R Under development (unstable) (2024-01-07 r85787 ucrt) -- "Unsuffered Consequences"
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> if (require(RUnit)) {
+   library(FeatureHashing)
+   df <- data.frame(a = rnorm(3), b = c("1,2", "1,,3", ",2,3"), stringsAsFactors = FALSE)
+   m <- hashed.model.matrix(~ a * split(b), df, create.mapping = TRUE)
+   mapping <- attr(m, "mapping")
+   checkTrue(!"b" %in% ls(mapping), "the output contains the column corresponding to empty string")
+   df$b <- factor(df$b)
+   m <- hashed.model.matrix(~ a * split(b), df, create.mapping = TRUE)
+   mapping <- attr(m, "mapping")
+   checkTrue(!"b" %in% ls(mapping), "the output contains the column corresponding to empty string")
+ }
Loading required package: RUnit
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   1.93    0.21    2.14