R-NULL-as-list-values-processing.R

# Dealing with NULLs as list values

library(tidyverse)

# As advertised on the tin:
is.null(NULL)
#> [1] TRUE

# is.null() is not vectorized?
is.null(list(NULL))
#> [1] FALSE

# And you can't vectorize it over a list
Vectorize(is.null)(list(NULL, NULL))
#> [1] FALSE

# But it can be mapped over
list(NULL, NULL) %>% map_lgl(is.null)
#> [1] TRUE TRUE



# Make a nested list, with some nulls
lstt <- list(parms = list(a=1, b=NULL, c="LETTER"), b=2, c=NULL, d="four")

# nulls are kept as entries
lstt %>% str()
#> List of 4
#>  $ parms:List of 3
#>   ..$ a: num 1
#>   ..$ b: NULL
#>   ..$ c: chr "LETTER"
#>  $ b    : num 2
#>  $ c    : NULL
#>  $ d    : chr "four"

# But I lose nulls when I unlist them
unlist(lstt)
#>  parms.a  parms.c        b        d 
#>      "1" "LETTER"      "2"   "four"

# I can recurse over list if I know what I am doing...
rapply(lstt, is.numeric) # All FALSE, because again, NULLS lost
#> parms.a parms.c       b       d 
#>    TRUE   FALSE    TRUE   FALSE
rapply(lstt, function(x) x) # Identity function, still loses nulls
#>  parms.a  parms.c        b        d 
#>      "1" "LETTER"      "2"   "four"

# Alas, `rapply` can't be coerced to do this stuff.
# Here is a solution:
# https://stackoverflow.com/questions/38950005/how-to-manipulate-null-elements-in-a-nested-list

replaceInList <- function (x, FUN, ...) {
  if (is.list(x)) {
      for (i in seq_along(x)) {
          x[i] <- list(replaceInList(x[[i]], FUN, ...))
      }
      x
  }
      else FUN(x, ...)
}


replaceInList(lstt, function(x) if (is.null(x)) NA else x) %>%
  unlist()
#>  parms.a  parms.b  parms.c        b        c        d 
#>      "1"       NA "LETTER"      "2"       NA   "four"

# And added bonus, flattening list and returning data frame
replaceInList(lstt, function(x) if (is.null(x)) NA else x) %>%
  unlist() %>%
  enframe() %>%
  spread(name, value)
#> # A tibble: 1 x 6
#>   b     c     d     parms.a parms.b parms.c
#>   <chr> <chr> <chr> <chr>   <chr>   <chr>  
#> 1 2     <NA>  four  1       <NA>    LETTER