Multiple Sum of Series in a Variable - Syntax Problem

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Christos Tsallis 2021-3-24

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评论： Jan 2021-3-25

Hello,

I got a problem expressing the equations bellow, in matlab. There is a dat variable which is a 7x2 table and has values in it. I must create a function wich will have "dat" as an input and "a" as an output. So if we go to the first "symsum" of SSxy equation I want to express "for j=1 to P do the sum of series of Xj*Yj" as it is shown bellow in the code. The problem is that I got errors and I must have the syntax of symsum wrong. Can someone help me understand my mistake? Thanks for your time, appreciate your help!

dat - variable

1000 79

3000 71

9000 59

50000 49

93000 39

223000 33

510000 24

function [a]=Example(dat)
x=log10(dat(:,1));
y=dat(:,2); 
P=length(dat); 
syms x y 
SSxy=(symsum(x(j)*y(j),j,1,P))-(((symsum(x(j),j,1,P))*(symsum(y(j),j,1,P)))/P);
SSxx=(symsum(x(j).^2,j,1,P))-(((symsum(x(j),j,1,P))^2)/P); 
a=SSxy/SSxx;
end 

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Answer 1

Jan 2021-3-24

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编辑：Jan 2021-3-24

在 MATLAB Online 中打开

Why do you try a symbolic sum, when your input is numerical? Here a numerical summation:

function a = Example(dat)
x = log10(dat(:, 1));
y = dat(:, 2); 
P = size(dat, 1);   % Safer than length(dat)
% X.' * y is the dot product, same as sum(x .* y)
SSxy = x.' * y - sum(x) * sumy(y) / P;
SSxx = x.' * x - sum(x)^2 / P;
a    = SSxy / SSxx;
end 

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Christos Tsallis 2021-3-24

My thoughts were more complex than they should be I guess... Can I do the same with the use of symsum? Really thanks for your reply, appreciate it!

Jan 2021-3-25

symsum creates a symbolic formula. You could create it and evaluate the result for numeric input. But this is ways slower than the direct numeric summation.

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