Combining dataframes into a list

问题

I'm trying to store multiple dataframes in a list. However, at some point, the dataframes end up getting converted into lists, and so I end up with a list of lists.

All I'm really trying to do is keep all my dataframes together in some sort of structure.

Here's the code that fails:

all_dframes <- list() # initialise a list that will hold a dataframe as each item
for(file in filelist){ # load each file
    dframe <- read.csv(file) # read CSV file
    all_dframes[length(all_dframes)+1] <- dframe # add to the list
}

If I now call, for example, class(all_dframes[1]), I get 'list', whereas if I call class(dframe) I get 'data.frame'!

回答1:

Of course, the class of all_dframes[1] is list since all_dframes is a list. The function [ returns a subset of the list. In this example, the length of the returned list is one. If you want to extract the data frame you have to use [[, i.e., all_dframes[[1]].

回答2:

May I suggest this:

library(data.table)
all_dframes <- vector("list",length(filelist))
for(i in 1:length(filelist)){ # load each file
       all_dframes[[i]]<-fread(filelist[i])
}

Is this what you need?

来源：https://stackoverflow.com/questions/26730970/combining-dataframes-into-a-list