How to get AST as a list in R

问题

I have strings, which describe mathematical formulas and I would like to convert it into lists of meaningful parts. The function ast_ does know how to parse it, to displays it as an Abstract Syntax Tree but does not return the AST. I am looking for a function which does return the tree.

bb <- "(media.urin_A + media.urin_B)/2"
lazyeval::ast_(rlang::parse_expr(bb))

> lazyeval::ast_(rlang::parse_expr(bb))
┗ ()
 ┗ `/
 ┗ ()
  ┗ `(
  ┗ ()
   ┗ `+
   ┗ `media.urin_A
   ┗ `media.urin_B
 ┗  2 

回答1:

You can construct one recursively using as.list() and map_if from purrr:

getAST <- function(ee) purrr::map_if(as.list(ee), is.call, getAST)

# Example usage on expressions:
getAST( quote(log10(a+5)/b) )
# List of 3
#  $ : symbol /
#  $ :List of 2
#   ..$ : symbol log10
#   ..$ :List of 3
#   .. ..$ : symbol +
#   .. ..$ : symbol a
#   .. ..$ : num 5
#  $ : symbol b

# Example usage on strings:
getAST( str2lang("(media.urin_A + media.urin_B)/2") )
# List of 3
#  $ : symbol /
#  $ :List of 2
#   ..$ : symbol (
#   ..$ :List of 3
#   .. ..$ : symbol +
#   .. ..$ : symbol media.urin_A
#   .. ..$ : symbol media.urin_B
#  $ : num 2

来源：https://stackoverflow.com/questions/60083614/how-to-get-ast-as-a-list-in-r