Use an arrow assignment function as R purrr map2-CodePudding

I have a list of rasters:

filelist <- as.list(c("rasterA", "rasterB") # name them
rasterlist <- lapply(filelist, function(x) raster::raster(x)) # read them in

And a list of CRSes corresponding to those rasters:

crslist <- as.list(c("crsA.Rds", "crsB.Rds")) # name them
crslist %<>% lapply(function(x) readRDS(x)) # read them in

To fill the @crs slot in a raster you use (e.g.):

raster::crs(rasterA) <- crsA

But I can't work out how to use this arrow assignment in a purrr::map2 call. The example in the R cheat sheet is map2(x, y, sum), but sum works on vectors, is directionless, and contains all of its terms within its brackets.

Reading the map2 help, I tried the formula format:

purrr::map2(rasterlist, crslist, ~ raster::crs(.x) <- .y)

But no dice:

Error in as_mapper(.f, ...) : object '.y' not found

Does anyone know how I'd do this? Currently I'm using a loop which is simple, but I'm trying to force myself to learn mapping and keep hitting these kinds of walls.

for (i in length(rasterlist)) {
 crs(rasterlist[[i]]) <- crslist[[i]]
}

Thanks!

Starting condition:

After for loop:

After raster_assigner function instead:

Hopefully this is salvagable because I've got other use cases like this. Cheers!

CodePudding user response：

Could do a dedicated function:

raster_assigner <- function(raster, crs_input){
  raster::crs(raster) <- crs_input
  raster
}
purrr::map2(rasterlist, crslist, raster_assigner)

CodePudding user response：

We may need

purrr::map2(rasterlist, crslist, ~ {crs(.x) <- .y;.x})

It is not related to the raster - as the simple example below shows the same error

> map2(list(1, 2, 3), list(3, 4, 5), ~ names(.x) <- .y)
Error in as_mapper(.f, ...) : object '.y' not found
> map2(list(1, 2, 3), list(3, 4, 5), ~ {names(.x) <- .y; .x})
[[1]]
3 
1 

[[2]]
4 
2 

[[3]]
5 
3

However this won't return an error in Map (but it is not correct because the output returned is without the names, unless we wrap it with {} and return the x

> Map(function(x, y) names(x) <- y, list(1, 2, 3), list(3, 4, 5))
[[1]]
[1] 3

[[2]]
[1] 4

[[3]]
[1] 5