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
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
