规范化单个多边形的光栅

library(raster)
library(rnaturalearth)
library(terra)

r <- raster::getData('CMIP5', var='tmin', res=10, rcp=45, model='HE', year=70)
r <- r[[1]]    
shp <- rnaturalearth::ne_countries()

newcrs <- "+proj=robin +datum=WGS84"

r <- rast(r)
shp <- vect(shp)
r_pr <- terra::project(r, newcrs)            
shp_pr <- terra::project(shp, newcrs)            

对于 shp_pr ,我想规范化底层光栅 在0-1的范围内。这意味着将一个单元格除以一个国家边界内所有单元格的总和,并对所有国家重复此操作。我的做法如下:

country_vec <- shp$sovereignt

temp_ls <- list()

for(c in seq_along(country_vec)){
  
  country_ref <- country_vec[c]
  
  if(country_ref == "Antarctica") { next }      
  
  shp_ct <- shp[shp$sovereignt == country_ref]

  r_country <- terra::crop(r, shp_ct) # crops to the extent of boundary 
  r_country <- terra::extract(r_country, shp_ct, xy=T) 
  r_country$score_norm <- r_country$he45tn701/sum(na.omit(r_country$he45tn701))
  r_country_norm_rast <- rasterFromXYZ(r_country[ , c("x","y","score_norm")])
  
  temp_ls[[c]] <- r_country_norm_rast
  
  rm(shp_ct, r_country, r_country_norm_rast)
  }    

m <- do.call(merge, temp_ls)

我想知道这是否是最有效/最正确的方法,即没有任何for循环,有人有任何建议吗?