[Rd] A trap for young players with the lapply() function.

Rolf Turner r.turner at auckland.ac.nz
Tue Mar 28 23:30:17 CEST 2017

On 28/03/17 15:26, Charles C. Berry wrote:
> On Mon, 27 Mar 2017, Rolf Turner wrote:
>> From time to time I get myself into a state of bewilderment when using
>> apply() by calling it with FUN equal to a function which has an
>> "optional" argument named "X".
>> E.g.
>>    xxx <- lapply(y,function(x,X){cos(x*X)},X=2*pi)
>> which produces the error message
>>> Error in get(as.character(FUN), mode = "function", envir = envir) :
>>>   object 'y' of mode 'function' was not found
>> This of course happens because the name of the first argument of
>> lapply() is "X" and so it takes the value of this first argument to be
>> the supplied X (2*pi in the foregoing example) and then expects what
>> the user has denoted by "y" to be the value of FUN, and (obviously!)
>> it isn't.
> The lapply help page addresses this issue in `Details' :
> "it is good practice to name the first two arguments X and FUN if ... is
> passed through: this both avoids partial matching to FUN and ensures
> that a sensible error message is given if arguments named X or FUN are
> passed through ..."
> So that advice suggests something like:
> xxx <- lapply( X=y, FUN=function(X,x){cos(X*x)}, x=2*pi )

That is of course very sound advice, but it pre-supposes that the user 
is *aware* that there is a pitfall to be avoided.  I was hoping for 
something that would protect dweebs like myself from the pitfall given 
that we are too obtuse to be cognizant of its existence.

I think that the suggestion I made, in response to a posting by Barry 
Rowlingson, that the first argument of lapply() be given the name of 
".X" rather than just-plain-X, would be (a) effective, and (b) harmless.



Technical Editor ANZJS
Department of Statistics
University of Auckland
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