Btukfyl

Can I create a function with the default value of an argument set to the value of a variable at the time of creation?

Something like,

a=1

fn = function(arg1 = a) {

  print (arg1+1)

}

fn would show

function(arg1 = 1) {

  print (arg1+1)

}

One way to do this is to use the global options in R:

fn <- function(arg1 = getOption("arg1", 1)) {

     print(arg1 + 1)

}



fn() # returns 2

options(arg1 = 5)

fn() # returns 6

fn(2) # returns 3

options(arg1 = NULL)

fn() # returns 2 again

I think the above solution is cleaner compared to using a global variable in .GlobalEnv, but here is also how you can do it with a global variable in .GlobalEnv:

fn2 <- function(arg1 = if( is.null(.GlobalEnv[["a"]]) ) 1 else .GlobalEnv[["a"]]) {

  print(arg1+1)

}



fn2() # this returns an empty vector

a <- 5

fn2() # this returns 6

Helped by help for bquote function. Here is what I found working for me.

It does not look straight forward to me but it works.

a=1

fn <- eval(bquote( function(arg1 = .(a)) {

    print (arg1+1)

} ))

fn

fn(3)

eval(bquote()) and .(a) are the point.

I found the hows but I don't think I fully understood it.
So anyone can help me understand how it works, I will be glad to take it as an answer.