/periodogram.html

The number of points, nfft, in the discrete Fourier transform (DFT) is the maximum of 256 or the next power of two greater than the signal length

So above, the 129 is related to 2^7 = 128

Try using pxx = periodogram(x,window,nfft)

Subscripted assignment dimension mismatch - Periodogram MATLAB

1 Answers