Adding data frames into a list within a forloop - r

Question

I have a for loop that generates a dataframe every time it loops through. I am trying to create a list of data frames but I cannot seem to figure out a good way to do this.
For example, with vectors I usually do something like this:
my_numbers <- c()
for (i in 1:4){
my_numbers <- c(my_numbers,i)
}
This will result in a vector c(1,2,3,4). I want to do something similar with dataframes, but accessing the list of data frames is quite difficult when i use:
my_dataframes <- list(my_dataframes,DATAFRAME).
Help please. The main goal is just to create a list of dataframes that I can later on access dataframe by dataframe. Thank you.

Answer 1

I'm sure you've noticed that list does not do what you want it to do, nor should it. c also doesn't work in this case because it flattens data frames, even when recursive=FALSE.
You can use append. As in,
data_frame_list = list()
for( i in 1:5 ){
d = create_data_frame(i)
data_frame_list = append(data_frame_list,)
}
Better still, you can assign directly to indexed elements, even if those elements don't exist yet:
data_frame_list = list()
for( i in 1:5 ){
data_frame_list[[i]] = create_data_frame(i)
}
This applies to vectors, too. But if you want to create a vector c(1,2,3,4) just use 1:4, or its underlying function seq.
Of course, lapply or the *lply functions from plyr are often better than looping depending on your application.

Answer 2

Continuing with your for loop method, here's a little example of creating and accessing.
> my_numbers <- vector('list', 4)
> for (i in 1:4) my_numbers[[i]] <- data.frame(x = seq(i))
And we can access the first column of each data frame with,
> sapply(my_numbers, "[", 1)
# $x
# [1] 1
#
# $x
# [1] 1 2
#
# $x
# [1] 1 2 3
#
# $x
# [1] 1 2 3 4
Other ways of accessing the data is my_numbers[[1]] for the first data set,
lapply(my_numbers, "[", 1,) to access the first row of each data frame, etc.

Answer 3

You can use operator [[ ]] for this purpose.
l <- list()
df1 <- data.frame(name = 'df1', a = 1:5 , b = letters[1:5])
df2 <- data.frame(name = 'df2', a = 6:10 , b = letters[6:10])
df3 <- data.frame(name = 'df3', a = 11:20 , b = letters[11:20])
df <- rbind(df1,df2,df3)
for(df_name in unique(df$name)){
l[[df_name]] <- df[df$name == df_name,]
}
In this example, there are three separate data frames and in order to store them
in a list using a for loop, we place them in one. Using the operator [[ we can even name the data frame in the list as we want and store it in the list normally.