[R-sig-Geo] indexing multi-layer rasters

[Ben Tupper]

Hi,

I usually avoid using logical indexes with multilayer rasters (stacks
and bricks), but I have never understood why indexing ala `[[` with
logicals isn't supported.  Below is an example that shows how it
doesn't work like the typical indexing with logicals.  I'm not asking
to have logical indices considered (although it would be handy), but
instead I really just want to understand why it is the way it is.  I
scanned over the introductory vignette (https://rspatial.org/raster)
and issues (https://github.com/rspatial/raster/issues) but found
nothing there.

Thanks and cheers,
Ben

### START
library(raster)

logo <- raster::brick(system.file("external/rlogo.grd", package="raster"))

# red-red-red
logo[[c(TRUE, TRUE, TRUE)]]
# class      : RasterStack
# dimensions : 77, 101, 7777, 3  (nrow, ncol, ncell, nlayers)
# resolution : 1, 1  (x, y)
# extent     : 0, 101, 0, 77  (xmin, xmax, ymin, ymax)
# crs        : +proj=merc +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
# names      : red.1, red.2, red.3
# min values :     0,     0,     0
# max values :   255,   255,   255

# red-red
logo[[c(TRUE, TRUE)]]
# class      : RasterStack
# dimensions : 77, 101, 7777, 2  (nrow, ncol, ncell, nlayers)
# resolution : 1, 1  (x, y)
# extent     : 0, 101, 0, 77  (xmin, xmax, ymin, ymax)
# crs        : +proj=merc +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
# names      : red.1, red.2
# min values :     0,     0
# max values :   255,   255

# red
logo[[c(TRUE)]]
# class      : RasterLayer
# band       : 1  (of  3  bands)
# dimensions : 77, 101, 7777  (nrow, ncol, ncell)
# resolution : 1, 1  (x, y)
# extent     : 0, 101, 0, 77  (xmin, xmax, ymin, ymax)
# crs        : +proj=merc +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
# source     : /usr/lib64/R/library/raster/external/rlogo.grd
# names      : red
# values     : 0, 255  (min, max)

logo[[c(TRUE, FALSE, TRUE)]]
#Error in .local(x, ...) : not a valid subset


#sessionInfo()
# R version 3.5.1 (2018-07-02)
# Platform: x86_64-redhat-linux-gnu (64-bit)
# Running under: CentOS Linux 7 (Core)
#
# Matrix products: default
# BLAS/LAPACK: /usr/lib64/R/lib/libRblas.so
#
# locale:
#   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
# [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
LC_PAPER=en_US.UTF-8       LC_NAME=C
# [9] LC_ADDRESS=C               LC_TELEPHONE=C
LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
#
# attached base packages:
#   [1] stats     graphics  grDevices utils     datasets  methods   base
#
# other attached packages:
#   [1] raster_3.0-7 sp_1.3-2
#
# loaded via a namespace (and not attached):
#   [1] compiler_3.5.1   rgdal_1.4-8      tools_3.5.1      yaml_2.2.0
     Rcpp_1.0.3       codetools_0.2-15
# [7] grid_3.5.1       lattice_0.20-35

### END




-- 
Ben Tupper
Bigelow Laboratory for Ocean Science
West Boothbay Harbor, Maine
http://www.bigelow.org/
https://eco.bigelow.org