Subsetting in General

R has three subsetting operators ([, [[, and $). The behavior of these operators depends on the object (class) they are being used with.

In general there are 6 different types of subsetting that can be performed:

Positive Integer subsetting

Returns elements at the given location(s)

x = c(1,4,7)

x[1]

[1] 1

x[c(1,3)]

[1] 1 7

x[c(1,1)]

[1] 1 1

x[c(1.9,2.1)]

[1] 1 4

y = list(1,4,7)

str( y[1] )

List of 1
 $ : num 1

str( y[c(1,3)] )

List of 2
 $ : num 1
 $ : num 7

str( y[c(1,1)] )

List of 2
 $ : num 1
 $ : num 1

str( y[c(1.9,2.1)] )

List of 2
 $ : num 1
 $ : num 4

Negative Integer subsetting

Excludes elements at the given location(s)

x = c(1,4,7)

x[-1]

[1] 4 7

x[-c(1,3)]

[1] 4

x[c(-1,-1)]

[1] 4 7

y = list(1,4,7)

str( y[-1] )

List of 2
 $ : num 4
 $ : num 7

str( y[-c(1,3)] )

List of 1
 $ : num 4

x[c(-1,2)]

Error in x[c(-1, 2)]: only 0's may be mixed with negative subscripts

y[c(-1,2)]

Error in y[c(-1, 2)]: only 0's may be mixed with negative subscripts

Logical Value Subsetting

Returns elements that correspond to TRUE in the logical vector. Length of the logical vector is coerced to be the same as the vector being subsetted.

x = c(1,4,7,12)

x[c(TRUE,TRUE,FALSE,TRUE)]

[1]  1  4 12

x[c(TRUE,FALSE)]

[1] 1 7

y = list(1,4,7,12)

str( y[c(TRUE,TRUE,FALSE,TRUE)] )

List of 3
 $ : num 1
 $ : num 4
 $ : num 12

str( y[c(TRUE,FALSE)] )

List of 2
 $ : num 1
 $ : num 7

x[x %% 2 == 0]

[1]  4 12

str( y[y %% 2 == 0] )

Error in y%%2: non-numeric argument to binary operator

Empty Subsetting

Returns the original vector, this is not the same as subsetting with NULL

x = c(1,4,7)

x[]

[1] 1 4 7

x[NULL]

numeric(0)

y = list(1,4,7)

str(y[])

List of 3
 $ : num 1
 $ : num 4
 $ : num 7

str(y[NULL])

 list()

Zero subsetting

Returns an empty vector (of the same type), this is the same as subsetting with NULL

x = c(1,4,7)

x[0]

numeric(0)

y = list(1,4,7)
str(y[0])

 list()

x[c(0,1)]

[1] 1

y[c(0,1)]

[[1]]
[1] 1

x[c(0,-1)]

[1] 4 7

y[c(0,-1)]

[[1]]
[1] 4

[[2]]
[1] 7

Character subsetting

If the vector has names, selects elements whose names correspond to the values in the name vector.

x = c(a=1, b=4, c=7)

x["a"]

a 
1 

x[c("a","a")]

a a 
1 1 

x[c("b","c")]

b c 
4 7 

y = list(a=1,b=4,c=7)

str(y["a"])

List of 1
 $ a: num 1

str(y[c("a","a")])

List of 2
 $ a: num 1
 $ a: num 1

str(y[c("b","c")])

List of 2
 $ b: num 4
 $ c: num 7

Out of bounds

x = c(1,4,7)

x[4]

[1] NA

x[-4]

[1] 1 4 7

x["a"]

[1] NA

x[c(1,4)]

[1]  1 NA

y = list(1,4,7)

str(y[4])

List of 1
 $ : NULL

str(y[-4])

List of 3
 $ : num 1
 $ : num 4
 $ : num 7

str(y["a"])

List of 1
 $ : NULL

str(y[c(1,4)])

List of 2
 $ : num 1
 $ : NULL

Missing values

x = c(1,4,7)

x[NA]

[1] NA NA NA

x[c(1,NA)]

[1]  1 NA

y = list(1,4,7)

str(y[NA])

List of 3
 $ : NULL
 $ : NULL
 $ : NULL

str(y[c(1,NA)])

List of 2
 $ : num 1
 $ : NULL

NULL and empty vectors (length 0)

This final type of subsetting follows the rules for length coercion with a 0-length vector (i.e. the vector being subset gets coerced to having length 0 if the subsetting vector has length 0)

x = c(1,4,7)

x[NULL]

numeric(0)

x[integer()]

numeric(0)

x[character()]

numeric(0)

y = list(1,4,7)

y[NULL]

list()

y[integer()]

list()

y[character()]

list()

Subsetting and assignment

Subsets can also be used with assignment to update specific values within an object (in-place).

x = c(1, 4, 7, 9, 10, 15)

x[2] = 2
x

[1]  1  2  7  9 10 15

x %% 2 != 0

[1]  TRUE FALSE  TRUE  TRUE FALSE  TRUE

x[x %% 2 != 0] = (x[x %% 2 != 0] + 1) / 2
x

[1]  1  2  4  5 10  8