How can I view the source code for a function?

Question

I want to look at the source code for a function to see how it works. I know I can print a function by typing its name at the prompt:

> t
function (x) 
UseMethod("t")
<bytecode: 0x2332948>
<environment: namespace:base>

In this case, what does UseMethod("t") mean? How do I find the source code that's actually being used by, for example: t(1:10)?

Is there a difference between when I see UseMethod and when I see standardGeneric and showMethods, as with with?

> with
standardGeneric for "with" defined from package "base"

function (data, expr, ...) 
standardGeneric("with")
<bytecode: 0x102fb3fc0>
<environment: 0x102fab988>
Methods may be defined for arguments: data
Use  showMethods("with")  for currently available ones.

In other cases, I can see that R functions are being called, but I can't find the source code for those functions.

> ts.union
function (..., dframe = FALSE) 
.cbind.ts(list(...), .makeNamesTs(...), dframe = dframe, union = TRUE)
<bytecode: 0x36fbf88>
<environment: namespace:stats>
> .cbindts
Error: object '.cbindts' not found
> .makeNamesTs
Error: object '.makeNamesTs' not found

How do I find functions like .cbindts and .makeNamesTs?

In still other cases, there's a bit of R code, but most of work seems to be done somewhere else.

> matrix
function (data = NA, nrow = 1, ncol = 1, byrow = FALSE, dimnames = NULL) 
{
    if (is.object(data) || !is.atomic(data)) 
        data <- as.vector(data)
    .Internal(matrix(data, nrow, ncol, byrow, dimnames, missing(nrow), 
        missing(ncol)))
}
<bytecode: 0x134bd10>
<environment: namespace:base>
> .Internal
function (call)  .Primitive(".Internal")
> .Primitive
function (name)  .Primitive(".Primitive")

How do I find out what the .Primitive function does? Similarly, some functions call .C, .Call, .Fortran, .External, or .Internal. How can I find the source code for those?

Answer 1

UseMethod("t") is telling you that t() is a (S3) generic function that has methods for different object classes.

The S3 method dispatch system

For S3 classes, you can use the methods function to list the methods for a particular generic function or class.

> methods(t)
[1] t.data.frame t.default    t.ts*       

   Non-visible functions are asterisked
> methods(class="ts")
 [1] aggregate.ts     as.data.frame.ts cbind.ts*        cycle.ts*       
 [5] diffinv.ts*      diff.ts          kernapply.ts*    lines.ts        
 [9] monthplot.ts*    na.omit.ts*      Ops.ts*          plot.ts         
[13] print.ts         time.ts*         [<-.ts*          [.ts*           
[17] t.ts*            window<-.ts*     window.ts*      

   Non-visible functions are asterisked

"Non-visible functions are asterisked" means the function is not exported from its package's namespace. You can still view its source code via the ::: function (i.e. stats:::t.ts), or by using getAnywhere(). getAnywhere() is useful because you don't have to know which package the function came from.

> getAnywhere(t.ts)
A single object matching ‘t.ts’ was found
It was found in the following places
  registered S3 method for t from namespace stats
  namespace:stats
with value

function (x) 
{
    cl <- oldClass(x)
    other <- !(cl %in% c("ts", "mts"))
    class(x) <- if (any(other)) 
        cl[other]
    attr(x, "tsp") <- NULL
    t(x)
}
<bytecode: 0x294e410>
<environment: namespace:stats>

The S4 method dispatch system

The S4 system is a newer method dispatch system and is an alternative to the S3 system. Here is an example of an S4 function:

> library(Matrix)
Loading required package: lattice
> chol2inv
standardGeneric for "chol2inv" defined from package "base"

function (x, ...) 
standardGeneric("chol2inv")
<bytecode: 0x000000000eafd790>
<environment: 0x000000000eb06f10>
Methods may be defined for arguments: x
Use  showMethods("chol2inv")  for currently available ones.

The output already offers a lot of information. standardGeneric is an indicator of an S4 function. The method to see defined S4 methods is offered helpfully:

> showMethods(chol2inv)
Function: chol2inv (package base)
x="ANY"
x="CHMfactor"
x="denseMatrix"
x="diagonalMatrix"
x="dtrMatrix"
x="sparseMatrix"

getMethod can be used to see the source code of one of the methods:

> getMethod("chol2inv", "diagonalMatrix")
Method Definition:

function (x, ...) 
{
    chk.s(...)
    tcrossprod(solve(x))
}
<bytecode: 0x000000000ea2cc70>
<environment: namespace:Matrix>

Signatures:
        x               
target  "diagonalMatrix"
defined "diagonalMatrix"

There are also methods with more complex signatures for each method, for example

require(raster)
showMethods(extract)
Function: extract (package raster)
x="Raster", y="data.frame"
x="Raster", y="Extent"
x="Raster", y="matrix"
x="Raster", y="SpatialLines"
x="Raster", y="SpatialPoints"
x="Raster", y="SpatialPolygons"
x="Raster", y="vector"

To see the source code for one of these methods the entire signature must be supplied, e.g.

getMethod("extract" , signature = c( x = "Raster" , y = "SpatialPolygons") )

It will not suffice to supply the partial signature

getMethod("extract",signature="SpatialPolygons")
#Error in getMethod("extract", signature = "SpatialPolygons") : 
#  No method found for function "extract" and signature SpatialPolygons

Functions that call unexported functions

In the case of ts.union, .cbindts and .makeNamesTs are unexported functions from the stats namespace. You can view the source code of unexported functions by using the ::: operator or getAnywhere.

> stats:::.makeNamesTs
function (...) 
{
    l <- as.list(substitute(list(...)))[-1L]
    nm <- names(l)
    fixup <- if (is.null(nm)) 
        seq_along(l)
    else nm == ""
    dep <- sapply(l[fixup], function(x) deparse(x)[1L])
    if (is.null(nm)) 
        return(dep)
    if (any(fixup)) 
        nm[fixup] <- dep
    nm
}
<bytecode: 0x38140d0>
<environment: namespace:stats>

Functions that call compiled code

Note that "compiled" does not refer to byte-compiled R code as created by the compiler package. The <bytecode: 0x294e410> line in the above output indicates that the function is byte-compiled, and you can still view the source from the R command line.

Functions that call .C, .Call, .Fortran, .External, .Internal, or .Primitive are calling entry points in compiled code, so you will have to look at sources of the compiled code if you want to fully understand the function. This GitHub mirror of the R source code is a decent place to start. The function pryr::show_c_source can be a useful tool as it will take you directly to a GitHub page for .Internal and .Primitive calls. Packages may use .C, .Call, .Fortran, and .External; but not .Internal or .Primitive, because these are used to call functions built into the R interpreter.

Calls to some of the above functions may use an object instead of a character string to reference the compiled function. In those cases, the object is of class "NativeSymbolInfo", "RegisteredNativeSymbol", or "NativeSymbol"; and printing the object yields useful information. For example, optim calls .External2(C_optimhess, res$par, fn1, gr1, con) (note that's C_optimhess, not "C_optimhess"). optim is in the stats package, so you can type stats:::C_optimhess to see information about the compiled function being called.

Compiled code in a package

If you want to view compiled code in a package, you will need to download/unpack the package source. The installed binaries are not sufficient. A package's source code is available from the same CRAN (or CRAN compatible) repository that the package was originally installed from. The download.packages() function can get the package source for you.

download.packages(pkgs = "Matrix", 
                  destdir = ".",
                  type = "source")

This will download the source version of the Matrix package and save the corresponding .tar.gz file in the current directory. Source code for compiled functions can be found in the src directory of the uncompressed and untared file. The uncompressing and untaring step can be done outside of R, or from within R using the untar() function. It is possible to combine the download and expansion step into a single call (note that only one package at a time can be downloaded and unpacked in this way):

untar(download.packages(pkgs = "Matrix",
                        destdir = ".",
                        type = "source")[,2])

Alternatively, if the package development is hosted publicly (e.g. via GitHub, R-Forge, or RForge.net), you can probably browse the source code online.

Compiled code in a base package

Certain packages are considered "base" packages. These packages ship with R and their version is locked to the version of R. Examples include base, compiler, stats, and utils. As such, they are not available as separate downloadable packages on CRAN as described above. Rather, they are part of the R source tree in individual package directories under /src/library/. How to access the R source is described in the next section.

Compiled code built into the R interpreter

If you want to view the code built-in to the R interpreter, you will need to download/unpack the R sources; or you can view the sources online via the R Subversion repository or Winston Chang's github mirror.

Uwe Ligges's R news article (PDF) (p. 43) is a good general reference of how to view the source code for .Internal and .Primitive functions. The basic steps are to first look for the function name in src/main/names.c and then search for the "C-entry" name in the files in src/main/*.