| extractList {IRanges} | R Documentation |
relist and split are 2 common ways of grouping the elements
of a vector-like object into a list-like object. The IRanges and
S4Vectors packages define relist and split methods
that operate on a Vector object and return a List object.
Note that the split methods defined in
S4Vectors delegate to the splitAsList function defined in
IRanges and documented below.
Because relist and 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
IRanges package introduces the extractList generic function
for performing arbitrary groupings.
## relist() ## -------- ## S4 method for signature 'ANY,List' relist(flesh, skeleton) ## S4 method for signature 'Vector,list' relist(flesh, skeleton) ## splitAsList() ## ------------- splitAsList(x, f, drop=FALSE, ...) ## extractList() ## ------------- extractList(x, i) ## regroup() ## --------- regroup(x, g)
flesh, x |
A vector-like object. |
skeleton |
A list-like object. Only the "shape" (i.e. element lengths) of
|
f |
An atomic vector or a factor (possibly in Rle form). |
drop |
Logical indicating if levels that do not occur should be dropped (if
|
i |
A list-like object. Unlike for |
g |
A Grouping or an object coercible to one. For
|
... |
Arguments to pass to methods. |
relist, split, and 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 relistToClass
and is based only on the class of the original object.
By default, extractList(x, i) is equivalent to:
relist(x[unlist(i)], i)
An exception is made when x is a data-frame-like object. In that
case x is subsetted along the rows, that is, extractList(x, i)
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.
relist and split (or splitAsList) can be seen as
special cases of extractList:
relist(flesh, skeleton) is equivalent to
extractList(flesh, PartitioningByEnd(skeleton))
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
by relist or split. Whenever possible, using relist
or 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 relist or split
is always reversible (via unlist and unsplit, respectively),
but not the transformation performed by extractList (in general).
The regroup function splits the elements of unlist(x)
into a list according to the grouping g. Each element of
unlist(x) inherits its group from its parent element of
x. regroup is different from relist and
split, because x is already grouped, and the goal is to
combine groups.
The relist methods behave like utils::relist except that they
return a List object. If skeleton has names, then they are
propagated to the returned value.
splitAsList behaves like base::split except that the
former returns a List object instead of an ordinary list.
extractList returns a list-like object parallel to i and with
the same "shape" as i (i.e. same element lengths).
If i has 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 x. relistToClass gives
the exact class of the returned object.
Hervé Pagès
The unlist and relist
functions in the base and utils packages, respectively.
The split methods defined in the
S4Vectors package.
Vector, List,
Rle, and DataFrame objects
in the S4Vectors package.
relistToClass is documented in the man
page for List objects.
Ranges objects.
## 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)