Recombining a list of Data.frames into a single data frame.

Question

Hello everyone

I am sorry if this question has been answered already. Also, this is my first time on stackoverflow.

I have a beginner R question concerning lists , data frames and merge and/or rbind.

I started with a Panel that looks like this

COUNTRY YEAR VAR
A         1
A         2
B         1
B         2

For efficiency purposes, I created a list that consists of one data frame for each country and performed a variety of calculations on each individual data.frame. However, I cannot seem to combine the individual data frames into one large frame again.

rbind and merge both tell me that only replacement of elements is allowed.

Could someone tell me what I am doing wrong/ and how to actually recombine the data frames?

Thank you

Answer 1

Hi there,

I suggest you to go and study the plyr package. (Here is a nice little tutorial for plyr. Here is a longer tutorial to read)

Then gave a look at the ldply function.

Answer 2

There might be a better way to do this, but this seems to work and it's straightforward. (My code has four lines so that it's easier to see the steps; these four could easily be combined.)

# first re-create your data frame:
A = matrix( ceiling(10*runif(8)), nrow=4)
colnames(A) = c("country", "year_var")
dfa = data.frame(A)

# now re-create the list you made from the individual rows of the data frame:
df1 = dfa[1,]
df2 = dfa[2,]
df3 = dfa[3,]
df4 = dfa[4,]
df_all = list(df1, df2, df3, df4)

# to recreate your original data frame:
x = unlist(df_all)         # from your list create a single 1D array 
A = matrix(x, nrow=4)      # dimension that array in accord w/ your original data frame
colnames(A) = c("country", "year_var")     # put the column names back on
dfa = data.frame(A)        # from the matrix, create your original data frame

Answer 3

Hello,

Maybe you want to do something like

do.call( rbind, my.df.list )

Answer 4

plyr is probably best. Another useful approach if the data frames can be different is to use reshape:

library(reshape)
data <- merge_recurse(listofdataframes)

Look at my answer to this related question on merging data frames.