This is truly bizarre behavior I can't explain.

library(terra)

r1 = rast(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
ext(r2) = ext(r2)/2
ext(r2)
ext(r1)

What you see is that the extent of 'r1' has also been cut in half. Whyyyyyyy??? By the way, you can change the values of 'r2' without affecting the values of 'r1'. As far as I can tell it only applies to the extent.

This also doesn't happen when doing the same process using the 'raster' package.

library(raster)
    
r1 = raster(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
extent(r2) = extent(r2)/2
extent(r2)
extent(r1)

This is truly bizarre behavior I can't explain.

library(terra)

r1 = rast(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
ext(r2) = ext(r2)/2
ext(r2)
ext(r1)

What you see is that the extent of 'r1' has also been cut in half. Whyyyyyyy??? By the way, you can change the values of 'r2' without affecting the values of 'r1'. As far as I can tell it only applies to the extent.

This also doesn't happen when doing the same process using the 'raster' package.

library(raster)
    
r1 = raster(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
extent(r2) = extent(r2)/2
extent(r2)
extent(r1)

• r
• raster
• r-raster
• terra

2 Answers 2

That is a bug that has now been fixed in the development version.

Here is a work-around with as.vector():

library(terra)
r1 = rast(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
ext(r2) = as.vector(ext(r2)/2)
ext(r2)
#SpatExtent : -90, 90, -45, 45 (xmin, xmax, ymin, ymax)
ext(r1)
#SpatExtent : -180, 180, -90, 90 (xmin, xmax, ymin, ymax)

And with the development version it works as expected

library(terra)
#terra 1.8.24
r1 = rast(nrow = 2, ncol = 2)
values(r1) = c(0, 1, 2, 3)
r2 = r1
ext(r2) = ext(r2)/2
ext(r2)
#SpatExtent : -90, 90, -45, 45 (xmin, xmax, ymin, ymax)
ext(r1)
#SpatExtent : -180, 180, -90, 90 (xmin, xmax, ymin, ymax)

You can install the development version with

install.packages('terra', repos='https://rspatial.r-universe.dev')

A simple assignment in terra creates a shallow copy, meaning that the original object can be changed if the derived one is edited by a function that makes an in-place change. The solution if you want to avoid that behaviour is to use deepcopy. The documentation of deepcopy shows a similar behaviour for set.ext() as you discovered for ext(): https://rspatial.github.io/terra/reference/deepcopy.html

Edit: note the answer by @Robert Hijmans saying that the dev version of terra now behaves differently, without the need of a deepcopy