R - Function Equals Function

Question

My question is fairly difficult to search for, as I just come across APIs for the "equals" function instead!

If I have the following line, working upon the matrix Gl:

diag(Gl)=diag(Gl)+1

Is this equivalent to modifying Gl itself?

Answer 1

Yes, this is modifying the diagonal of Gl in place. This is an example of a moderately obscure R language feature called a replacement function. From the R language manual (section 3.4.4, about the equivalent operation for names):

The same mechanism can be applied to functions other than [. The replacement function has the same name with <- pasted on. Its last argument, which must be called value, is the new value to be assigned. For example,

names(x) <- c("a","b")

is equivalent to

   `*tmp*` <- x
   x <- "names<-"(`*tmp*`, value=c("a","b"))
   rm(`*tmp*`)

(sorry about formatting). You can print the `diag<-` function to see (surround the name in back-ticks ` `) so the parser doesn't get confused, or getAnywhere("diag<-"))

Answer 2

Yes, it's assigning. Simple example:

 > g1 <- matrix(1:4, nrow = 2, ncol = 2)
 > g1
     [,1] [,2]
[1,]    1    3
[2,]    2    4
 > diag(g1) = diag(g1) + 1
 > g1
     [,1] [,2]
[1,]    2    3
[2,]    2    5

When you type "dia", then RStudio autofills with the options of diag() and diag<-, with some info on the replacement part.