r - The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe

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Top 5 Answer for r - The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe

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100

The R Language Definition is handy for answering these types of questions:

R has three basic indexing operators, with syntax displayed by the following examples

     x[i]     x[i, j]     x[[i]]     x[[i, j]]     x$a     x$"a" 

For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x.

For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements.

The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

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89

The significant differences between the two methods are the class of the objects they return when used for extraction and whether they may accept a range of values, or just a single value during assignment.

Consider the case of data extraction on the following list:

foo <- list( str='R', vec=c(1,2,3), bool=TRUE ) 

Say we would like to extract the value stored by bool from foo and use it inside an if() statement. This will illustrate the differences between the return values of [] and [[]] when they are used for data extraction. The [] method returns objects of class list (or data.frame if foo was a data.frame) while the [[]] method returns objects whose class is determined by the type of their values.

So, using the [] method results in the following:

if( foo[ 'bool' ] ){ print("Hi!") } Error in if (foo["bool"]) { : argument is not interpretable as logical  class( foo[ 'bool' ] ) [1] "list" 

This is because the [] method returned a list and a list is not valid object to pass directly into an if() statement. In this case we need to use [[]] because it will return the "bare" object stored in 'bool' which will have the appropriate class:

if( foo[[ 'bool' ]] ){ print("Hi!") } [1] "Hi!"  class( foo[[ 'bool' ]] ) [1] "logical" 

The second difference is that the [] operator may be used to access a range of slots in a list or columns in a data frame while the [[]] operator is limited to accessing a single slot or column. Consider the case of value assignment using a second list, bar():

bar <- list( mat=matrix(0,nrow=2,ncol=2), rand=rnorm(1) ) 

Say we want to overwrite the last two slots of foo with the data contained in bar. If we try to use the [[]] operator, this is what happens:

foo[[ 2:3 ]] <- bar Error in foo[[2:3]] <- bar :  more elements supplied than there are to replace 

This is because [[]] is limited to accessing a single element. We need to use []:

foo[ 2:3 ] <- bar print( foo )  $str [1] "R"  $vec      [,1] [,2] [1,]    0    0 [2,]    0    0  $bool [1] -0.6291121 

Note that while the assignment was successful, the slots in foo kept their original names.

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80

Double brackets accesses a list element, while a single bracket gives you back a list with a single element.

lst <- list('one','two','three')  a <- lst[1] class(a) ## returns "list"  a <- lst[[1]] class(a) ## returns "character" 
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69

From Hadley Wickham:

From Hadley Wickham

My (crappy looking) modification to show using tidyverse / purrr:

enter image description here

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59

[] extracts a list, [[]] extracts elements within the list

alist <- list(c("a", "b", "c"), c(1,2,3,4), c(8e6, 5.2e9, -9.3e7))  str(alist[[1]])  chr [1:3] "a" "b" "c"  str(alist[1]) List of 1  $ : chr [1:3] "a" "b" "c"  str(alist[[1]][1])  chr "a" 

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