r <- raster(nc=5, nr=5)
values(r) <- runif(ncell(r)) * 2
s <- setValues(r, runif(ncell(r)) * 3)
crosstab(r,s)
rs <- r/s
r[1:5] <- NA
s[20:25] <- NA
x <- stack(r, s, rs)
crosstab(x, useNA=TRUE, long=TRUE)
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