I am getting different values of fft for the same variable as demonstrated by a simple example below...

E=[1 2 4 6;3 6 3 9;7 5 8 9]

E = 3x4

1 2 4 6 3 6 3 9 7 5 8 9

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EF=fftshift(abs(fft(E)))

EF = 3x4

4.5826 3.0000 5.2915 3.6056 15.0000 24.0000 11.0000 13.0000 4.5826 3.0000 5.2915 3.6056

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ES=fftshift(abs(fft(E(:,1))))

ES = 3x1

5.2915 11.0000 5.2915

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shouldn't ES be equal to the first row of EF...

Answer 1

[Paul]

I think you were expecting ES be equal to the first column of EF.

If desired for fftshift to work along a specific dimension of the input, then that dimension needs to be specified

E=[1 2 4 6;3 6 3 9;7 5 8 9];
EF=fftshift(abs(fft(E)),1)   % specify the dimension

EF = 3x4

5.2915 3.6056 4.5826 3.0000 11.0000 13.0000 15.0000 24.0000 5.2915 3.6056 4.5826 3.0000

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ES=fftshift(abs(fft(E(:,1))))

ES = 3x1

5.2915 11.0000 5.2915

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If that second argument to fftshift is not specified, then the the 2D input is assumed to be a 2D fft, as might be produced from fft2, and the shifting is done in both dimensions.