returning from list to data.frame after lapply

Question

I have a very simply question about lapply. I am transitioning from STATA to R and I think there is some very basic concept that I am not getting about looping in R. But I have been reading about it all afternoon and can't figure out a reasonable way to do this very simple thing.

I have three data frames df1, df2, and df3 that all have the same column names, in the same order, etc.

I want to rename their columns all at once.

I put the data frames in a list:

dflist <- list(df1, df2, df3)

What I want the new names to be:

varlist <- c("newname1", "newname2", "newname3")

Write a function that replaces names with those in varlist, and lapply it over the data frames

ChangeNames <- function(x) {
  names(x) <- varlist 
  return(x)
}

dflist <- lapply(dflist, ChangeNames)

So, as far as I understand, R has changed the names of the copies of the data frames that I put in the list, but not the original data frames themselves. I want the data frames themselves to be renamed, not the elements of the list (which are trapped in a list).

Now, I can go

df1 <- as.data.frame(dflist[1])
df2 <- as.data.frame(dflist[2])
df2 <- as.data.frame(dflist[3])

But that seems weird. You need a loop to get back the elements of a loop?

Basically: once you've put some data frames in a list and run your function on them via lapply, how do you get them back out of the list, without starting back at square one?

Answer 1

I am not sure I understand correctly, do you want to rename the columns of the data frames or the components of the list that contain the data frames?

If it is the first, please always search before asking, the question has been asked here.

So what you can easily do in case you have even more data frames in the list is:

# Creating some sample data first
> dflist <- list(df1 = data.frame(a = 1:3, b = 2:4, c = 3:5),
+ df2 = data.frame(a = 4:6, b = 5:7, c = 6:8),
+ df3 = data.frame(a = 7:9, b = 8:10, c = 9:11))

# See how it looks like
> dflist
    $df1
  a b c
1 1 2 3
2 2 3 4
3 3 4 5

$df2
  a b c
1 4 5 6
2 5 6 7
3 6 7 8

$df3
  a  b  c
1 7  8  9
2 8  9 10
3 9 10 11

# And do the trick
> dflist <- lapply(dflist, setNames, nm = c("newname1", "newname2", "newname3"))

# See how it looks now
> dflist

$df1
  newname1 newname2 newname3
1        1        2        3
2        2        3        4
3        3        4        5

$df2
  newname1 newname2 newname3
1        4        5        6
2        5        6        7
3        6        7        8

$df3
  newname1 newname2 newname3
1        7        8        9
2        8        9       10
3        9       10       11

So the names were changed from a, b and c to newname1, newname2and newname3 for each data frame in the list.

If it is the second, you can do this:

> names(dflist) <- c("newname1", "newname2", "newname3")

Answer 2

If you just want to change the names, that isn't too hard in R. Bear in mind that the assignment operator, <-, can be applied in sequence. Hence:

names(df1) <- names(df2) <- names(df3) <- c("newname1", "newname2", "newname3")