Modifying large arrays slower than modifying structure arrays?
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Joshua Robinson
il 11 Set 2019
Modificato: Joshua Robinson
il 11 Set 2019
I'm having issues with my code where I noticed manipulating large matrix arrays is extremely slow.
In my original implementation, I have to read in some data from a binary file using a for loop. I noticed that if I inserted the data into an array of structures instead of putting the data directly into an initialized matrix array, my performance was much better. I'm not sure why this is happening? Could someone provide me with some insight?
Below is a representative example of the performance issues I'm seeing.
% set up raw data
n = 10000;
rawNum = (1:n);
rawData = magic(n); % if n==10000, data is about 0.75 GB
% struct example
simpleStruct.num = [];
simpleStruct.data = [];
S(n,1) = simpleStruct;
tic
for i=1:n
S(i).num = rawNum(i);
S(i).data = rawData(i,:);
end
fprintf("Time took to populate struct array: %1.3f seconds.\n",toc);
% array example
arrayNum = zeros(n,1);
arrayData = zeros(n,n);
tic
for i=1:n
arrayNum(i) = rawNum(i);
arrayData(i,:) = rawData(i,:);
end
fprintf("Time took to populate plain ole array: %1.3f seconds.\n",toc);
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Walter Roberson
il 11 Set 2019
Row versus column ordering effects.
>> tic;for i = 1:n; arrayNum(i) = rawNum(i); arrayData(i,:) = rawData(i,:); end;toc
Elapsed time is 2.254735 seconds.
>> tic;for i = 1:n; arrayNum(i) = rawNum(i); arrayData(:,i) = rawData(:,i); end;toc
Elapsed time is 0.196039 seconds.
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