Finding the Source

The thing that makes R different from commercial packages like SAS or Excel is that it's open-source. This means that if you ever wonder exactly how R calculates a certain value, or you want to adapt an existing R routine for your own purposes, you can always find the source code R uses and take a look yourself.

Much of R is written in the R programming language itself: if you want to see how a function is R is defined, all it usually takes is to type the name of the function at the command line.  For example, to see how the sd (standard deviation) function in R works, just type its name:

> sd
function (x, na.rm = FALSE) 
{
    if (is.matrix(x)) 
        apply(x, 2, sd, na.rm = na.rm)
    else if (is.vector(x)) 
        sqrt(var(x, na.rm = na.rm))
    else if (is.data.frame(x)) 
        sapply(x, sd, na.rm = na.rm)
    else sqrt(var(as.vector(x), na.rm = na.rm))
}

This works because in R, functions are just like any variable like a matrix or vector, in the sense that they have a value. (In the usual parlance, functions are "first-class objects" in R.) The value displayed is the definition of the function, in the R language.

There are some wrinkles in finding the source code for some functions: some are in other packages or namespaces, and some are implemented in the core R sources in C or Fortran.  You can find detailed instructions on finding the source code in these cases in a 2006 R-News article by Uwe Ligges: Accessing the Sources (see pages 44-45).  But in general, the source is always there for you to look at if you need it.  

Comments

[Luis]

While I wholeheartedly agree with the concept, the example is not very good. The only thing we learn from that function call is that if we use sd R calls a function (written in c or fortran) called sd, so we are back at the starting point.

[wcw]

Well, no, it mostly calls 'var' -- which is where you hit the .Internal call. If it hits a matrix or a data frame, it calls itself in a way that then calls 'var'.

Still true, mind you, that 'sd' calls 'var' which makes an .Internal call. I think these are mostly coded in C, but I really neither know nor want to know.