How to calculate reveral summations

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Xuefei Sun
Xuefei Sun 2021-2-23
编辑： Matt J 2021-2-25
I need to calculate sigma=sum(u(t)*u(t+a)), from a to 600(t=a to 600), and a is from 1 to 600. So I should get 600 different sigma.
How to do it?
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Walter Roberson
Walter Roberson 2021-2-24
Is it correct that u is least length 600+h, so that u(t+h) can be in range for indexing when t becomes 600 ?
Xuefei Sun
Xuefei Sun 2021-2-24
Sorry, the "h" should be a.
The data is u. For example u1 to u1500. And when a=1, I will get sigma(1)=u1*u(1+1)+u(2)*u(2+1)+...u(600)*u(600+1), and then a=2, I will get sigma(2)=u2*u(2+2)+u(3)*u(3+2)+...u(600)*u(600+2). ... a=600, sigma(600)=u600*u(600+600)

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回答（2 个）

Matt J
Matt J 2021-2-23
Use conv() or xcorr().
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Walter Roberson
Walter Roberson 2021-2-24
Y = circshift(A,K) circularly shifts the elements in array A by K positions. If K is an integer, then circshift shifts along the first dimension of A whose size does not equal 1. If K is a vector of integers, then each element of K indicates the shift amount in the corresponding dimension of A.
I don't think you have 600 dimensions to your u .
If you did not get an error when you did the xcorr() then that implies that u and y must both have been vectors.
The example in doc xcorr for Cross-Correlation of Two Vectors shows a vector of length 16 correlated with a modified version of itself. The output is length 31, reflecting a lag up to (16-1) before, and a lag up to (16-1) after, for a total of (16-1) + 1 + (16-1) = 2*16-1 .
Therefore for a vector of length 600, you should expect a length of 2*600-1 = 1199 for the output.
Xuefei Sun
Xuefei Sun 2021-2-24
Yes, I think that is my problem, since my u data is 600x1, my y data is also 600x1. Both of them are Vrctors.
So how to fix it?

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Matt J
Matt J 2021-2-24
编辑：Matt J 2021-2-24
T=triu(ones(600));
I=(1:600).' + (1:600);
ur=u(:).';
sigma= (T.*ur(I))*ur(1:600).';
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Xuefei Sun
Xuefei Sun 2021-2-24
It won't use xcorr() anymore? But when I type this code, it said: "Index exceeds the number of array elements (600)"
Matt J
Matt J 2021-2-25
编辑：Matt J 2021-2-25
Ran in:
Works fine when I run it.
u=rand(1,1500); %Example data
T=triu(ones(600));
I=(1:600).' + (1:600);
ur=u(:).';
sigma= (T.*ur(I))*ur(1:600).';
whos sigma
Name Size Bytes Class Attributes sigma 600x1 4800 double

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