What is the precise meaning of the quoted square bracket passed as a parameter to apply functions? [duplicate]

Question

In the programming language R what, precisely, is the meaning of

'['

which serves as a parameter to sapply() and lapply() in the following portion of code:

dd <- data.frame(
    A = c(1L, 2L, 3L), 
    B = c(4L, 5L, 6L), 
    C = c("X1=7;X2=8;X3=9",
          "X1=13;X2=14",
          "X1=5;X2=1;X3=8")
)
namev <- function(x) {
    a <- strsplit(x,"=")
    setNames(sapply(a,'[',2), sapply(a,'[',1))
}

vv <- lapply(strsplit(as.character(dd$C),";"), namev)

nm <- unique(unlist(sapply(vv, names)))

#extract data from all rows for every column
nv <- do.call(rbind, lapply(vv, '[', nm))

dd$C [1] X1=7;X2=8;X3=9 X1;; X1=13;X2=14
Levels: X1;; X1=13;X2=14 X1=7;X2=8;X3=9

@Henrik The answer to the two answers are the same but the questions are different. The question for which this has been marked duplicate (Using '[' square bracket as a function for lapply in R ) presupposes a knowledge that [ is a function which is not self evident to us R newbies.

Answer 1

[ is a function. In the examples below it is used with two arguments.

L <- list(a = 1:4, b = 1:3)

sapply(L, `[`, 2)
## a b 
## 2 2 

The above sapply is the same as either of these:

sapply(L, function(x) `[`(x, 2))

sapply(L, function(x) x[2])

It is a primitive function in R whose R source is the following, i.e. it punts to the underlying C code.

`[`
## .Primitive("[")

S3 methods can be written for it. For example these methods are available in vanilla R.

> methods("[")
 [1] [,nonStructure-method [.acf*                [.AsIs               
 [4] [.bibentry*           [.data.frame          [.Date               
 [7] [.difftime            [.Dlist               [.factor             
[10] [.formula*            [.getAnywhere*        [.hexmode            
[13] [.listof              [.noquote             [.numeric_version    
[16] [.octmode             [.pdf_doc*            [.person*            
[19] [.POSIXct             [.POSIXlt             [.raster*            
[22] [.roman*              [.SavedPlots*         [.simple.list        
[25] [.table               [.terms*              [.ts*                
[28] [.tskernel*           [.warnings           
see '?methods' for accessing help and source code

For example, try the following to see the R source code for these methods:

`[.data.frame`

`[.Date`

Answer 2

[ is a function.

iris[1,2] is the equivalent of '['(iris,1,2).

It needs to be quoted to be used this way as it's not a syntactically valid name (see ?make.names).

You could quote any function though :

'head'(iris)

  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa

FYI the magrittr package includes the extract and extract2 functions that are identical to functions [ and [[ but might be more readable for some (and one can use them without the quotes).

[<- and [[<- are functions too, that you use when assigning to an element of a vector/matrix/data.frame/list and have aliases inset and inset2 in magrittr