[R] NULL or NA for missing function arguments?
Hi, I am troubled by the use of NULL or NA to indicate missing/non-specified function arguments. In the R code that I have looked at, it seems that both forms are used (NULL seems to be used more often though). Sometimes both variants are in the same declaration, e.g. format.default - function(x, trim = FALSE, digits = NULL, nsmall = 0, justify = c(left, right, centre, none), width = NULL, na.encode = TRUE, scientific = NA, big.mark = , big.interval = 3, small.mark = , small.interval = 5, decimal.mark = ., zero.print = NULL, ...) Is there a right way? And if both forms are used, how do I know which one is right? Thanks a lot and best regards, Hans-Peter __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] NULL or NA for missing function arguments?
There is also a third way, namely use the missing function in the code: f - function(x) if (missing(x)) print(missing) else print(x) f() On 10/16/06, Hans-Peter [EMAIL PROTECTED] wrote: Hi, I am troubled by the use of NULL or NA to indicate missing/non-specified function arguments. In the R code that I have looked at, it seems that both forms are used (NULL seems to be used more often though). Sometimes both variants are in the same declaration, e.g. format.default - function(x, trim = FALSE, digits = NULL, nsmall = 0, justify = c(left, right, centre, none), width = NULL, na.encode = TRUE, scientific = NA, big.mark = , big.interval = 3, small.mark = , small.interval = 5, decimal.mark = ., zero.print = NULL, ...) Is there a right way? And if both forms are used, how do I know which one is right? Thanks a lot and best regards, Hans-Peter __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] NULL or NA for missing function arguments?
On 10/16/2006 8:47 AM, Hans-Peter wrote: Hi, I am troubled by the use of NULL or NA to indicate missing/non-specified function arguments. In the R code that I have looked at, it seems that both forms are used (NULL seems to be used more often though). Sometimes both variants are in the same declaration, e.g. format.default - function(x, trim = FALSE, digits = NULL, nsmall = 0, justify = c(left, right, centre, none), width = NULL, na.encode = TRUE, scientific = NA, big.mark = , big.interval = 3, small.mark = , small.interval = 5, decimal.mark = ., zero.print = NULL, ...) Is there a right way? And if both forms are used, how do I know which one is right? As Gabor said, the third way is to give no default, but test missing() in the code. There are differences between the options, but I don't think there's a single right way. Some differences: If you want to allow a vector of parameters, some of which are missing and some are not, then you'd probably want NA, so that something like c(1,2,NA) was possible. The length of NA is 1, but the length of NULL is 0, so it would be harder to expand NULL to the same length as x. Taking the length of a truly missing parameter, or trying to change it, will trigger an error. That is, rep(param, length(x)) will work for NA, but not the others. It's also convenient to declare an error if length(scientific) != 1. Using NULL or NA is a little clearer to a user who just takes a quick look at the function header, rather than carefully reading the man page, to find what parameters are needed. NA is logical, NULL is NULL. So in format.default, there could be a test is.logical(scientific) which will default to TRUE. So generally my advice would be: - Be consistent with similar existing functions. - Choose what you think will be convenient in current and predicted future versions of your function. Duncan Murdoch __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] NULL or NA for missing function arguments?
2006/10/16, Duncan Murdoch [EMAIL PROTECTED]: As Gabor said, the third way is to give no default, but test missing() in the code. I forgot this one, thank you. In my case it is probably not suited as I just pass the arguments to a C (Pascal) function and do the checking there. [explanations snipped] So generally my advice would be: - Be consistent with similar existing functions. - Choose what you think will be convenient in current and predicted future versions of your function. Ok, thank you. - Until now I always used NA but will (apart from your advice) prefer NULL from now on. In C (Pascal) code NULL is also easier to check than NA* Thanks again and best regards, Hans-Peter Suter -- * function riIsNull( _s: pSExp ): aRBoolean; cdecl; vs. function IsNaScalar( _x: pSExp ): boolean; begin result:= (riLength( _x ) = 1) and (riTypeOf( _x ) in [setLglSxp, setRealSxp]) and (rIsNa( riReal( riCoerceVector( _x, setRealSxp ) )[0] ) 0); end; __ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] NULL or NA for missing function arguments?
On 10/16/06, Hans-Peter [EMAIL PROTECTED] wrote:
2006/10/16, Duncan Murdoch [EMAIL PROTECTED]:
As Gabor said, the third way is to give no default, but test missing()
in the code.
I forgot this one, thank you. In my case it is probably not suited as
I just pass the arguments to a C (Pascal) function and do the checking
there.
The R interface need not be identical to the C or Pascal interface.
Its pretty easy to convert making use of the fact that a nonexistent
else leg returns NULL:
f - function(x) { x - if (!missing(x)) x; x }
f() # NULL
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Re: [R] NULL or NA for missing function arguments?
On 10/16/2006 5:06 PM, Brahm, David wrote:
Hans-Peter [EMAIL PROTECTED] wrote:
I am troubled by the use of NULL or NA to indicate
missing/non-specified function arguments.
I suggest using NULL for arguments which are vectors or lists of
unspecified length, and NA for scalars (arguments whose length
should always be one, such as na.rm). I rarely use missing arguments,
as they are harder to pass down to other functions.
To be a little more precise: they are easy to pass down, but hard to do
anything else with. For example:
f - function(x) if (missing(x)) print(missing!) else print(x)
g - function(x) f(x)
h - function(x) { y - x; f(y) }
g()
[1] missing!
h()
Error in h() : argument x is missing, with no default
Duncan Murdoch
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