How to find out which package version is loaded in R?

You can use utils::packageVersion to see what version of a package is installed:

> packageVersion("snow")
[1] ‘0.3.9’

Note that

A package will not be ‘found’ unless it has a DESCRIPTION file which contains a valid Version field. Different warnings are given when no package directory is found and when there is a suitable directory but no valid DESCRIPTION file.

Although it sounds like you want to see what version of R you are running, in which case @Justin's sessionInfo suggestion is the way to go.

To check the version of R execute : R --version

Or after you are in the R shell print the contents of version$version.string

EDIT

To check the version of installed packages do the following.

After loading the library, you can execute sessionInfo ()

But to know the list of all installed packages:

packinfo <- installed.packages(fields = c("Package", "Version"))
packinfo[,c("Package", "Version")]

OR to extract a specific library version, once you have extracted the information using the installed.package function as above just use the name of the package in the first dimension of the matrix.

packinfo["RANN",c("Package", "Version")]
packinfo["graphics",c("Package", "Version")]

The above will print the versions of the RANN library and the graphics library.

You can use sessionInfo() to accomplish that.

> sessionInfo()
R version 2.15.0 (2012-03-30)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8    LC_PAPER=C                 LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] graphics  grDevices utils     datasets  stats     grid      methods   base     

other attached packages:
[1] ggplot2_0.9.0  reshape2_1.2.1 plyr_1.7.1    

loaded via a namespace (and not attached):
 [1] colorspace_1.1-1   dichromat_1.2-4    digest_0.5.2       MASS_7.3-18        memoise_0.1        munsell_0.3       
 [7] proto_0.3-9.2      RColorBrewer_1.0-5 scales_0.2.0       stringr_0.6       
> 

However, as per comments and the answer below, there are better options

> packageVersion("snow")

[1] ‘0.3.9’

Or:

"Rmpi" %in% loadedNamespaces()

Technically speaking, all of the answers at this time are wrong. packageVersion does not return the version of the loaded package. It goes to the disk, and fetches the package version from there.

This will not make a difference in most cases, but sometimes it does. As far as I can tell, the only way to get the version of a loaded package is the rather hackish:

asNamespace(pkg)$`.__NAMESPACE__.`$spec[["version"]]

where pkg is the package name.

EDIT: I am not sure when this function was added, but you can also use getNamespaceVersion, this is cleaner:

getNamespaceVersion(pkg)