R – What are the main differences between R data files

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

• http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

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.

The difference in assignment operators is clearer when you use them to set an argument value in a function call. For example:

median(x = 1:10)
x   
## Error: object 'x' not found

In this case, x is declared within the scope of the function, so it does not exist in the user workspace.

median(x <- 1:10)
x    
## [1]  1  2  3  4  5  6  7  8  9 10

In this case, x is declared in the user workspace, so you can use it after the function call has been completed.

There is a general preference among the R community for using <- for assignment (other than in function signatures) for compatibility with (very) old versions of S-Plus. Note that the spaces help to clarify situations like

x<-3
# Does this mean assignment?
x <- 3
# Or less than?
x < -3

Most R IDEs have keyboard shortcuts to make <- easier to type. Ctrl + = in Architect, Alt + - in RStudio (Option + - under macOS), Shift + - (underscore) in emacs+ESS.

If you prefer writing = to <- but want to use the more common assignment symbol for publicly released code (on CRAN, for example), then you can use one of the tidy_* functions in the formatR package to automatically replace = with <-.

library(formatR)
tidy_source(text = "x=1:5", arrow = TRUE)
## x <- 1:5

The answer to the question "Why does x <- y = 5 throw an error but not x <- y <- 5?" is "It's down to the magic contained in the parser". R's syntax contains many ambiguous cases that have to be resolved one way or another. The parser chooses to resolve the bits of the expression in different orders depending on whether = or <- was used.

To understand what is happening, you need to know that assignment silently returns the value that was assigned. You can see that more clearly by explicitly printing, for example print(x <- 2 + 3).

Secondly, it's clearer if we use prefix notation for assignment. So

x <- 5
`<-`(x, 5)  #same thing

y = 5
`=`(y, 5)   #also the same thing

The parser interprets x <- y <- 5 as

`<-`(x, `<-`(y, 5))

We might expect that x <- y = 5 would then be

`<-`(x, `=`(y, 5))

but actually it gets interpreted as

`=`(`<-`(x, y), 5)

This is because = is lower precedence than <-, as shown on the ?Syntax help page.