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A general description is here.
R now uses a ‘stringsAsFactors = FALSE’ default, and hence by default no longer converts strings to factors in calls to ‘data.frame()’ and ‘read.table()’.
A large number of packages relied on the previous behaviour and so have needed/will need updating.
For ‘head(x, n)’ and ‘tail()’ the default and other S3 methods notably for _vector_ ‘n’, e.g. to get a “corner” of a matrix, has been extended to ‘array’'s of higher dimension thanks to the patch proposal by Gabe Becker in PR#17652. Consequently, optional argument ‘addrownums’ is deprecated and replaced by the (more general) argument ‘keepnums’. An invalid second argument ‘n’ now leads to typically more easily readable error messages.
Where they are available, ‘writeBin()’ allows long vectors.