why arrayfun does NOT improve my struct array operation performance
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here is the input data:
% @param Landmarks:
% Landmarks should be 1*m struct.
% m is the number of training set.
% Landmark(i).data is a n*2 matrix
old function:
function Landmarks=CenterOfGravity(Landmarks)
% align center of gravity
for i=1 : length(Landmarks)
Landmarks(i).data=Landmarks(i).data - ones(size(Landmarks(i).data,1),1)...
*mean(Landmarks(i).data);
end
end
new function which use arrayfun:
function [Landmarks] = center_to_gravity(Landmarks)
Landmarks = arrayfun(@(struct_data)...
struct('data', struct_data.data - repmat(mean(struct_data.data), [size(struct_data.data, 1), 1]))...
,Landmarks);
end %function center_to_gravity
when using profiler, I find the usage of time is NOT what I expected:
Function Total Time Self Time*
CenterOfGravity 0.011s 0.004 s
center_to_gravity 0.029s 0.001 s
Can someone tell me why?
采纳的回答
Jan
2012-6-23
ARRAYFUN is not more efficient than a FOR loop, because it has a FOR loop internally.
Another idea:
for i = 1 : length(Landmarks)
data = Landmarks(i).data;
Landmarks(i).data= bsxfun(@minus, data, sum(data, 1) / size(data, 1));
end
Reduce the repeated access to a field, SUM/LENGTH is faster than MEAN, BSXFUN avoid the creation of a temporary array.
Or:
for i = 1 : length(Landmarks)
data = Landmarks(i).data;
m = sum(data, 1) / size(data, 1);
data(:, 1) = data(:, 1) - m(1);
data(:, 2) = data(:, 2) - m(2);
Landmarks(i).data = data;
end
更多回答（1 个）
Walter Roberson
2012-6-23
1 个投票
Also, profile disables a number of optimizations, so you cannot use profiler to determine full execution rate. Try timeit
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