[英]How to speed up a double loop in matlab
以下代碼需要花費大量時間來遍歷。 您對加快流程有什么建議嗎? 變量z
的大小為479x1672,其他大小約為479x12000。
z = HongKongPrices;
zmat = false(size(z));
r = size(z,1);
c = size(z,2);
for k = 1:c
for i = 5:r
if z(i,k) == z(i-4,k) && z(i,k) == z(i-3,k) && z(i,k) == z(end,k)
zmat(i-3:i,k) = 1
end
end
end
z(zmat) = NaN
我目前正在具有3.2 Intel i5和16 GB DDR3的iMac上通過MatLab R2014b運行它。
您可以在此處使用logical indexing
來代替IF-conditional
語句,並使用一個小循環-
%// Get size parameters
[r,c] = size(z);
%// Get logical mask with ones for each column at places that satisfy the condition
%// mentioned as the IF conditional statement in the problem code
mask = z(1:r-4,:) == z(5:r,:) & z(2:r-3,:) == z(5:r,:) & ...
bsxfun(@eq,z(end,:),z(5:r,:));
%// Use logical indexing to map entire z array and set mask elements as NaNs
for k = 1:4
z([false(k,c) ; mask ; false(4-k,c)]) = NaN;
end
%// Size parameters
nrows = 479;
ncols = 12000;
max_num = 10;
num_iter = 10; %// number of iterations to run each approach,
%// so that runtimes are over 1 sec mark
z_org = randi(max_num,nrows,ncols); %// random input data of specified size
disp('--------------------------------- With proposed approach')
tic
for iter = 1:num_iter
z = z_org;
[..... code from the proposed approach ...]
end
toc, clear z k mask r c
disp('--------------------------------- With original approach')
tic
for iter = 1:num_iter
z = z_org;
[..... code from the problem ...]
end
toc
案例1: z
為479 x 1672
( num_iter = 50
)
--------------------------------- With proposed approach
Elapsed time is 1.285337 seconds.
--------------------------------- With original approach
Elapsed time is 2.008256 seconds.
案例2: z
為479 x 12000
( num_iter = 10
)
--------------------------------- With proposed approach
Elapsed time is 1.941858 seconds.
--------------------------------- With original approach
Elapsed time is 2.897006 seconds.
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