Using trace

Here is a partial solution:

trace(fact,quote({cat(sprintf("x= %i\n",x));return}),print=T)

[1] "fact"

fact(4)

Tracing fact(4) on entry
x= 4
Tracing fact(x - 1) on entry
x= 3
Tracing fact(x - 1) on entry
x= 2
Tracing fact(x - 1) on entry
x= 1
Tracing fact(x - 1) on entry
x= 0
[1] 24


2009/4/17 Stavros Macrakis <macrakis at alum.mit.edu>:

Well, yes, of course I could add the code to the function by hand. ?I
could also calculate square roots by hand. ?But -- as in every other
basic programming environment -- there exists an R function 'trace'
which appears to automate the process, and I can't figure out how to
use it to handle this most elementary and standard case. ?Clearly I'm
missing something.

? ? ? ? ? ? ?-s

On Thu, Apr 16, 2009 at 9:26 PM, ronggui <ronggui.huang at gmail.com> wrote:

Can you just print what you need to know? For example:

fact <- function(x) {

+ if(x<1) ans <- 1 else ans <- x*fact(x-1)
+ print(sys.call())
+ cat(sprintf("X is %i\n",x))
+ print(ans)
+ }

fact(4)

fact(x - 1)
X is 0
[1] 1
fact(x - 1)
X is 1
[1] 1
fact(x - 1)
X is 2
[1] 2
fact(x - 1)
X is 3
[1] 6
fact(4)
X is 4
[1] 24


2009/4/13 Stavros Macrakis <macrakis at alum.mit.edu>:

I would like to trace functions, displaying their arguments and return
value, but I haven't been able to figure out how to do this with the
'trace' function.

After some thrashing, I got as far as this:

? ?fact <- function(x) if(x<1) 1 else x*fact(x-1)
? ?tracefnc <- function() dput(as.list(parent.frame()), ?#
parent.frame() holds arg list
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?control=NULL)
? ?trace("fact",tracer=tracefnc,print=FALSE)

but I couldn't figure out how to access the return value of the
function in the 'exit' parameter. ?The above also doesn't work for
"..." arguments. ?(More subtly, it forces the evaluation of promises
even if they are otherwise unused -- but that is, I suppose, a weird
and obscure case.)

Surely someone has solved this already?

What I'm looking for is something very simple, along the lines of
old-fashioned Lisp trace:

defun fact (i) (if (< i 1) 1 (* i (fact (+ i -1)))))

FACT

(trace fact)

(FACT)

(fact 3)

?1> (FACT 3)
? ?2> (FACT 2)
? ? ?3> (FACT 1)
? ? ? ?4> (FACT 0)
? ? ? ?<4 (FACT 1)
? ? ?<3 (FACT 1)
? ?<2 (FACT 2)
?<1 (FACT 6)
6

Can someone help? Thanks,

? ? ? ? -s

--
HUANG Ronggui, Wincent
PhD Candidate
Dept of Public and Social Administration
City University of Hong Kong
Home page: http://asrr.r-forge.r-project.org/rghuang.html

--
HUANG Ronggui, Wincent
PhD Candidate
Dept of Public and Social Administration
City University of Hong Kong
Home page: http://asrr.r-forge.r-project.org/rghuang.html