[R-sig-Geo] Add +1 to current value of a cell in a raster in R

[Vijay Lulla]

Maybe something like this?

R> r <- raster(nrows=5,ncols=5)
R> r[] <- 0
R> idx <- matrix(c(1,1,1,5,5,1,5,5),ncol=2,byrow=TRUE) # four corners of
the raster!
R> r[idx] <- r[idx] + 1
R> image(r)

>From R's ?`[` : When indexing arrays by '[' a single argument 'i' can be a
matrix with as many columns as there are dimensions of 'x'; the result is
then a vector with elements corresponding to the sets of indices in each
row of 'i'.


On Sun, Jul 10, 2016 at 5:59 PM, Ben Tupper <btupper at bigelow.org> wrote:

> Hi,
>
> I stub my toe on this all the time (and wish I didn't).  The behavior
> isn't limited to raster - the replacement operator `[` in general doesn't
> update the left had side like my wild imagination might allow.  It's just a
> function and so it must make a copy of the input.
>
> This note seen here ?`[` sort of points out the issue... "Subassignment is
> done sequentially, so if an index is specified more than once the latest
> assigned value for an index will result."  Between that, the phase of the
> moon and the pass-by-value stuff my brain starts to jumble.
>
> On the other hand, if you have dependable indices and are simply
> incrementing you can leverage table() for your purpose.
>
> x <- rep(0,5)
> i <- c(1,1,1,4)
> ti <- table(i)
> tindex <- as.numeric(names(ti))
> x[tindex] <- x[tindex] + ti
>
> Likewise for raster
>
> library(raster)
> r <- raster(ncol=5,nrow=5)
> r[] <- 0
> vec <- c(1,1,1,3,4,5,6,7,8,8,8,9)
> tvec <- table(vec)
> tindex <- as.integer(names(tvec))
> r[tindex] <- r[tindex] + tvec
>
> I hope there are smarter/faster ways, but in the meantime it might speed
> things along for you.
>
>
> Ben
>
>
> > On Jul 10, 2016, at 3:34 PM, Giacomo May <Giacomo_May94 at gmx.de> wrote:
> >
> > Dear R-users,
> > I am trying to add +1 to the cell value of certain cells in a raster. I
> have the cell numbers of the cells on which I'd like to perform said
> computation saved in a vector. I have a working code but with a large
> vector and raster it takes ages:
> >
> > r <- raster(ncol=5,nrow=5)
> > r[] <- 0
> > vec <- c(1,1,1,3,4,5,6,7,8,8,8,9)
> > for(i in 1:length(vec))
> > {
> > r[vec[i]] <- r[vec[i]] + 1
> > }
> >
> > I have already tried to use r[vec] <- r[vec] + 1 but that doesn't work
> properly, it just sets the values of the cells stored in the vector to 1.
> Does anybody know a faster way to do this ? Thanks in advance!
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo at r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> Ben Tupper
> Bigelow Laboratory for Ocean Sciences
> 60 Bigelow Drive, P.O. Box 380
> East Boothbay, Maine 04544
> http://www.bigelow.org
>
> Report Gulf of Maine jellyfish sightings to jellyfish at bigelow.org or
> tweet them to #MaineJellies -- include date, time, and location, as well as
> any descriptive information such as size or type.  Learn more at
> https://www.bigelow.org/research/srs/nick-record/nick-record-laboratory/mainejellies/
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>

	[[alternative HTML version deleted]]