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A general description is here.
sum() now has a method for "difftime" objects (via the Summary group).
max(), min() and range() applied to "difftime" object(s) now return a result in the common units of the objects, if there is one.
installed.packages() is now tolerant of malformed DESCRIPTION files in installed packages (it ignores them, with a warning).
R CMD build now also uses a Makevars file for cleaning up src/.
add1.glm() no longer warns if applied to a binomial glm with a factor response.
aggregate.data.frame() now does something sensible with a zero-column 'x' rather than failing with an obscure error message. It gives an explicit error message if called with a zero-row 'x'.
qt() now works for 0 < df < 1.
integrate() now accepts integer results from f(): as documented, but only double was accepted.
save() (optionally, but by default) checks for the existence of objects before opening the file/connections (wish of PR#12543).
str()'s default method gains a 'formatNum' function argument which is used for formatting numeric vectors. Note that this is very slightly not backward compatible, and that its default may change before release.
update.packages() gains an argument 'oldPkgs', where new.packages() and old.packages() get 'instPkgs'. These allow to consideration of only subsets of packages instead of all installed ones.
The default "pkgType" option for non-CRAN builds of R on Mac OS X is now correctly "source" as documented.
The windows() device now works around the total clipping of semi-transparent rectangles and circles that extend outside the device region.