Question

I want to write a function that creates new variables in a dataframe based on strings read from a SQL table. The function should take two arguments, df and str, and return df with an extra column appended. str will be string of the form "a = f(B)", where a is a column of df, B is a set of columns of df, and f is the transformation function.

Example:

df <- data.frame(EventID = c(111,112,113), Day = c(6,8,15))

What I want is transform(df, Week = ceiling(Day / 7)), but I can’t figure out how to pass the string "Week = ceiling(Day / 7)" (as a variable of type character) to the ... arguments of transform.

I’ve tried various combinations of parse and eval, but I can’t figure out how to change a character object into what the R documentation describes as a “tagged vector expression.”

Answer 1

Here is one way:

fun <- function(df, str.expression) within(df, eval(parse(text=str.expression)))

fun(df, "Week = ceiling(Day / 7)")
#   EventID Day Week
# 1     111   6    1
# 2     112   8    2
# 3     113  15    3