[R] Difference between a[[i]] and a[i]

Wacek Kusnierczyk Waclaw.Marcin.Kusnierczyk at idi.ntnu.no
Tue Feb 3 09:55:20 CET 2009

Rolf Turner wrote:
> On 3/02/2009, at 12:45 PM, David Epstein wrote:
>> I'm sure I've read about the difference between a[[i]] and a[i] in R,
>> but I
>> cannot recall what I read. Even more disturbing is the fact that I don't
>> know how to search the newsgroup for this. All the different
>> combinations I
>> tried were declared not to be valid search syntax.
> Essentially: a is a list; a[i] is a list of length 1 whose sole entry is
> the i-th entry of a; a[[i]]] is the i-th entry of a.  If you are of a
> mathematical bent it may be illuminating to think of this as being
> analogous
> to, say, {3} being a subset of the set {1,2,3,4,5} and 3 being an element
> of this set.
>> 1. What sort of object can the operators [] and [[]] be applied to?
>> How do
>> they differ? I mean objects in standard R, not in packages that provide
>> conceivable overloading of these operators (if that's possible).
> Essentially lists.  Note that any vector can be considered to be a list.

what do you mean by 'can be considered to be a list'?  by whom?  not by
the r interpreter, it seems:

v = 1:10
# a vector
l = as.list(1:10)
# a list

# no
# yes

is(v, "list")
# no
is(l, "list")
# yes

# "integer" "vector" "numeric" "data.frameRowLabels"
# "list" "vector"

d = data.frame(x=1:10)
aggregate(d, by=v)
# error: 'by' must be a list
aggregate(d, by=l)
# fine

# error: have you called 'sort' on a list?
# fine, no error

# and the best of all
# error: have you called 'sort' on a list?
# fine, no error

perhaps you'd care to explain what 'can be considered to be a list' is
supposed to mean precisely, or else you're messing things up.


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