detecting entry into a recursive function

On 02/17/2013 12:55 PM, Benjamin Tyner wrote:

Given a function that calls itself, what's the best way to detect the
entry point? The best I came up with is:

IsEntryPoint <- function(){

par <- sys.call(-1L)[[1]]
grandpar <- sys.call(-2L)[[1]]

!identical(par, grandpar)
}

but this won't work for functions that don't directly call themselves;
for example, in this one the paste gets inserted in the call stack, so i
is always TRUE.

f <- function(d){

i <- IsEntryPoint()

if(d > 1L) paste(d, f(d-1L))
}

Any ideas?

Hi Ben,
There are a number of ways to do this, from simply having a flag that 
has one value at the initial call to the function and another value 
when the function calls itself. for an example of this, see the code 
for the "sizetree" function in the plotrix package, in particular the 
"firstcall" argument.

Jim