R devel - Mar 2017 - A trap for young players with the lapply() function.

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.

Once one realises what is going on, it's all quite obvious, and usually
pretty easy to fix.  OTOH there are lots of functions around with second
or third arguments whose formal name is "X", and these can trip one up
until the penny drops.

This keeps happening to me, over and over again (with sufficiently long
intervals between occurrences so that my ageing memory forgets the 
previous occurrence).

Is there any way to trap/detect the use of an optional argument called 
"X" and thereby issue a more perspicuous error message?

This would be helpful to those users who, like myself, are bears of very 
little brain.

Failing that (it does look impossible) might it not be a good idea to 
add a warning to the help for lapply(), to the effect that if FUN has an 
optional argument named "X" then passing this argument via
"..." will
cause this argument to be taken as the first argument to lapply() and 
thereby induce an error?

cheers,

Rolf Turner

-- 
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

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 )

Best,

Chuck

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.

cheers,

Rolf

-- 
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276