Match polygon and dataframe IDs after raster::extract

Hi all,

I'm trying to look at correlation between two raster layers, for
different polygons. So I use raster::extract to get all the raster
values for every polygon, do the calculation and feed the output back to
a SpatialPolygonDataFrame.
I got it working, but I have a doubt regarding the order of the rows;
and it doesn't look like I can use match.ID = TRUE.

See the example below.

library(raster)
library(dplyr)

# Create brick with 2 layers
b <- brick(ncol=36, nrow=18, nl=2)
b[[1]] <- rnorm(ncell(b))
b[[2]] <- rnorm(ncell(b))

# Create sp
cds1 <- rbind(c(-180,-20), c(-160,5), c(-60, 0), c(-160,-60), c(-180,-20))
cds2 <- rbind(c(80,0), c(100,60), c(120,0), c(120,-55), c(80,0))
cds3 <- rbind(c(-10,20), c(50,60), c(70,-10))
polys <- spPolygons(cds1, cds2, cds3)

# Extract all values
df0 <- extract(b, polys, df = TRUE)

# Compute correlation betwen the two layers for every polygons (sorry
for the pipe)
df1 <- group_by(df0, ID) %>%
  summarise(cor = cor(layer.1, layer.2)) %>%
  data.frame()

# Attach to df to spdf
spdf <- SpatialPointsDataFrame(polys, df1)



How do I know for sure that the order of the rows in the dataframe did
not get mixed up? Can I just assume that they will remain in the same
order?

pts = matrix(1:4,2,2,dimnames=list(c("a", "b"), NULL))
pts

df = data.frame(a = 2:3, row.names = c("b", "a"))
df

SpatialPointsDataFrame(pts, df)

SpatialPointsDataFrame(pts, df, match.ID = FALSE)

The dataframe returned by extract has an ID column, but the IDs do not
correspond to the polygons IDs. For instance if I remove the second
polygon (which has the ID "2"), the IDs in the df extracted are still 1
and 2 (instead of 1 and 3).

# Quick function to get polygons IDs
getPolyID <- function(x) {
  sapply(x at polygons, function(x) {x at ID} )
}


getPolyID(polys)
# [1] "1" "2" "3"
getPolyID(polys[-2])
# [1] "1" "3"

unique(extract(b, polys[-2], df = TRUE)$ID)
# [1] 1 2


Any suggestions?

Thanks,
Lo?c