Question: why does levels() return a list object?

[Anatolii Tsyplenkov]

Hey, guys

I am just curious, why does levels() return a list object? Is there any practical reason for this, or is it just a legacy from the {raster} package?

library(terra)
#> terra 1.7.71
set.seed(0)
r <- rast(nrows = 10, ncols = 10)
values(r) <- sample(3, ncell(r), replace = TRUE)
cls <- data.frame(id = 1:3, cover = c("forest", "water", "urban"))
levels(r) <- cls

class(levels(r))
#> [1] "list"

^{Created on 2024-05-09 with reprex v2.1.0}

[Jason Flower]

I'd say this is because if you have a multi-layer raster, the levels for each raster are separate elements in the list:

library(terra)
#> terra 1.7.71

set.seed(0)
r <- rast(nrows = 10, ncols = 10)
values(r) <- sample(3, ncell(r), replace = TRUE)
cls <- data.frame(id = 1:3, cover = c("forest", "water", "urban"))
levels(r) <- cls

class(levels(r))
#> [1] "list"

#multi-layer raster
r2 <- c(r,r)

levels(r2)
#> [[1]]
#>   id  cover
#> 1  1 forest
#> 2  2  water
#> 3  3  urban
#> 
#> [[2]]
#>   id  cover
#> 1  1 forest
#> 2  2  water
#> 3  3  urban

^{Created on 2024-05-09 with reprex v2.1.0}