User specified function with operators in R

Question

I want to use a user-specified function and apply the function to a list of values. I envision that the user will give a 'formula' as a character string containing the names of variable and operators, e.g. "a * b %% c - d / e ^ f + g %/% h".

The following toy example works

prmlist <- list(a=1:10, b=21:30, c=31:40, d=4, e=5, f=6, g=7, h=8) 
with(prmlist, a * b %% c - d / e ^ f + g %/% h)

The problem starts when I want to use this approach within a function. To do that I must get the 'formula' specified by the user inside the function. A character string seems the obvious route. The question is how to evaluate it inside the function. do.call() doesn't seem to be suited because the operators are each really a function. I hoped something simple like

my.formula <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval(my.formula)) 

would work but it doesn't.

Answer 1

You can envoke your operators using substitute() instead:

my.formula <- substitute(a * b %% c - d / e ^ f + g %/% h)
with(prmlist, eval(my.formula)) 
 [1]  20.99974  43.99974  68.99974  95.99974 124.99974 155.99974 188.99974
 [8] 223.99974 260.99974 299.99974

Update: If the command is a string you can use parse:

myCmd <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval( parse(text=myCmd) ))
 [1]  20.99974  43.99974  68.99974  95.99974 124.99974 155.99974 188.99974
 [8] 223.99974 260.99974 299.99974