Using Loop variable to access and write specific data.frames - r

Question

I wrote a script, that reads CSV-Data with help of user input. For example when the user enters "20 40 160" the CSV files 1, 2 and 3 are read and saved as the data.frames d20, d40 and d160 in my global enviroment/workspace. The variable vel has the values for the user input.
Now for the actual question:
Im trying to manipulate the read data in a loop with the vel variable. For example:
for (i in vel)
{
newVariable"i" <- d"i"[6]
}
I know thats not the correct syntax for the programming, but what im trying to do ist to write a newVariable with a specific row from a specific data frame d.
The result should be:
newVariable20 = d20[20]
newVariable40 = d40[20]
newVariable160 = d160[20]
So I think the actual question is, how do I use the Loop Variable for calling out the names of the created data frames and for writing new variables.

Answer 1

There are a couple of ways to do this. One is to store all of your dataframes in a list originally. There are a couple ways to do this. Start with an empty list and then put each df into the next position in the list. Note that you have to use list(df) because a dataframe is actually already a list and gets messed up if you don't do this.
list_of_df <- list();
list_of_df[1] <- list(df1);
list_of_df["df20"] <- list(df2)
This makes it easy to loop through the dataframes. If you want column 4 of dataframe 2 you just put in
list_of_df[[2]][,4]
# Same thing different code
list_of_df[["df20"]][,4]
The double brackets [[2]] give you the value that is stored in the list at position 2 (instead of [2] which gives you a list containing the value and metadata). The next [,4] says that from the dataframe we just got the value of, we now want to get every row of the 4th column. Note that this will output a vector and not a dataframe.
Or in a loop:
for(df in list_of_df) {
print(df)
}