[R] converting "call" objects into character

[Samuel Le]

Hi,

David and Douglas, thanks for the effort in helping me.
It seems that deparse(match.call()) is doing the trick.

I learned that the class "call" is not easy to handle in R.

Samuel

-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net]
Sent: 03 April 2011 18:56
To: Douglas Bates
Cc: Samuel Le; r-help at r-project.org
Subject: Re: [R] converting "call" objects into character


On Apr 3, 2011, at 1:22 PM, Douglas Bates wrote:

> On Sun, Apr 3, 2011 at 11:42 AM, David Winsemius <dwinsemius at comcast.net
> > wrote:
>>
>> On Apr 3, 2011, at 12:14 PM, Samuel Le wrote:
>>
>>> Dear all,
>>>
>>>
>>>
>>> I would like to log the calls to my functions. I am trying to do
>>> this
>>> using the function match.call():
>>
>> fTest<-function(x)
>>
>> {  theCall<-match.call()
>>      print(theCall)
>>      return(list(x=x, logf = theCall))
>> }
>>
>>>
>>> fTest(x=2)$x
>> [1] 2
>>> fTest(x=2)$logf
>> fTest(x = 2)
>>> str(fTest(x=2)$logf)
>>  language fTest(x = 2)
>>
>> You may want to convert that  call component to a character object,
>> since:
>>
>>> cat(fTest(x=2)$logf)
>> Error in cat(list(...), file, sep, fill, labels, append) :
>>  argument 1 (type 'language') cannot be handled by 'cat'
>
> If you want to examine a call object you need to ensure that it is not
> evaluated.  Evaluating a number or a character string is not a problem
> because
>
> eval(4)
>
> is the same as
>
> 4
>
> However, evaluating a function call should be different from the call
> itself.  As David shows, the str function is careful not to evaluate
> the call object.  (Martin and I found ourselves going around in
> circles when looking at the structure of a fitted model object that
> included a call and he kindly changed the behavior of str().)
>
> So you need to decide when a function, such as print(), evaluates its
> arguments or when it doesn't, which can get kind of complicated.  An
> alternative is to use match.call() repeatedly instead of trying to
> save the value, as in
>
>> fTest
> function(x) {
>    print(match.call())
>    list(x=x, logf = match.call())
> }
>> fTest(x=2)
> fTest(x = 2)
> $x
> [1] 2
>
> $logf
> fTest(x = 2)
>
> The trick there is that the value of match.call() is the unevaluated
> call whereas
>
> myCall <- match.call()
> print(myCall)
>
> evaluates myCall in the call to print, thereby evaluating the function
> fTest again.
>
> Is this sufficiently confusing?  :-)

Yes, I am now sufficiently confused^W , ... er, motivated to look for
another route. I think the way out of the confusion is to turn the
call into text and since as.character doesn't do a very neat a job, I
would suggest instead: deparse()

 > fTest <- function(x) {
+    print(match.call())
+    list(x=x, logf = deparse(match.call()))
+ }
 > fTest(x=3)$logf
fTest(x = 3)
[1] "fTest(x = 3)"
 > cat(fTest(x=3)$logf)
fTest(x = 3)
fTest(x = 3)

cat() is a convenient test of the capacity of an object to be written
to a file. It has an append parameter that implies it could serve the
logging function requested by the OP.


>>>
>>> I can see "theCall" printed into the console, but I don't manage to
>>> convert it into a character to write it into a log file with other
>>> informations.
>>>
>>> Can anyone help?


David Winsemius, MD
West Hartford, CT



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