How do I use a for-loop to do fft and plot the power?

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Emma
Emma 2014-12-16
评论： Emma 2014-12-17
Hello,
I have a data set (time and variable Be) and I want to do a fft on one part of the data at a time (1000 years). To do this, I would like to use a for-loop that plots the power of the period for each of the different ffts. How do I do this? I'm new to Matlab but I have tried the code below:
hold on
for ts=(0:1000:5000)'; % I have data between 0 and 5000 years ago.
A=fft(Be); % Do FFT on the Be data.
A(1)=[];
n=length(A);
power=abs(A(1:floor(n/2))).^2;
nyquist=1/2;
freq=(1:n/2)/(n/2)*nyquist;
period=1./freq;
plot(period,power);
end
When I do this, I get only one plot. What am I doing wrong?
Thanks!

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Thorsten
Thorsten 2014-12-17
编辑：Thorsten 2014-12-17
May be that's because they are the same? I get different curves using random input.
BTW: note that n is always 1000 in your case, so you can compute "period" outside the loop; further, as you have an offset of 350 now, a could start from 0; just be(0) is wrong syntax in Matlab because matrix indices start with 1.
figure;
be_i_dt = rand(1, 20000);
hAxes = gca;
hold( hAxes, 'on' )
offset = 305;
n = 1000;
n=length(A);
nyquist=1/2;
freq=(1:n/2)/(n/2)*nyquist;
period=1./freq;
for a = 1:9
A = fft(be_i_dt(offset+a*n:offset+(a+1)*n));
A(1)=[];
power=abs(A(1:floor(n/2))).^2;
plot(hAxes,period,power)
end
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Thorsten
Thorsten 2014-12-17
Yes, you probably have to few data points. The last value
offset+(a+1)*n
for the highest a (9 in your case) in your for loop must always be lower or equal to
numel(be_i_dt)

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更多回答（2 个）

David Young
David Young 2014-12-16
编辑：David Young 2014-12-16
The variable Be is not changed between iterations, so A will also be the same each time, so each plot will be the same as the previous one. If Be is a vector with all 5000 data points, then you could replace the first line in the loop with
A = fft(Be(ts+1:ts+999));
to select 1000 data points starting from ts.
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Emma
Emma 2014-12-17
Thanks for the tips!
There is only one plot. I did plot the ffts for each period separately (without the loop) and they look different, so the 5 plots should not look the same.
I also tried to add:
thisfig = figure();
to try to get 5 different plots, but then I get the same one as before plus an empty figure.
How come it has 6 iterations?
Thorsten
Thorsten 2014-12-17
编辑：Thorsten 2014-12-17
Could you please show your present code, and provide the Be matrix so that we can try it and see what went wrong?

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Sudharsana Iyengar
Sudharsana Iyengar 2014-12-17
HI,
Try this and let me know.
for a= 0:4
A=fft(Be(a*1000:(a+1)*1000));
A(1)=[];
n=length(A);
power=abs(A(1:floor(n/2))).^2;
nyquist=1/2;
freq=(1:n/2)/(n/2)*nyquist;
period=1./freq;
figure()
plot(period,power)
end
  1 个评论
Emma
Emma 2014-12-17
Hi!
Thanks! I tried it but got an error message (Subscript indices must either be real positive integers or log) so I changed a to
a=1:5
and then it plots 5 different figures (no error message=good) but it's the same plot in each figure. So the plot doesn't change.
I have changed some things (the period I'm lookin at is now from year 305 to 9305 before present, be is now called be_i_dt) and the code looks like this now:
for a= 1:9
A=fft(be_i_dt(305+a*1000:305+(a+1)*1000));
A(1)=[];
n=length(A);
power=abs(A(1:floor(n/2))).^2;
nyquist=1/2;
freq=(1:n/2)/(n/2)*nyquist;
period=1./freq;
figure()
plot(period,power)
end
And I still get 9 figures that look the same.

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