Add file names to columns in a list of dataframes - r

Question

I am importing a list of several dataframes using a custom function. I want to take the name of the imported file (e.g. file1 from file1.csv) and add it onto all of the column names in that dataframe. In this example, all column names will look like this:
# Column names as they are
q1 q2 q3
# Column names with added name of the file they come from
q1_file1 q2_file1 q3_file1
This is what I've tried, but it doesn't work (the list ends up having 0 dataframes):
my_function<- function (x) {
df <- read.csv(x)
tag <- sub('\\.csv$', '', x)
colnames(df) <- paste0(tag, colnames(df))
}
lapply(my_list, my_function)
Thanks!

Answer 1

It can be:
#Code
tucson_function<- function (x) {
df <- read.csv(x)
tag <- sub('\\.csv$', '', x)
df$tag <- tag
}
Or:
#Code
tucson_function<- function (x) {
df <- read.csv(x)
tag <- sub('\\.csv$', '', x)
names(df) <- paste0(tag,'.',names(df))
return(df)
}

Answer 2

We can use transform with tools::file_path_sans_ext to create a column
my_function<- function(x) {
df <- read.csv(x)
transform(df, tag = tools::file_path_sans_ext(x))
}
and then call the function with lapply
lapply(my_list, my_function)
In the OP's function the issue seems to be that the return is the last assignment i.e. the column names assignment. We need to return the data i.e. 'df'
my_function<- function (x) {
df <- read.csv(x)
tag <- sub('\\.csv$', '', x)
colnames(df) <- paste0(tag, colnames(df))
df
}