[R] Data frame from list of lists

[Gregory Jefferis]

This seems to be a simple problem, and I feel that there ought to be a
simple answer, but I can't seem to find it.

I have a function that returns a number of values as a heterogeneous list -
always the same length and same names(), but a number of different data
types, including character.  I want to apply it to many inputs, resulting in
a list of lists.

I would like to turn this list of lists into a single data frame in which
each row corresponds to one of the original sublists.

Here is a toy example:

myfunc=function(x) return(list(A=x,L=letters[x],T=Sys.time()))
ListOfLists=lapply(1:4,myfunc)
ListOfDataFrames=lapply(ListOfLists,as.data.frame)
df=do.call("rbind",ListOfDataFrames)

df

Which gives:

   A L                   T
1  1 a 2003-09-22 02:08:44
11 2 b 2003-09-22 02:08:44
12 3 c 2003-09-22 02:08:44
13 4 d 2003-09-22 02:08:44

Which is what I want (bar the rownames).  The problem is that this can be
very slow, particularly the last rbind step, when I have a large data set
(e.g. 5000 rows x20 cols).

I thought that one improvement might be to preassign the data frame since I
know how big it should be and then make assignments row by row.  But it
turns out that I can't then assign rows to the data frame one at a time - I
get errors because factor levels don't exist e.g.:

df[5:10,]=df[4,] 
for (i in 5:10){   
    df[i,]=as.data.frame(myfunc(i))
}

I presume that rbind.data.frame normally looks after adding extra levels to
factors as they appear in the new rows being appended to the data frame. If
anyone has a solution that is quick (and/or elegant), I would be extremely
grateful,

Greg Jefferis.

__________________________________________________________________________
Greg Jefferis,                          Lab Address: Liqun Luo, Herrin 144
Neurosciences PhD Programme &                e-mail: jefferis at stanford.edu
Dept Biological Sciences,                       Lab: (650) 725 5809
Gilbert Biology Building,                       Fax: (650) 723 0589
371 Serra Mall,
Stanford, CA 94305-5020.                       Home: (650) 326 9597