Difference between `names(df[1]) <- ` and `names(df)[1] <- `

Question

Consider the following:

df <- data.frame(a = 1, b = 2, c = 3) names(df[1]) <- "d" ## First method ##  a b c ##1 1 2 3  names(df)[1] <- "d" ## Second method ##  d b c ##1 1 2 3 

Both methods didn't return an error, but the first didn't change the column name, while the second did.

I thought it has something to do with the fact that I'm operating only on a subset of df, but why, for example, the following works fine then?

df[1] <- 2  ##  a b c ##1 2 2 3 

Answer 1

What I think is happening is that replacement into a data frame ignores the attributes of the data frame that is drawn from. I am not 100% sure of this, but the following experiments appear to back it up:

df <- data.frame(a = 1:3, b = 5:7) #   a b # 1 1 5 # 2 2 6 # 3 3 7  df2 <- data.frame(c = 10:12) #    c # 1 10 # 2 11 # 3 12  df[1] <- df2[1]   # in this case `df[1] <- df2` is equivalent 

Which produces:

#    a b # 1 10 5 # 2 11 6 # 3 12 7 

Notice how the values changed for df, but not the names. Basically the replacement operator `[<-` only replaces the values. This is why the name was not updated. I believe this explains all the issues.

In the scenario:

names(df[2]) <- "x" 

You can think of the assignment as follows (this is a simplification, see end of post for more detail):

tmp <- df[2] #   b # 1 5 # 2 6 # 3 7  names(tmp) <- "x" #   x # 1 5 # 2 6 # 3 7  df[2] <- tmp   # `tmp` has "x" for names, but it is ignored! #    a b # 1 10 5 # 2 11 6 # 3 12 7 

The last step of which is an assignment with `[<-`, which doesn't respect the names attribute of the RHS.

But in the scenario:

names(df)[2] <- "x" 

you can think of the assignment as (again, a simplification):

tmp <- names(df) # [1] "a" "b"  tmp[2] <- "x" # [1] "a" "x"  names(df) <- tmp #    a x # 1 10 5 # 2 11 6 # 3 12 7 

Notice how we directly assign to names, instead of assigning to df which ignores attributes.

df[2] <- 2 

works because we are assigning directly to the values, not the attributes, so there are no problems here.

---

EDIT: based on some commentary from @AriB.Friedman, here is a more elaborate version of what I think is going on (note I'm omitting the S3 dispatch to `[.data.frame`, etc., for clarity):

Version 1 names(df[2]) <- "x" translates to:

df <- `[<-`(   df, 2,    value=`names<-`(   # `names<-` here returns a re-named one column data frame     `[`(df, 2),            value="x" ) )  

Version 2 names(df)[2] <- "x" translates to:

df <- `names<-`(   df,   `[<-`(      names(df), 2, "x" ) ) 

---

Also, turns out this is "documented" in R Inferno Section 8.2.34 (Thanks @Frank):

right <- wrong <- c(a=1, b=2) names(wrong[1]) <- 'changed' wrong # a b # 1 2 names(right)[1] <- 'changed' right # changed b # 1 2