# [R] Modifying a function programmatically

Søren Højsgaard sorenh at math.aau.dk
Mon Feb 4 11:00:50 CET 2013

```Dear list

# I have a function
ff  <- function(a,b=2,c=4){a+b+c}
# which I programmatically want to modify to a more specialized function in which a is replaced by 1
ff1 <- function(b=2,c=4){1+b+c}

# I do as follows:
vals  <- list(a=1)
(expr1 <- as.expression(body(ff)))
expression({
a + b + c
})
(expr2 <- do.call("substitute", list(expr1[[1]], vals)))
{
1 + b + c
}

# This "works",
eval(expr2, list(b=10,c=12))
[1] 23

# - but I would like a function. Hence I do:
ll  <- alist(b=, c=, eval(expr2))
ff2 <- as.function(ll)
ff2(b=10,c=12)
[1] 23

# BUT I am only half-way where I want to be because the alist(b=, c=, ...)
# requires me plugin the remaining formals by hand. I do:
newformals <-setdiff(names(formals(ff)), names(vals))
vv         <- vector("list", length(newformals))
names(vv)  <- newformals
(hh <- c(vv, expr2))
\$b
NULL
\$c
NULL
[[3]]
{
1 + b + c
}

(ff3 <- as.function(hh))
function (b = NULL, c = NULL)
{
1 + b + c
}
ff3(10,12)
[1] 23
# Hence I get the function that returns what I want (given the correct input)
# but the arguments will have default values that I don't want.
# I want to retain the original default values (if any)
ff1()
[1] 7
ff3()
numeric(0)

I have two questions:
1) How to fix the above?
2) Aren't there more elegant alternatives?

Any help would be appreciated.
Regards
Søren

```