This is the missing Matlab code:

function[fc_matrix]=grw_permute(fc_vector)



n=length(fc_vector);



fc_matrix=zeros(2,n^2);



index=1;

for i=1:n

    for j=1:n

    fc_matrix(index)=fc_vector(i);

    fc_matrix(index+1)=fc_vector(j);

    index=index+2;

    end

end






On Wed, Apr 30, 2008 at 4:47 PM, Ray Brownrigg <Ray.Brownrigg@mcs.vuw.ac.nz>
wrote:

> On Thu, 01 May 2008, Zhandong Liu wrote:
> > I am switching from Matlab to R, but I found that R is 200 times slower
> > than matlab.
> >
> > Since I am newbie to R, I must be missing some important programming
> tips.
> >
> > Please help me out on this.
> >
> > Here is the function:
> > ## make the full pair-wise permutation of a vector
> > ## input_fc=c(1,2,3);
> > ## output_fc=(
> > 1 1 1 2 2 2 3 3 3
> > 1 2 3 1 2 3 1 2 3
> > );
> >
> > grw_permute = function(input_fc){
> >
> > fc_vector = input_fc
> >
> > index = 1
> >
> > k = length(fc_vector)
> >
> > fc_matrix = matrix(0,2,k^2)
> >
> > for(i in 1:k){
> >
> > for(j in 1:k){
> >
> > fc_matrix[index]  =  fc_vector[i]
> >
> > fc_matrix[index+1]  =  fc_vector[j]
> >
> > index = index+2
> >
> > }
> >
> > }
> >
> > return(fc_matrix)
> >
> > }
> >
> > For an input vector of size 300. It took R 2.17 seconds to run.
> >
> > But the same code in matlab only needs 0.01 seconds to run.
>
> I am not a MATLAB user, but I suspect it wasn't "the same code" that
> produced
> an answer in MATLAB, but you don't provide your MATLAB code, nor do you
> specify what version of R, of MATLAB, or what hardware and OS you are
> using.
>
> I get {NetBSD, R version 2.6.0 (2007-10-03), Core 2 Duo, 3.x GHz}:
> > input_fc <- sample(1:600)
> > unix.time(a1 <- grw_permute(input_fc))
>   user  system elapsed
>  3.279  -0.001   3.280
> > unix.time({n <- length(input_fc); a2 <- matrix(c(rep(input_fc, each=n),
> rep(input_fc, n)), 2, n*n, byrow = T)})
>   user  system elapsed
>  0.019   0.020   0.040
> > all.equal(a1, a2)
> [1] TRUE
> >
> A sample of length 300 took less than 1 second using your grw_permute()
> (so
> your OS may be making a difference as well).
> >
> > Am I missing sth in R.. Is there a away to optimize.  ???
> >
> Yes. Loops are not efficient in R.
>
> > Thanks
>
> HTH,
> Ray Brownrigg
>



-- 
Zhandong Liu

Genomics and Computational Biology
University of Pennsylvania

616 BRB II/III, 421 Curie Boulevard
University of Pennsylvania School of Medicine
Philadelphia, PA 19104-6160

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