transfer character in function to the function in r

Question

I have such a problem, I want to define the grad function as an input of the function. Please see the example below.

f<-function(grad="2*m-4"){
y=grad
return(y)
}
f(3)=2

user_3358272 · Answer

I'm usually not a fan of using eval(parse(text=..)), but it does do this with a character string:

f <- function(m, grad="2*m-4"){
  eval(parse(text = grad))
}
f(3)
# [1] 2

The two take-aways:

if your grad formula requires variables, you should make them arguments of your function; and
parse(text=..) parse the string as if the user typed it in to R's interpreter, and it returns an expression:
```
parse(text="2*m - 4")
# expression(2*m - 4)
```
This expression can then be evaluated with eval.