R help - Jun 2012 - Making a Subset Assignment function ("[<-")

Greetings,
    I am trying to understand how to making a subset assignment function.
Take the example where of making a "last" generic function.

last <- function(x,...) UseMethod("last");
last.numeric <- function(x, ...) x[length(x)] 
last.list <- function(x, ...) x[[length(x)]] 
.....

Now we run into the problem at hand if we want to assign something to the
last item. Of course we could define a separate setter function, but I am
not interested in this problem specifically, but understanding the method
that would let me write the function to allow the following reasonable
behavior.

vec <- 1:5
last(vec)<-10
vec $ 10


Thank you for your time and expertise!

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On 28/06/2012 10:39, dj hawthorne wrote:
> Greetings,
>      I am trying to understand how to making a subset assignment function.
> Take the example where of making a "last" generic function.
>
> last <- function(x,...) UseMethod("last");
> last.numeric <- function(x, ...) x[length(x)]
> last.list <- function(x, ...) x[[length(x)]]
> .....
>
> Now we run into the problem at hand if we want to assign something to the
> last item. Of course we could define a separate setter function, but I am
> not interested in this problem specifically, but understanding the method
> that would let me write the function to allow the following reasonable
> behavior.
>
> vec <- 1:5
> last(vec)<-10
> vec $ 10
[Dunno what the last means: it is not valid R syntax.]

To do that you define a replacement function (not a 'setter function'). 
  Most of those in R are primitive, but here's an example:

`last<-` <- function(x, value) UseMethod("last<-")
`last<-.default` <- function(x, value) { x[length(x)] <- value; x }
`last<-.list` <- function(x, value) { x[l[ength(x)]] <- value; x }
> Thank you for your time and expertise!

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595