tidyr 0.4.0

Nested data frames

nest() and unnest() have been overhauled to support a useful way of structuring data frames: the nested data frame. In a grouped data frame, you have one row per observation, and additional metadata define the groups. In a nested data frame, you have one row per group, and the individual observations are stored in a column that is a list of data frames. This is a useful structure when you have lists of other objects (like models) with one element per group.

• nest() now produces a single list of data frames called "data" rather
  than a list column for each variable. Nesting variables are not included
  in nested data frames. It also works with grouped data frames made
  by dplyr::group_by(). You can override the default column name with .key.
• unnest() gains a .drop argument which controls what happens to
  other list columns. By default, they're kept if the output doesn't require
  row duplication; otherwise they're dropped.
• unnest() now has mutate() semantics for ... - this allows you to
  unnest transformed columns more easily. (Previously it used select semantics).

Expanding

• expand() once again allows you to evaluate arbitrary expressions like
  full_seq(year). If you were previously using c() to created nested
  combinations, you'll now need to use nesting() (#85, #121).
• nesting() and crossing() allow you to create nested and crossed data
  frames from individual vectors. crossing() is similar to
  base::expand.grid()
• full_seq(x, period) creates the full sequence of values from min(x) to
  max(x) every period values.

Minor bug fixes and improvements

• fill() fills in NULLs in list-columns.
• fill() gains a direction argument so that it can fill either upwards or
  downwards (#114).
• gather() now stores the key column as character, by default. To revert to
  the previous behaviour of using a factor (which allows you to preserve the
  ordering of the columns), use key_factor = TRUE (#96).
• All tidyr verbs do the right thing for grouped data frames created by
  group_by() (#122, #129, #81).
• seq_range() has been removed. It was never used or announced.
• spread() once again creates columns of mixed type when convert = TRUE
  (#118, @jennybc). spread() with drop = FALSE handles zero-length
  factors (#56). spread()ing a data frame with only key and value columns
  creates a one row output (#41).
• unite() now removes old columns before adding new (#89, @krlmlr).
• separate() now warns if defunct ... argument is used (#151, @krlmlr).