getVarsFromFormula: Parse a formula to get the variable list
In madlogos/aseskit: ASES Generic Toolkit

Description Usage Arguments Details Value Author(s) Examples

Get x, y, weight, series from a formula expression. If expr cannot be interpreted as formula, it will be returned as a character vector as-is.

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getVarsFromFormula(expr, ...)

parse_formula(expr, ...)

`expr`	An expression to coerce to formula.
`...`	Other arguments to pass to the function.

There are two forms of arguments:

Put all the arguments in expr and ignore ..., then the function will analyze the format and parse elements automatically: left-hand side to "~" is y, right-hand side to "~" is independent variables, of which symbols connected with + or - is x, symbols to the right of * is weight and symbols to the right of | is series.
Put arguments along with expr and ..., then the function will wrap them up as-is and output in a named vector. In this case, the result totally depends on the order you input the arguments.

If the parsing succeeds, then a named vector of x, y, series and weight, the missing element will be output as NA
If the parsing fails, then the expr as a character
If expr is NULL or NA/NaN, then NULL or NA

If more than one asterisks or pipes are detected, the function will only extract the last match as series or weight.

Yiying Wang, wangy@aetna.com

## Not run: 
## Identify ingredients in a formula.
getVarsFromFormula("y+z~x*w+m|d")  ## return
  \tabular(cccc){
   x \tab y \tab series \tab weight \cr
   "x + m" \tab "y + z" \tab "d"  \tab  "w"
  }
  
## it is equivalent to
parse_formula(y+z~x*w+m|d)

## another form
parse_formula(y+z, x+m, d, w)


## End(Not run)