Finding layer with maximum value for each pixel in terra

Question

I have a raster like this here: https://drive.google.com/drive/u/0/folders/1AEKu4TKpzJg95oXv8dxep24xpXaesr0F

The tif has 11 Layers (timestamps). For each pixel I want to find the timestamp with the maximum value. For some reason one approach works with that data, one does not.

r = rast("au.tif")
terra::app(r, which.max) # works
terra::app(r, function(x){which.max(x)}) # does not work

For the second approach I get this error which I am not really sure about:

Error: Not compatible with requested type: [type=list; target=double].

If I am using another raster, both work. So I was wondering why that might be:

b = rast(system.file("ex/logo.tif", package="terra"))
terra::app(b, which.max) # works
terra::app(b, function(x){which.max(x)}) # works also

Becua

Answer 1

Its because there are pixels that are all NA.

b = rast(system.file("ex/logo.tif", package="terra"))
terra::app(b, function(x){which.max(x)} 

works but...

b[[1]][1,1] = NA
b[[2]][1,1] = NA
b[[3]][1,1] = NA
terra::app(b, function(x){which.max(x)}) 

fails:

Error: Not compatible with requested type: [type=list; target=double].

I think this is because R will be calling which.max on a vector of NAs and returning a length-0 vector, which messes up how app puts things together.

Also, you can just do which.max(b) since there's a method for terra rasters.