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

[Ben Tupper]

Hi,

dplyr is faster for larger datasets compared to using table.  I tried adjusting the value of n below and for larger n (my typical situtation) dplyr stayed very fast while tablulate/table took more time.  

A good lesson for me - start using dplyr everyday!

Thanks,
Ben

library(raster)
library(dplyr)

# @param r raster
# @param cells cell indices to be incremented, repeats are honored
# @return updated raster
increment_by_tabulate <- function(r, cells){
   n <- ncell(r)
   tbl <- tabulate(cells, n)
   tcell <- 1:n
   r[tcell] <- r[tcell] + tbl
   r
}

# @param r raster
# @param cells cell indices to be incremented, repeats are honored
# @return updated raster
increment_by_table <- function(r, cells){
   tbl <- table(cells)
   tcell <- as.integer(names(tbl))
   r[tcell] <- r[tcell] + tbl
   r
}

# @param r raster
# @param cells cell indices to be incremented, repeats are honored
# @return updated raster
increment_by_dplyr <- function(r, cells){
   cel <- data_frame(cell = cells)
   tab <- cel %>% group_by(cell) %>% summarize(val = n())
   r[tab$cell] <- tab$val
   r
}

n <- 500
r <- raster(ncol=n,nrow=n)
r[] <- 0
cells <- sample(1:(n*n), 2*n*n, replace = TRUE)
system.time( r1 <- increment_by_tabulate(r, cells) )
system.time( r2 <- increment_by_table(r, cells) )
system.time( r3 <- increment_by_dplyr(r, cells) )
identical(r1,r2)
identical(r1,r3)

> On Jul 11, 2016, at 1:21 PM, Giacomo May <Giacomo_May94 at gmx.de> wrote:
> 
> Hi Ben,
> I am happy if I can be of help. So I have tried the two suggestions (one from stackexchange, which i provided, and one from the mailing list, from Loic to be exact) and the only one working properly was the one from stackexchange. it was very fast, it took 1 second max. If you want me to try anything else out for you just let me know.
> best,
> Giacomo
>  
> Gesendet: Montag, 11. Juli 2016 um 18:33 Uhr
> Von: "Ben Tupper" <btupper at bigelow.org>
> An: "Giacomo May" <Giacomo_May94 at gmx.de>
> Cc: r-sig-geo at r-project.org
> Betreff: Re: [R-sig-Geo] Add +1 to current value of a cell in a raster in R
> Hi Giacomo,
>  
> I am interested to know which you find fastest - perhaps you could let us know some timing results?
>  
> Thanks,
> Ben
>  
> On Jul 11, 2016, at 12:21 PM, Giacomo May <Giacomo_May94 at gmx.de <>> wrote:
>  
> Thanks for all the replies. In case anyone is interested, I asked the same question on stackexchange and got the following answer:
> 
> library(dplyr)
> cel <- data_frame(cell = vec)
> tab <- cel %>% group_by(cell) %>% summarize(val = n())
> ## update your raster
> r[tab$cell] <- tab$val
> 
> I will try the soultions you guys suggested as well, thanks a lot again !
> Best,
> Giacomo
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> Ben Tupper
> Bigelow Laboratory for Ocean Sciences
> 60 Bigelow Drive, P.O. Box 380
> East Boothbay, Maine 04544
> http://www.bigelow.org <http://www.bigelow.org/>
>  
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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/


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