R：关于LHS和RHS的分配和操作顺序的混淆

Question

I am having some fundamental confusion with R. I have a snippet of R code.

> m <- 1:10
> m
 [1]  1  2  3  4  5  6  7  8  9 10
> dim(m) <- c(2,5)
> m
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    3    5    7    9
[2,]    2    4    6    8   10

Now I am a C/Python programmer and the line dim(m) <- c(2,5) is incredibly confusing to me. I realize that it effectively changed a vector into a matrix, however looking at it I do not understand the logic/order of operation.

<- is the assignment operator in R. So to me, logically the order of operation is : assign (2,5) to the output of dim(m). Since the output of dim(m) isn't assigned to a variable, the output would be lost.

Could someone explain how I should read the line dim(m) <- c(2,5) ? What is the order of operation? It seems that the order of operation using <- to changes depending on the LHS and RHS of the equation.

Answer 1

These are special functions called Replacement Functions. I quote from Hadley's Advanced-R book:

Replacement functions act like they modify their arguments in place, and have the special name xxx<-. They typically have two arguments (x and value), although they can have more, and they must return the modified object. For example, the following function allows you to modify the second element of a vector:

`second<-` <- function(x, value) {
  x[2] <- value
  x
}
x <- 1:10
second(x) <- 5L
x
#>  [1]  1  5  3  4  5  6  7  8  9 10

When R evaluates the assignment second(x) <- 5, it notices that the left hand side of the <- is not a simple name, so it looks for a function named second<- to do the replacement.

You can check the full chapter here under the Replacement Functions title.