# why doubling is done from 2 to end-1 only in plotting single sided spectrum: P1(2:end-1) = 2*P1(2:end-1); ?

14 views (last 30 days)
Abhishek Maurya on 27 Feb 2018
Commented: Rena Berman on 19 Mar 2018
The full code is:
Y = fft(X);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
plot(f,P1);
title('Single-Sided Amplitude Spectrum of X(t)');

The 1st indexed point is the 0-frequency, the end point is the nyquist frequency. These remain unchanged as they are not part of the repeated spectrum
Abhishek Maurya on 28 Feb 2018
Thankyou to Adam,John D'Errico and everybody else who showed their interest in answering questions.
Rena Berman on 19 Mar 2018

John D'Errico on 27 Feb 2018
Edited: John D'Errico on 27 Feb 2018
end (when used as a subscript) is a shorthand to indicate the last position for the corresponding index.
So V(end) refers to the last element of a vector V. V(2:end) takes all elements but the first in a vector. And V(2:end-1) is the 2nd through penultimate element. This also applies to array indexing.
Therefore
P1(2:end-1) = 2*P1(2:end-1);
merely doubles the indicated elements in the vector P1.
I suppose use of the end keyword might have been confusing to some. They might have created some new keyword name for use in subscripting. They might, for example, have given us a lastIndex keyword, which might be clearer to some. But that in turn is longer to write, and establishes an entirely new keyword, possibly disabling some older code. Since end already existed as a keyword, but one that had no valid context in a subscript, it was deemed safe to use for indexing.

KL on 27 Feb 2018
Edited: KL on 27 Feb 2018
simple example,
>> A = 1:5
A =
1 2 3 4 5
>> A(end)
ans =
5
>> A(end-1)
ans =
4