R: How to replace values in polygon geometry without a for loop?

I have over 60,000 building polygons footprints in shapefile format (polygonZ) where I need to replace the z values (for each ring) for all the polygon vertices, so that all the Z values for the polygon are the same. The issue is that we have 3D polygons that have not been edited in 3D so some rings have correct Z values for all the vertices while other rings have a few erroneous Z values which need to be corrected.

I am able to do it via a for loop but it takes a good 45 minutes to complete as it needs to iterate through all the polygons.

I was wondering if anyone has any ideas on how to speed this up?

I did try the following, thinking I could replace all the values at once via indexing but it didn't work. I assume its due to there being a different number of vertices per polygon compared to the "new values" I want to use as replacement.

> myshape$geometry[][[1]][[1]][[1]][,2] <- new_values[]

Error in myshape$geometry[][[1]][[1]][[1]][, 2] <- new_values[] : 
  number of items to replace is not a multiple of replacement length

Here is a working example using stock R data. I am replacing the X instead of Z for this example.

library(sf)
library(tictoc) 

# load test data
myshape <- st_read(system.file("shape/nc.shp", package = "sf"))

# recast to polygon (my actual data are polygons, not multipolygons)
myshape <- st_cast(myshape, "POLYGON")

# make some random data
new_values <- runif(nrow(myshape))

# replace some values
tic()
for (row in 1:nrow(myshape)) {
  # replace
  myshape$geometry[row][[1]][[1]][,2] <- new_values[row]
}
toc()

edit: clarified that each ring will have the same Z value, recast to polygons (my actual data are polygons), provided background on my problem and also fixed an error in my for loop.

Solution

There's something going on that maybe someone else can explain--possibly the whole geometry column is being overwritten with every iteration using the for loop. A Map approach is orders of magnitude faster with a larger sf object:

library(sf)

# load test data
myshape <- st_read(system.file("shape/nc.shp", package = "sf"))[sample(100, 1e4, 1),]

# make some random data
new_values <- runif(1e4)
myshape$geometry2 <- myshape$geometry3 <- myshape$geometry

system.time({
  for (row in 1:nrow(myshape)) {
    # replace
    myshape$geometry2[row][[1]][[1]][[1]][,2] <- new_values[row]
  }
})
#>    user  system elapsed 
#>  105.19    0.39  105.64

system.time({
  f <- function(g, x) {
    g[[1]][[1]][,2] <- x
    g
  }
  
  myshape$geometry3[] <- Map(f, myshape$geometry, new_values)
})
#>    user  system elapsed 
#>    0.07    0.00    0.06

identical(myshape$geometry2, myshape$geometry3)
#> [1] TRUE