What do 'names<-' and 'class<-' do in R codes below?

Question

I read the following codes in a book. Not quite sure of the 'names<-' part in last line.

Could someone provide some reference for this? Is this related to the functions forms mentioned in section 6.8 of https://adv-r.hadley.nz/functions.html ? Or they define the attributes in some way? Thanks.

bs.quantile <- function(v, p=c(0, 0.25, 0.5, 0.75, 1))
{
  b <- 1-p
  k <- floor((ps <- p*length(v))+b)
  beta <- ps+b-k
  'names<-'((1-beta)*(v <- sort(v))[k]+beta*(ifelse(k<length(v), v[k+1], v[k])), p)
}

There are several other places where the author use this approach.

‘class<-‘(tree, "dectree.frac")

‘class<-‘(list(prior=cc/sum(cc),cond=sapply(avc, 
function(avc1) t(apply(avc1, 1, "/", colSums(avc1))))),"nbc")
}

Good find, @Henrik. Questions on functions/issues that include specials like `<-` can be especially hard to find since most search engines seem to optimize them out. — r2evans, Sep 14 '20 at 18:26
Ken, the answers in Henrik's link give a little more info than I did here in my answer. I'm going to mark this question as a dupe (not a bad thing), but it would be nice if you accepted this anyway. Thanks! — r2evans, Sep 14 '20 at 18:28
Regarding @r2evans's point 1, ‘class<-‘ was due to pure laziness. I just copied the code from the book directly. — Ken, Sep 14 '20 at 23:22

r2evans · Accepted Answer · 2020-09-14T23:47:32.577

‘class<-‘ (with single quotes, whether fancy ‘ or straight-ascii ') is wrong, likely a side-effect of copying and pasting and/or misinterpreting somebody else's code. (MS Word does this a lot ... never never never write/edit R code in Word, R is intolerant of these fancy-niceties :-).

Typically, it should be backticks instead, as in `class<-` and `names<-`.
Typically, one would see something like
```
vec <- c(1, 3, 11)
names(vec) <- c("a", "BB", "quux")
vec
#    a   BB quux 
#    1    3   11 
```
That last call is actually calling a special form of a function call `names<-` (not names itself), which tells R that it should do something different when names(...) is called on the left-hand side (LHS) of an assignment operator (= or <- in R).

Sometimes, coders try to be slick (code-golf) by calling the function directly so that they can get the returned value without necessarily changing the original variable's value. For instance:
```
vec <- c(1, 3, 11)
`names<-`(vec, c("a", "BB", "quux"))
#    a   BB quux 
#    1    3   11 
vec
# [1]  1  3 11
```
With `class<-`, it is changing the class of an object "inline".

What do 'names<-' and 'class<-' do in R codes below?

1 Answers1