"Warning: Integer operands are required for colon operator when used as index" in fft

Question

Guglielmo Giambartolomei 2016-10-11

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https://ww2.mathworks.cn/matlabcentral/answers/306726-warning-integer-operands-are-required-for-colon-operator-when-used-as-index-in-fft

评论： Guglielmo Giambartolomei 2016-10-11

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!

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Answer 1

Walter Roberson 2016-10-11

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https://ww2.mathworks.cn/matlabcentral/answers/306726-warning-integer-operands-are-required-for-colon-operator-when-used-as-index-in-fft#answer_238456

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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Guglielmo Giambartolomei 2016-10-11

Thank you Walter, I changed the code line with xdft1=xdft1(1:floor(N1/2)+1) and the warning disappeared. Now I have to understand better what the code do.

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