Most efficient way for repeating a vector in Matlab

Question

What is the most efficient way for repeating a vector?

I'm currently under the impression that repmat is superior to any other methods? Or am I horribly wrong with this mindset?

Would it be possible to produce the exact same result as repmat does, using a different technique? Perhaps ordinary matrix multiplication?

I would like to express my greatest gratitude for all your interest and supportive answers!

AER

Answer 1

The bottom line, bsxfun is faster than the two you asked for if the start vector is long or the # of repeats is big enough (see below), otherwise matrix multiplication is more efficient. Between the two you've asked it looks like matrix multiplication+reshape wins in efficiency by a factor of ~3 over repmat . I've used timeit the following way, I've created a random vector of 1e5 elements and checked how long it takes to create 100 repeats of it:

v=rand(1e5,1);
f1=@()repmat(v,[100,1])
f2=@() reshape(v*ones(1,100),[],1);

timeit(f1)
ans =
     0.1675

timeit(f2)
ans =
    0.0516

however bsxfun is even faster:

f3=@() reshape(bsxfun(@times,v,ones(1,100)),[],1) 

timeit(f3)

 ans =
     0.0374

Here's a more careful study of this observation:

Given a vector is 1000 elements long, repeating it 10 to 1e5 times yield the following performance times:

For smaller # of repeats there is little difference between bsxfun and matrix multiplication but as the # of repeats passes ~1e3, bsxfun wins clearly.

However, taking a mere 10 elements long vector with the same range of repeats, shows that matrix multiplication is more efficient. bsxfun starts to be better only after 10^5 repeats, but even then it is only ~5% faster (not shown) :

so it depends really what you're after. Further discussion is found in Loren on the Art of MATLAB blog .

Answer 2

No, currently repmat mostly is slow er than plain indexing, which works as follows:

Classis repmat :

a = [ 1 2 3 ];
repmat(a,4,1)
>> 
     [1 2 3;
      1 2 3;
      1 2 3;
      1 2 3];

indexing version of this:

a(ones(4,1),:);

which gives just the same result.

Answer 3

it really depends what sort of verctor you want. for example if you want aa specialized vector such as row of ones, then

tic
repmat([1],3,5)
toc

tic
ones(3,5)
toc

will tell you that using "ones" is about 3 times faster (at least on my machine). You can always use tic and toc to measure which method is faster. Lets try another one...

tic
repmat([1,2,3],1,2)
toc

tic
cat(2,[1,2,3],[1,2,3]) 
toc

now cat is 3 times faster again but it has its own obvious limitation. Hope this helps