1.8 KiB
1.8 KiB
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| Subsetting Data in R |
What is subsetting?
Subsetting is the selection or extraction of specific parts of larger data. We can subset on various kinds of data objects: vectors, lists, and data frames.
Subsetting operators
There are three subsetting operators: [, [[ and $.
[[ is similar to [, except it can only return a single value and it allows you to pull elements out of a list.
$ is a useful shorthand for [[ combined with character subsetting.
You need [[ when working with lists. This is because when [ is applied to a list, it always returns a list; it never gives you the contents of the list.
The following are the examples of subsetting of various R objects:
1. Vectors
x <- c(2.1, 4.2, 3.3, 5.4)
x[c(3, 1)] # Subsetting using positive integers: return elements at the specified positions.
## [1] 3.3 2.1
x[-c(3, 1)] # Subsetting using positive integers: return elements at the specified positions.
## [1] 4.2 5.4
x[c(TRUE, TRUE, FALSE, FALSE)] # # Subsetting using logical vectors.
## [1] 2.1 4.2
2. Lists
a <- matrix(1:9, nrow = 3)
colnames(a) <- c("A", "B", "C")
a[1:2, ]
## A B C
## [1,] 1 4 7
## [2,] 2 5 8
3. Data Frames
df <- data.frame(x = 1:3, y = 3:1, z = letters[1:3])
df[df$x == 2, ]
## x y z
## 2 2 2 b
df[c(1, 3), ]
## x y z
## 1 1 3 a
## 3 3 1 c
To get content of a list use [[ operator like:
a <- list(a = 1, b = 2)
a[[1]]
## [1] 1
a[["a"]]
## [1] 1