MATLAB: Improve performance in ‘expansion of data’ function

Question

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

Answer 1

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