[R] Making routine faster by using apply instead of for-loop

[Peter Ehlers]

Your code is doing too many needless things.
The following takes about one second on my slow Vista laptop.

n <- 500
mat <- matrix(1:(n*n), n)
v <- n:1
z <- 2*1:(n/2)
w <- c(z, rev(z))
for(i in seq_len(n)){
   for(j in seq_len(n)){
     if(v[j] + w[i] <= n)(mat[i,j] <- NA)
   }
}
rownames(mat) <- v
colnames(mat) <- w

str(mat)

You end up with matrix, but if you really want a data.frame
with duplicate names, that's easy to get. Do you actually
want those row/col names or are they just used to identify
the cells that get NA?

Depending on what you really need, the following may be
good enough; takes about 0.1 seconds.

n <- 500
mat <- matrix(1:(n*n), n)
for(i in 1:(n/2)){mat[i, -(1:(2*i))] <- mat[n+1-i, -(1:(2*i))] <- NA}

  -Peter Ehlers

Etienne Stockhausen wrote:
> Hey everybody,
> 
> I have a small problem with a routine, which prepares some data for
> plotting.
> I've made a small example:
> 
>    c=10
>    mat=data.frame(matrix(1:(c*c),c,c))
>    row.names(mat)=seq(c,1,length=c)
>    names(mat)=c(seq(2,c,length=c/2),seq(c,2,length=c/2))
>    v=as.numeric(row.names(mat))
>    w=as.numeric(names(mat))
>    for(i in 1:c)
>    { for(j in 1:c)
>    {
>    if(v[j]+w[i]<=c)(mat[i,j]=NA)
>    }}
> 
> This produces exactly the data I need to go on, but if I increase the
> constant c ,to for instance 500 , it takes a very long time to set the 
> NA's.
> I've heard there is a much faster way to set the NA's using the command
> apply( ), but I don't know how.
> I'm looking forward for any ideas or hints, that might help me.
> 
> Best regards
> 
> Etienne
> 
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> 
> 

-- 
Peter Ehlers
University of Calgary
403.202.3921