Expand stars-raster in R

Question

In the r-package terra there is the function terra::extend. To my understanding it just expands the raster with no data values on each side right?

I wanted to do something similar, but using a stars object. My approach kind of looked like this:

  # add.y and add.y are scalars between 0 and 1 to extend the number of cols/rows by e.g. add.x percent

  grid_vals = star[[1]]
  ncol = ncol(grid_vals)
  nrow = nrow(grid_vals)
  new_cols = floor(ncol * add.x); if(!new_cols %% 2 == 0) new_cols = new_cols -1
  new_rows = floor(nrow * add.y); if(!new_rows %% 2 == 0) new_rows = new_rows -1

  # new columns
  for (i in 1:new_cols) {
    if (i %% 2 == 0) {
      grid_vals = cbind(grid_vals, rep(NA, nrow))
    } else{
      grid_vals = cbind(rep(NA, nrow), grid_vals)
    }
  }

  # new rows
  cols = ncol(grid_vals)
  
  for (j in 1:new_rows) {
    if (j %% 2 == 0) {
      grid_vals = rbind(grid_vals, rep(NA, cols))
    } else{
      grid_vals = rbind(rep(NA, cols), grid_vals)
    }
  }

I can create a new matrix with this. However, I do not know how I could assign this to my old stars-object that has a different extent and bounding-box etc...

Answer 1

With this stars object

library(stars)
f = system.file("ex/elev.tif", package="terra")
s = read_stars(f)

You can do

library(terra)
r = rast(s)
r = extend(r, ext(r)+2)

And then coerce it back to stars

x = st_as_stars(r)