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Asked by Stephen on 15 Jan 2013

Hi,

In my application i have a performance bottleneck in the following function. I have experimented a little and improved it a bit but im wondering if it can be made any faster. Its right on the edge of acceptable and too slow still.

I need a function to do the following:

if I = [1 2 3 4] then expanding it by 4 would give [1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4]

I am using the data generated from the code below to apply to another set of data

%Expand I and Q Data to longer arrays %Expands the original data by copying each number a few times function [I_mod Q_mod] = expandIQ(I,Q,expandfactor)

expandfactor = round(expandfactor);

I_mod = []; Q_mod = [];

for i=1:expandfactor:(length(I)*expandfactor) I_mod = [I_mod I((i-1)/expandfactor+1)*ones(1,expandfactor)]; Q_mod = [Q_mod Q((i-1)/expandfactor+1)*ones(1,expandfactor)]; end

if(length(I_mod) ~= length(I)*expandfactor) error('Unexpected length of I_mod') end

I have tried 'pre-allocating for speed' as matlab suggested, however this reduced it drastically.

I have been testing the performance using:

I = ones(1,length_of_I); Q = ones(1,length_of_I); tic expandIQ(I,Q,expandfactor); toc

where length_of_I << expandfactor

eg. length_of_I = 16, expandfactor = 500000 or length_of_I = 500, expandfactor = 50000

Im sure there is a faster way but i can't think of any. I am more comfortable in C than matlab so any suggestions/ideas before i try mex?

Thanks, Stephen

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Answer by Cedric Wannaz on 16 Jan 2013

Accepted answer

Hi Stephen,

See if the following brings an improvement in your specific context:

function [I_mod Q_mod] = expandIQ(I,Q,expandfactor) expandfactor = round(expandfactor) ; I_mod = reshape(repmat(I, expandfactor, 1), 1, []) ; Q_mod = reshape(repmat(Q, expandfactor, 1), 1, []) ; end

It's not too relevant here, but there is a profiler in the Tools menu.

Cheers,

Cedric

## 2 Comments

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/58880#comment_122790

Ahhh, this is much better :) Any improvements on this? Its still a bit slow :/

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/58880#comment_122805

I haven't tested your code, but I think this is what you are trying to achieve. It seems fairly fast for your length(I)=16, expandfactor = 500000 example: