Recursive functions and global vs. local variables

Question

I'm writing a recursive function in R, and I want it to modify a global variable such that I know how many instances of the function have been called. I don't understand why the following doesn't work:

i <- 1

testfun <- function( depth= 0 ) {

  i <- i + 1
  cat( sprintf( "i= %d, depth= %d\n", i, depth ) )
  if( depth < 10 ) testfun( depth + 1 )
}

Here is the output:

i= 2, depth= 0
i= 2, depth= 1
i= 2, depth= 2
i= 2, depth= 3
i= 2, depth= 4
i= 2, depth= 5
i= 2, depth= 6
i= 2, depth= 7
i= 2, depth= 8
i= 2, depth= 9
i= 2, depth= 10

Here is the expected output:

i=2, depth= 0
i=3, depth= 1
i=4, depth= 2
i=5, depth= 3
i=6, depth= 4
i=7, depth= 5
i=8, depth= 6
i=9, depth= 7
i=10, depth= 8
i=11, depth= 9
i=12, depth= 10

Answer 1

You can use the local function to do the same thing but without modifying the global environment:

testfun <- local({
  i <- 1
  function( depth= 0 ) {
    i <<- i + 1
    cat( sprintf( "i= %d, depth= %d\n", i, depth ) )
    if( depth < 10 ) testfun( depth + 1 )
  }
})

This very neatly wraps the testfun function in a local environment which holds i. This method should be acceptable in packages submitted CRAN, whereas modifying the global environment is not.

Answer 2

OK, so I'm not very bright. Here is the answer:

i <<- i + 1