for loop to matrix calculation

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john_arle 2018-12-3

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/433567-for-loop-to-matrix-calculation

评论： Jan 2018-12-4

Hello there,

I do not know how to optimize this calculation. It is very slow, since vector "my_vector" is made of 3 millions indeces.

I would like to perform the same calculation using matrices, but I really do not know how to do it!

Thank you!

Have a good one,

Andrea.

corr = zeros(1,n_A*p_A-1); % row vector of 3762799
v = residual(my_vector);
variance = mean(v.^2); 
index_corr = 0;
ref_length = 0;
for l = 1:(n_A*p_A-1)
    for j = 1:(n_A*p_A-l)
        corr(1,l) = corr(1,l)+v(1,j)*v(1,j+l);
    end
    corr(1,l) = corr(1,l)/((n_A*p_A-l)*variance);
    if (corr(1,l) < (1/exp(1)))
        if index_corr == 0
            ref_length = l;
        end
        index_corr = 1;
    end   
end

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Jan 2018-12-3

编辑：Jan 2018-12-3

It would be useful if you explain, what the code should do. Some meaningful comments in the code are required also to improve its quality. Please provide some meaningful inputs also, because this is more efficient than letting the readers guess the inputs.

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Answer 1

Jan 2018-12-3

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/433567-for-loop-to-matrix-calculation#answer_350304

编辑：Jan 2018-12-3

在 MATLAB Online 中打开

1/exp(1) is a very expensive calculation. Do this once before the loop.

The index "l" (lowercase L) looks like a "1", so I replaced it by "k".

corr = zeros(1, n_A*p_A-1); % row vector of 3762799
v = residual(my_vector);
variance = mean(v .^ 2); 
index_corr = 0;
ref_length = 0;
c = 1 / exp(1);
for k = 1:(n_A*p_A-1)
    n = n_A * p_A - k;
    % Use DOT product to calculate the sum:
    corr(1, k) = corr(1, k) + v(1, 1:n) * v(1, (1 + k):(n + k)).';
    % Alternative - assumed to be slower:
    % corr(1, k) = corr(1, k) + sum(v(1, 1:n) .* v(1, (1 + k):(n + k)));
    
    corr(1, k) = corr(1,k) / ((n_A*p_A-k) * variance);
    if corr(1,k) < c
        if index_corr == 0
            ref_length = k;
        end
        index_corr = 1;
    end
end

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john_arle 2018-12-4

在 MATLAB Online 中打开

Hello Jan,

thank you again for your kind response!

I get an error though (implementing dot product), and I do not know how to fix it!

Index in position 2 exceeds array bounds (must not exceed 1).
Error in example2 (line 67)
    corr(1, k) = corr(1, k) + v(1, 1:n) * v(1, (1 + k):(n + k)).';

Jan 2018-12-4

在 MATLAB Online 中打开

@Andrea: Unfortunately I cannot run your code, because you did not provide the input arguments. Then you have to debug the code by your own. Perhaps one of the inputs is a column vector? Try this:

...
v = V(:).';
for k = 1:(n_A*p_A-1)
    n = n_A * p_A - k;
    corr(k) = corr(k) + v(1:n) * v((1 + k):(n + k)).';
    corr(k) = corr(k) / ((n_A*p_A-k) * variance);
    if corr(k) < c
        if index_corr == 0
            ref_length = k;
        end
        index_corr = 1;
    end
end