find negative values of simple summation. code efficiency

Question

Ansgar Elles 2021-1-3

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/707718-find-negative-values-of-simple-summation-code-efficiency

回答： Walter Roberson 2021-1-5

Hello, I have the following statement which must be true: v1_before(1) == v1_transferred_vector(1) + v1_conserved_vector(1)

My goal is to find out which of the two elements on the right side are actually negative. The algebraic sign of "v1_before" is already known, as well as the absolute values of the two elements on the right side.

The indices refer to the x-value of the [x,y] vectors. So i will do exactly the same with (2) / y-values too.

I coded the following. But im wondering whether there is a more elegant and efficient way to do it. Many thanks in advance!!!!

%determine whether + / - for x-values
if v1_before(1) - v1_transferred_vector(1) - v1_conserved_vector(1) == 0       %==0 when difference is smaller then 1e-15/eps
    %do nothing, leave them positiv
elseif v1_before(1) + v1_transferred_vector(1)- v1_conserved_vector(1) == 0
    v1_transferred_vector(1) = - v1_transferred_vector(1);
elseif v1_before(1) - v1_transferred_vector(1) + v1_conserved_vector(1) == 0
    v1_conserved_vector(1) = - v1_conserved_vector(1);
else %v1_before(1) + v1_transferred_vector(1) + v1_conserved_vector(1) == 0
    v1_transferred_vector(1) = - v1_transferred_vector(1);
    v1_conserved_vector(1) = - v1_conserved_vector(1);
end

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Rik 2021-1-5

If this code only runs once it doesn't matter if it takes 1 ms or 100 ms, but if this code is executed thousands of times in a time-sensitive context, then it does start to matter.

The main point is to spend time optimizing the code that matters. You can spend a few hours here to save a few miliseconds, or spend your time on code that runs often and takes longer to complete.

Ansgar Elles 2021-1-5

ok thanks

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Answer 1

Walter Roberson 2021-1-5

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/707718-find-negative-values-of-simple-summation-code-efficiency#answer_591153

在 MATLAB Online 中打开

M = [1 1; 1 -1; -1 1; -1 -1];
m = M(v1_before(1)  == M*[v1_transferred_vector(1); v1_conserved_vector(1)],:);
v1_transfered_vector(1) = m(1)*v1_transfered_vector(1);
v1_conserved_vector(1) = m(2)*v1_conserved_vector(1);

This can be vectorized with a slight bit of work.