Subset a raster using row/column index in terra

Question

I'm trying, in the package terra, to subset a raster by row and column numbers. Apparently that is easy in raster, at least without a geographic extent and crs: Subset a raster using row/column index . But I can't get it to work in terra. There has to be an easy way. terra::subset only selects layers of a raster.

Expecting someone to ask why: I padded a raster with rows and columns before sampling an elevation raster and calculating slope and aspect, which relies on neighboring cells. Now I need to strip off those padded rows and columns.

library(terra)
EXT <- c( -108, -105, 39, 42 )
R <- rast( extent=EXT, ncol=14, nrow=14, crs="epsg:4326" )
R[] <- 1:ncell(R)

# Now try to strip off the outer 2 rows and columns
crop( x=R, y=ext( 3, 12, 3, 12 ) )
# Error: [crop] extents do not overlap

# Normal R-style subsetting also does not work,
# just gives values of that subset
R[ 3:12, 3:12 ]

Answer 1

You can subset with drop=FALSE

library(terra)
r <- rast( extent=c( -108, -105, 39, 42 ), ncol=14, nrow=14, crs="epsg:4326" )
values(r) <- 1:ncell(r)

x <- r[ 4:12, 3:10, drop=FALSE]
x
#class       : SpatRaster 
#dimensions  : 9, 8, 1  (nrow, ncol, nlyr)
#resolution  : 0.2142857, 0.2142857  (x, y)
#extent      : -107.5714, -105.8571, 39.42857, 41.35714  (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326) 
#source      : memory 
#name        : lyr.1 
#min value   :    45 
#max value   :   164 

Verify

xy <- xyFromCell(x, cells(x))
range(rowFromY(r, xy[,2]))
#[1]  4 12
range(colFromX(r, xy[,1]))
#[1]  3 10

As to the "why", I would use crop(slope, original) to remove the padded values.