MATLAB: “Warning: Integer operands are required for colon operator when used as index” in fft

fffinteger operandswarning

Hello everyone, I tried to write a script in order to perform a fft of an accelerometric signal. When I run the code the warning in object appears. Below the faulty code line (second line):

xdft1=fft(x1-mean(x1))/N1;
xdft1=xdft1(1:N1/2+1);

where x1 is the accelerometric signal and N1=length(x1). I deem the problem is in the operation N1/2 and I think I have to use the function "floor". Can you help me? Thank you very much!

Best Answer

N1 is odd, so N1/2 is a fraction. Add 1 to that and it is still a fraction. You are then trying to index from 1 to that fraction. That will give you a warning.

You need to either prevent N1 from being odd, or else you need to use floor() or fix() or round() on the N1/2 so that it is no longer a fraction

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MATLAB: Over-lapping & adding [FFT] – Error – Index exceeds matrix dimensions

Maith - if I run your code as a function (named g13.m) or as a script, I observe the following error

 Index exceeds matrix dimensions.
 Error in g13 (line 33)
 x1 = [x(i:i+n3-n2) zeros(1,n3-n2)];

So we know the line, but not which iteration. To make the debugging easier, run the following command before running your above code

 dbstop if error

This will cause the debugger to pause at the line which generates the error, and you will be able to look at all variables at that point and so you can get an idea of what is happening. When I do this, I notice that this fails for when i is 17 (as an aside, you may want to use a different index variable name as i and j are also used to represent the imaginary number). The code then fails because of

 x(i:i+n3-n2)

as i is 17, n3 is 7, and n2 is 4 with x only being a vector with 18 elements. With that in mind, how would you modify this code to handle the case where i+n3-n2 exceeds 18?

MATLAB: Continuous Time Fourier Transform of a signal matrix

If the first column of your matrix is just the time vector with increments of 0.02 seconds, then just take the Fourier transform of the 2nd column

Let X be your matrix

   xdft = fft(X(:,2));

If the signal is real-valued, you only need 1/2 the DFT to examine the amplitude spectrum.

The frequency vector can be formed as follows:

   freq = 0:50/100:25;

For example:

    t = 0:0.02:(100*0.02)-0.02;
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    X(:,1) = t';
    X(:,2) = x';
    % Now X is your matrix
    xdft = fft(X(:,2));
    xdft = xdft(1:length(xdft)/2+1);
    freq = 0:50/100:25;
    plot(freq,abs(xdft))
    xlabel('Hz'); ylabel('Magnitude')

Best Answer

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MATLAB: Over-lapping & adding [FFT] – Error – Index exceeds matrix dimensions

MATLAB: Continuous Time Fourier Transform of a signal matrix

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