[R] assignments to function return values

[Duncan Murdoch]

On 18/07/2009 1:08 PM, Barry Rowlingson wrote:
> On Sat, Jul 18, 2009 at 5:47 PM, Bernd<prof7bit at googlemail.com> wrote:
>> colnames(m) = c("a", "b")
>>
>> I am fairly new to R and trying to understand this language. Having
>> learned quite a few other programming languages the above statement
>> when i saw it first immediately led to two reactions:
>>
>> (1) wtf?
>>
>> (2) maybe they made the function return an object with overloaded
>> assignment operator, lets try this:
>>
>> x = colnames(m)
>> x = c("c","d")
>>
>> but this doesnt work this way and obviously there must be some other
>> mechanisms at work here. I opened the r-intro document
>> http://cran.r-project.org/doc/manuals/R-intro.html and read through
>> chapter 10 (Writing your own functions) but i didnt see one single
>> mention of this strange syntax, yet it is used in some example code
>> here (10.6.2) but the reader is left with a big question mark in his
>> mind and no single word mentions this outstanding unusualness while
>> all other aspects of functions that are pretty common with most other
>> languages in existance are dealt with in great depth.
>>
>> I then searched for some hints in the description of the assignment
>> operator and the asign() function but didnt find anything there also.
>>
>> In my oppinion the R documentation could be greatly improved by giving
>> each chapter a list of links to all other chapters of the
>> documentation needed to understand the example code of this chapter
>> (all things that are not immediately obvious to someone coming from a
>> more mainstream language) and thus making each chapter a self
>> contained possible entry point into the complete documentation. This
>> would of course lead to some redundance for the documentation as a
>> whole but it would IMHO greatly improve the learning experience.
>>
>> And here comes my question: How can the above mentioned syntax be
>> explained, how can i define such functions with can work in this
>> strange inverse manner, where should i have searched to find the
>> answer myself? Is there a document like "R for programmers" which
>> completely leaves out all the basic stuff and concentrates on the
>> obvious differences to most common mainstream languages only?
>>
> 
>  It's a sprinkle of syntactic sugar:
> 
>  colnames(x)=c("a","b")
> 
> is the same as:
> 
>  x = "colnames<-"(x,c("a","b"))
> 
> So there is a function called "colnames<-". See it by typing:
> 
> get("colnames<-")
> 
> and you'll see it has args x and value, and returns the modified x.
> 
> You can write your own foo<- functions and they operate like that.
> 
>  This is probably documented somewhere...

In the R Language Definition (which is sort of "R for Programmers"), but 
you need to search for "replacement function" to find it.

Duncan Murdoch