Extract data from a vector in a loop
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Hi everyone, First of all, I am a beginner.
I would like to make a function that would do the variance over specified number of samples "n". I need a variance of a signal (which is in a vector "x"). Basically, from a matrix "x", it takes first "n" number of samples, calculates variance and carries on to the next "n" samples. Now, I need those variance calculations in one vector, so I can plot it. That vector would have "length(x)/n" units. I made something like this:
function [a,b] = Variance( x, n )
a=zeros(1,length(x));
b=zeros(1,length(x));
for i=1:length(x)/n
a=x(i*n:(i+1)*n);
b=var(a);
end
end
This, of course does not work. I would appreciate if you could help out.
采纳的回答
Joseph Cheng
2014-3-14
So lets break it down. in your for loop
for i=1:length(x)/n
a=x(i*n:(i+1)*n);
b=var(a);
end
when i=1 you are getting x(1*n:2*n) and missing the the values of x from 1 to n. So you'll want to start with i-1 and go to the i value. example:
x=1:20;
for i=1:10
disp(x((i-1)*2+1:(i)*2))
end
Since you are assigning the calculations to a and b you should additionally be putting them into the correct array positions (i). so you'll have a(i) = __ ; and b(i) =var(a(i)); that way you get your length(x)/n arrays.
Another method of this is you can use reshape. example:
x=1:20
y = reshape(x,5,4);
b=var(y);
here you see that the reshape formed your buckets of n-samples for you.
~J
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