R Object elements: brackets [], double brackets [[]] and $
Many R objects are composed of multiple elements. There are various ways to extract one (or more) elements from an object, depending on the object itself.
Square brackets []
A simple vector for example is a 1-D object; you can get elements from a vector using the square brackets:
# Make a numeric vector > data1 <- c(3, 5, 7, 5, 3, 2, 6, 8, 5, 6, 9) > data1 [1] 3 5 7 5 3 2 6 8 5 6 9 > data1[1] # The first item [1] 3 > data1[3] # The third item [1] 7 > data1[1:4] # The first 4 items [1] 3 5 7 5 > data1[-1] # All except the first [1] 5 7 5 3 2 6 8 5 6 9 > data1[c(1, 3, 4, 8)] # The 1st, 3rd, 4th, 8th [1] 3 7 5 8 > data1[data1 > 3] # All items > 3 [1] 5 7 5 6 8 5 6 9 > data1[data1 < 5 | data1 > 7] # Items < 5 OR > 7 [1] 3 3 2 8 9
Multi-dimensional objects and brackets
If your object has 2 dimensions, such as a data.frame or a matrix you can use the same idea but now specify [rows, columns]. Extra dimensions can be supplied if needed (e.g. for a table).
> mymat <- matrix(1:30, ncol = 5, dimnames = list(letters[1:6], LETTERS[1:5])) > mymat A B C D E a 1 7 13 19 25 b 2 8 14 20 26 c 3 9 15 21 27 d 4 10 16 22 28 e 5 11 17 23 29 f 6 12 18 24 30 > mymat[2, 3] # Item from 2nd row and 3rd column [1] 14 > mymat[, 2] # All rows but only 2nd column a b c d e f 7 8 9 10 11 12 > mymat[3, ] # All columns but only 3rd row A B C D E 3 9 15 21 27 > mymat[-1, ] # All columns and all rows except the first A B C D E b 2 8 14 20 26 c 3 9 15 21 27 d 4 10 16 22 28 e 5 11 17 23 29 f 6 12 18 24 30 > mymat[, "B"] # All rows and the column named "B" a b c d e f 7 8 9 10 11 12
You can also use conditional statements just like for a vector.
The dollar symbol $
With some objects you can use the $, particularly data.frame and list objects:
> mydf <- data.frame(num = 1:12, mnths = month.abb[1:12], fac = gl(3, 4, labels = c("high", "mid", "low")), let = LETTERS[12:1])
> mydf
num mnths fac let
1 1 Jan high L
2 2 Feb high K
3 3 Mar high J
4 4 Apr high I
5 5 May mid H
6 6 Jun mid G
7 7 Jul mid F
8 8 Aug mid E
9 9 Sep low D
10 10 Oct low C
11 11 Nov low B
12 12 Dec low A
> mylist <- list(num = 1:6, let = letters[9:1], mnth = month.abb[1:7])
> mylist
$num
[1] 1 2 3 4 5 6
$let
[1] "i" "h" "g" "f" "e" "d" "c" "b" "a"
$mnth
[1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul"
The $ is used with the element name like so:
> mydf$let [1] L K J I H G F E D C B A Levels: A B C D E F G H I J K L > mydf$num [1] 1 2 3 4 5 6 7 8 9 10 11 12 > mylist$mnth [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul"
You can also use square brackets. For the data.frame the [row, column] syntax works as it did for the matrix. Giving a single value selects a column.
> mylist[1] $num [1] 1 2 3 4 5 6 > mydf[2] mnths 1 Jan 2 Feb 3 Mar 4 Apr 5 May 6 Jun 7 Jul 8 Aug 9 Sep 10 Oct 11 Nov 12 Dec > mydf[2,] num mnths fac let 2 2 Feb high K
Note that the $ does not work with a matrix object.
Double brackets [[]]
You can use double brackets to select elements in more or less the same way as single brackets. The difference between single and double is that with double brackets any element names are not displayed:
> mydf[[2]] [1] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Levels: Apr Aug Dec Feb Jan Jul Jun Mar May Nov Oct Sep > mydf[2] mnths 1 Jan 2 Feb 3 Mar 4 Apr 5 May 6 Jun 7 Jul 8 Aug 9 Sep 10 Oct 11 Nov 12 Dec mylist[3] $mnth [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" > mylist[[3]] [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul"
You don’t have to use an index value, the element name gives a similar result:
> mylist[["let"]] [1] "i" "h" "g" "f" "e" "d" "c" "b" "a" > mylist["let"] $let [1] "i" "h" "g" "f" "e" "d" "c" "b" "a"
Summary
So, the $ can be used with vector and data.frame objects and the [] with more or less any object. Use [[]] to suppress the element name.
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