Group elements of a vector-like object into a list-like object
split are 2 common ways of grouping the elements
of a vector-like object into a list-like object. The
that operate on a Vector object and return a List object.
Note that the
split methods defined in
splitAsList function defined in
split both impose restrictions on
the kind of grouping that they support (e.g. every element in the input
object needs to go in a group and can only go in one group), the
extractList generic function
for performing arbitrary groupings.
## relist() ## --------
## S3 method for class 'ANY,List': relist(flesh, skeleton)
## S3 method for class 'Vector,list': relist(flesh, skeleton)
## splitAsList() ## -------------
splitAsList(x, f, drop=FALSE, ...)
## extractList() ## -------------
## regroup() ## ---------
- flesh, x
- A vector-like object.
- A list-like object. Only the "shape" (i.e. element lengths) of
skeletonmatters. Its exact content is ignored.
- An atomic vector or a factor (possibly in Rle form).
- Logical indicating if levels that do not occur should be dropped (if
fis a factor).
- A list-like object. Unlike for
skeleton, the content here matters (see Details section below). Note that
ican be a Ranges object (a particular type of list-like object), and, in that case,
extractListis particularly fast (this is a common use case).
Groupingor an object coercible to one. For
ggroups the elements of
- Arguments to pass to methods.
extractList have in common that
they return a list-like object where each list element has the same class
as the original vector-like object. Thus they need to be able to select
the appropriate List concrete subclass to use for this returned
value. This selection is performed by
and is based only on the class of the original object.
extractList(x, i) is equivalent to:
An exception is made when
x is a data-frame-like object. In that
x is subsetted along the rows, that is,
is equivalent to:
relist(x[unlist(i), ], i)
This is more or less how the default method is implemented, except for
some optimizations when
i is a Ranges object.
splitAsList) can be seen as
special cases of
relist(flesh, skeleton) is equivalent to
split(x, f) is equivalent to
extractList(x, split(seq_along(f), f))
It is good practise to use
extractList only for cases not covered
split. Whenever possible, using
split is preferred as they will always perform more efficiently.
In addition their names carry meaning and are familiar to most R
users/developers so they'll make your code easier to read/understand.
Note that the transformation performed by
is always reversible (via
but not the transformation performed by
extractList (in general).
regroup function splits the elements of
into a list according to the grouping
g. Each element of
unlist(x) inherits its group from its parent element of
regroup is different from
x is already grouped, and the goal is to
relistmethods behave like
utils::relistexcept that they return a List object. If
skeletonhas names, then they are propagated to the returned value.
base::splitexcept that the former returns a List object instead of an ordinary list.
extractListreturns a list-like object parallel to
iand with the same "shape" as
i(i.e. same element lengths). If
ihas names, then they are propagated to the returned value.
All these functions return a list-like object where the list elements have the same class as
relistToClassgives the exact class of the returned object.
## On an Rle object: x <- Rle(101:105, 6:2) i <- IRanges(6:10, 16:12, names=letters[1:5]) extractList(x, i) ## On a DataFrame object: df <- DataFrame(X=x, Y=LETTERS[1:20]) extractList(df, i)