[R] Elementary sapply question

[Tony Plate]

At Monday 12:57 PM 6/21/2004, Ajay Shah wrote:
>[...snip...]
>I am aware of the "..." in sapply(). I am unable to understand how
>sapply will know where to utilise the x[i] values: as the 1st arg or
>the 2nd arg for f(x, y)?
>
>That is, when I say:
>
>      sapply(x, f, 3)
>
>how does sapply know that I mean:
>
>     for (i in 3:5) {
>         f(i, 3)
>     }
>
>and not
>
>     for (i in 3:5) {
>         f(3, i)
>     }
>
>How would we force sapply to use one or the other interpretation?

All the functions in the apply() family construct the call by just 
appending the additional arguments after the first.  If you supply argument 
names for the additional arguments, those will be supplied to the function 
called.  This can be used to force different interpretations of 
arguments.  E.g:

 > sapply(3:5, function(x, y) {return(y)}, 1)
[1] 1 1 1
 > sapply(3:5, function(x, y) {return(y)}, y=1)
[1] 1 1 1
 > sapply(3:5, function(x, y) {return(y)}, x=1)
[1] 3 4 5
 > sapply(3:5, function(x, y) {return(y)}, z=1)
Error in FUN(X[[as.integer(1)]], ...) : unused argument(s) (z ...)
 >

In the third example, the actual set of arguments in the call to the 
anonymous function is something like (3, x=1), so the standard argument 
interpretation rules result in the arguments having the values y=3, x=1.

hope this help,

Tony Plate




>Thanks,
>
>         -ans.
>
>--
>Ajay Shah                                                   Consultant
>ajayshah at mayin.org                      Department of Economic Affairs
>http://www.mayin.org/ajayshah           Ministry of Finance, New Delhi
>
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