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List: r-devel
Subject: Re: [Rd] Missing argument vs. empty argument
From: Renaud Gaujoux <renaud () mancala ! cbio ! uct ! ac ! za>
Date: 2011-01-25 11:01:38
Message-ID: 4D3EAD92.5020108 () cbio ! uct ! ac ! za
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My purpose is indeed to write a '[' method.
I will go for the `[.data.frame` solution then.
Thank you.
On 25/01/2011 12:53, Prof Brian Ripley wrote:
> On Tue, 25 Jan 2011, Renaud Gaujoux wrote:
>
>> Hi,
>>
>> is there an easy, robust, and/or recommended way to distinguish a
>> missing argument from an empty argument as in:
>
> An empty argument is a missing argument when argument matching is
> done, e.g.
>
>> foo <- function(i,j) match.call()
>> foo(i)
> foo(i = i)
>> foo(i,)
> foo(i = i)
>> foo(,j)
> foo(j = j)
>
> It is rather against the spirit of R to use the actual call rather
> than the matched call. Unless you are doing this to write a '['
> method I would suggest you find a different convention, e.g.
> distinguish f(i) and f(i, NULL). For the exception, look at
> `[.data.frame`, which does use nargs().
>
> (NB: what I have said does not apply to primitives like '[' itself,
> which do not do standard argument matching.)
>
>
>>
>> foo <- function(i, j){
>> print(missing(j))
>> print(nargs())
>> }
>>
>> foo(i) # TRUE, 1
>> foo(i,) # TRUE, 2
>>
>> I know I can work around with nargs, the list of arguments and the
>> names of the passed arguments, but I wish there is something already
>> in place for this.
>> This is specially important for '['-like methods where x[i,] is not
>> the same as x[i].
>> What I am looking for is a function that tells me if an argument has
>> actually been passed empty:
>>
>> foo <- function(i, j, k){
>> print( empty.arg(j) )
>> print(nargs())
>> }
>>
>> would result in:
>>
>> foo(i) # FALSE, 1
>> foo(i, ) # TRUE, 2
>> foo(i, j) # FALSE, 2
>> foo(i, k=2) # FALSE, 2
>> foo(i, k=2, ) # TRUE, 3
>>
>> Thank you for any help or pointer.
>>
>> Bests,
>> Renaud
>>
>>
>>
>>
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>
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