for loop to matrix calculation

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john_arle
john_arle 2018-12-3
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
  1 个评论
Jan
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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采纳的回答

Jan
Jan 2018-12-3
编辑：Jan 2018-12-3
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
  3 个评论
显示 1更早的评论
john_arle
john_arle 2018-12-4
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
Jan 2018-12-4
@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

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