Looping over the rows, as David did below, is one way.
You can also loop over the columns. This can be faster
when nrow(matrix)>>ncol(matrix). E.g., with the
following 2 functions
f0 <- function (x) {
apply(x, 1, function(aRow) which(aRow <= 5)[1])
}
f1 <- function (x) {
isGood <- !is.na(x) & x <= 5
retval <- rep(NA, nrow(x))
for (col in rev(seq_len(ncol(x)))) {
retval[isGood[, col]] <- col
}
retval
}
and a 1 million row (by 5 column) matrix
set.seed(1)
mBig <- matrix(sample(10,size=5*10^6,replace=TRUE), ncol=5, nrow=10^6)
mBig[sample(length(mBig), size=length(mBig)/10)] <- NA
I get
> system.time(z0 <- f0(mBig))
user system elapsed
17.84 0.02 17.36
> system.time(z1 <- f1(mBig))
user system elapsed
0.56 0.01 0.56
> identical(z0,z1)
[1] TRUE
In either case it can save time (and sometimes use extra memory)
to compute things outside of the loop. f1 already does that but
f0 would be a bit quicker if you moved some repeated calculations
out of the loop:
f0a <- function (x)
{
s <- seq_len(ncol(x))
apply(!is.na(x) & x <= 5, 1, function(aRow) s[aRow][1])
}
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
> -----Original Message-----
> From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
> Behalf Of David Winsemius
> Sent: Wednesday, July 20, 2011 1:35 PM
> To: gallon li
> Cc: R-help@r-project.org
> Subject: Re: [R] select element from each row of the matrix
>
>
> On Jul 20, 2011, at 4:23 PM, gallon li wrote:
>
> > I have a 5 column matrix like
> >
> > 12 10 8 6 3
> > 10 9 8 7 5
> > 14 NA 4 NA NA NA
> > 15 NA 10 NA 5
> > ...
>
> Probably something along the lines of
>
> aapply(mtx, 1, function(x) { c( x[ which(x <= 5)[1] ], # first row are
> the values
> which(x <= 5)[1]) } ) # second row
> the positions
>
> --
> David.
>
> >
> > I want to select the position of the first entry for each row <=5
> >
> > for example, for the first row, I want to select the last element
> > and return
> > its position as 5;
> > for th e third row, I want to select the third element and return its
> > position as 3;
> > similarly for the 4th row, I want to select the fifth element and
> > return its
> > position 5.
> >
> > I am wondering how to do this fast? Thanks a lot!
> >
> > [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
______________________________________________
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