[Rd] sprintf, check number of parameters

[Tomas Kalibera]

Dear Matthias,

On 2/6/21 2:11 PM, Matthias Gondan wrote:
> Dear developers,
>
> This is a follow-up from an earlier mail about warnings of unused arguments in sprintf:
>
> 1. This should obviously raise an error (and it does):
> sprintf('%i %i', 1)
> Fehler in sprintf("%i %i", 1) : zu wenig Argumente [= too few arguments]
>
> 2. This should, in my opinion, raise a warning about an unused argument (and I think it does in now R-devel):
> sprintf('%i', 1, 2)
yes, it does.
> 3. From the conversation below, it seems that this also raises a warning (in R-devel):
> sprintf('%1$i', 1, 2)
yes, it does as well
> I think that one should be suppressed. When I reported this a few months ago, I didn’t really have a use case for (3), but now I think I have found something. Suppose I have a function that calculates some descriptive statistics, mean, sd, available cases, missings, something like the one below:
>
> msnx = function(x, mask='%1$.1f (SD=%2$.1f, n=%3$i, NA=%4$i)')
> {
>    m = mean(x, na.rm=TRUE)
>    s = sd(x, na.rm=TRUE)
>    n = sum(!is.na(x))
>    na = sum(is.na(x))
>    
>    sprintf(mask, m, s, n, na)
> }
>
> The mask is meant to help formatting it a bit.
>
> msnx(T0)
> [1] "30.7 (SD=4.7, n=104, NA=0)"
>
> Now I want a „less detailed“ summary, so I invoke the function with something like
>
> msnx(T0, mask='%1$.1f (SD=%2$.1f)')
> [1] "30.7 (SD=4.7)"
>
> In my opinion, in the last example, sprintf should not raise the warning in (2) if all arguments in the mask are „dollared“. I am still a bit unsure since the example uses a function that calculate things that aren’t being used (n and na), and this could be considered bad programming style. But there might be other use cases, and it is, nevertheless, a deliberate choice to skip arguments 3$ and 4$.

Thanks for the example. I am sympathetic with your concerns about the 
programming style in it: the caller needs to know exactly how "mask" 
will be used, that it would be in a call to sprintf() and what would be 
the indices of the arguments.

The warning has been introduced a while ago and there has not been any 
report yet that it would break existing good style code (particularly 
CRAN packages have been tested extensively), which indicates that 
currently the R code base does not rely on unused $- arguments.

It is hence I think wise to keep the warning to prevent R code base from 
relying on that in the future, because gcc/clang already warn on unused 
$-arguments. Not only that gcc developers must have been thinking hard 
about the same thing before us getting to this conclusion: $- arguments 
are a POSIX extension and gcc/clang are the key compilers for POSIX 
systems, so it is safer to abide by their rules. In principle POSIX may 
mandate that $- arguments are used explicitly in the future (now it is 
rather vague, it seems unused are fine only when last), and even if not, 
deviations from gcc/clang could cause confusion for applications and 
developers using both C/C++ and R.

Best
Tomas



>
> Best wishes,
>
> Matthias
>
>
>
> Dear Matthias,
>
> thanks for the suggestion, R-devel now warns on unused arguments by
> format (both numbered and un-numbered). It seems that the new warning is
> useful, often it finds cases when arguments were accidentally passed to
> sprintf but had been meant for a different function.
>
> R allows combining both numbered and un-numbered references in a single
> format, even though it may be better to avoid and POSIX does not allow
> that.
>
> Best
> Tomas
>
> On 9/20/20 1:03 PM, Matthias Gondan wrote:
>> Dear R developers,
>>
>> I am wondering if this should raise an error or a warning.
>>
>>> sprintf('%.f, %.f', 1, 2, 3)
>> [1] "1, 2"
>>
>> I am aware that R has „numbered“ sprintf arguments (sprintf('%1$.f', …), and in that case, omissing of specific arguments may be intended. But in the usual syntax, omission of an argument is probably a mistake.
>>
>> Thank you for your consideration.
>>
>> Best wishes,
>>
>> Matthias
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