R - Change column name using get()

问题

I have several data.frames in my Global Environment that I need to merge. Many of the data.frames have identical column names. I want to append a suffix to each column that marks its originating data.frame. Because I have many data.frames, I wanted to automate the process as in the following example.

df1 <-  data.frame(id = 1:5,x = LETTERS[1:5])
df2 <-  data.frame(id = 1:5,x = LETTERS[6:10])

obj <- ls()

for(o in obj){
  s <- sub('df','',eval(o))
  names(get(o))[-1] <- paste0(names(get(o))[-1],'.',s)
}

# Error in get(o) <- `*vtmp*` : could not find function "get<-"'

But the individual pieces of the assignment work fine:

names(get(o))[-1]
# [1] "x"
paste0(names(get(o))[-1],'.',s)
# [1] "x.1"

I've used get in a similar way to write.csveach object to a file.

for(o in obj){
  write.csv(get(o),file = paste0(o,'.csv'),row.names = F)
}

Any ideas why it's not working in the assignment to change the column names?

回答1:

The error "could not find function get<-" is R telling you that you can't use <- to update a "got" object. You could probably use assign, but this code is already difficult enough to read. The better solution is to use a list.

From your example:

df1 <-  data.frame(id = 1:5,x = LETTERS[1:5])
df2 <-  data.frame(id = 1:5,x = LETTERS[6:10])

# put your data frames in a list
df_names = ls(pattern = "df[0-9]+")
df_names   # make sure this is the objects you want
# [1] "df1" "df2"
df_list = mget(df_names)

# now we can use a simple for loop (or lapply, mapply, etc.)
for(i in seq_along(df_list)) {
    names(df_list[[i]])[-1] =
        paste(names(df_list[[i]])[-1],
              sub('df', '', names(df_list)[i]),
              sep = "."
        )
}

# and the column names of the data frames in the list have been updated
df_list
# $df1
#   id x.1
# 1  1   A
# 2  2   B
# 3  3   C
# 4  4   D
# 5  5   E
# 
# $df2
#   id x.2
# 1  1   F
# 2  2   G
# 3  3   H
# 4  4   I
# 5  5   J

It's also now easy to merge them:

Reduce(f = merge, x = df_list)
#   id x.1 x.2
# 1  1   A   F
# 2  2   B   G
# 3  3   C   H
# 4  4   D   I
# 5  5   E   J

For more discussion and examples, see How do I make a list of data frames?

回答2:

You can use eval which evaluate an R expression in a specified environment.

df1 <- data.frame(id = 1:5,x = LETTERS[1:5])
df2 <- data.frame(id = 1:5,x = LETTERS[6:10])

obj <- ls()

for(o in obj) {
  s <- sub('df', '', o)
  new_name <- paste0(names(get(o))[-1], '.', s)
  eval(parse(text = paste0('names(', o, ')[-1] <- ', substitute(new_name))))
}

modify df1 and df2

回答3:

Using setnames from library(data.table) you can do

for(o in obj) {
  oldnames = names(get(o))[-1]
  newnames = paste0(oldnames, ".new")
  setnames(get(o), oldnames, newnames)
}

来源：https://stackoverflow.com/questions/47698342/r-change-column-name-using-get

标签

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