R: subsetting of data in r (#20163)

* subsetting in r

* Fix formatting and clarify content

guide/english/r/subsetting-data/index.md

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+---
+title: 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**
+
+```r
+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**
+
+```r
+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**
+
+```r
+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:
+
+```r
+a <- list(a = 1, b = 2)
+a[[1]]
+## [1] 1
+
+a[["a"]]
+## [1] 1
+```
+
+## Resources
+
+ * [Quick-R](https://www.statmethods.net/management/subset.html)
+ * [R Documentation](https://www.rdocumentation.org/packages/base/versions/3.5.1/topics/subset)
+ * [R Bloggers](https://www.r-bloggers.com/5-ways-to-subset-a-data-frame-in-r/)
+ * [Advanced R](http://adv-r.had.co.nz/Subsetting.html)