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

[Loïc Dutrieux]

This works fine for me:

library(raster)

r <- raster(ncol=5,nrow=5)
r[] <- 0
vec <- c(1,1,1,3,4,5,6,7,8,8,8,9)
a <- data.frame(table(vec))
a$vec <- as.numeric(a$vec) # Not very elegant coercion, there must be a 
better way
r[a$vec] <- r[a$vec] + a$Freq[a$vec]

Cheers,
Loïc

On 07/11/2016 02:58 AM, Vijay Lulla wrote:
> OOPS!  I appear to have misunderstood the question then, Ben.  I'm unsure
> how to increment a particular cell more than once.
>
> Please pardon the noise.
>
> On Sun, Jul 10, 2016 at 7:54 PM, Ben Tupper <btupper at bigelow.org> wrote:
>
>> Hi,
>>
>> Interesting.  How would you use the matrix approach to increment a
>> particular cell more than once with a single subassignment?  If I repeat
>> the upper left raster coord in the matrix 3 times it still yields a value
>> of 1 up there.
>>
>> library(raster)
>> r <- raster(nrows=5,ncols=5)
>> r[] <- 0
>> idx <- matrix(c(1,1,1,1,1,1,1,5,5,1,5,5),ncol=2,byrow=TRUE)
>> r[idx] <- r[idx] + 1
>> plot(r)
>> range(getValues(r))
>> #[1] 0 1
>>
>> I could be misreading the original posting, but I think the desire is that
>> the result will be increment for each repeated index using just one
>> subassignment.
>>
>> Cheers,
>> Ben
>>
>>
>> On Jul 10, 2016, at 6:45 PM, Vijay Lulla <vijaylulla at gmail.com> wrote:
>>
>> 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
>>>
>>
>>
>> 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/
>>
>>
>
> 	[[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>