[R] making a dataframe out of lapply() result

[Sundar Dorai-Raj]

Remko Duursma wrote:
> Dear R-helpers,
> 
> i have a question on how to vectorize this problem:
> 
> i have a dataframe:
> 
> tester <- data.frame(groups=c("A","A","B","B","C","C"), one=c(1,1,2,2,3,3), two=c(6,6,7,7,8,8))
> 
> # i split it into a list
> tester.L <- split(tester, tester$groups)
> 
> # And want to keep only the first item in each:
> lapply(tester.L, function(x) x <- x[1,] )
> 
> 
> How do i make a dataframe out of the last result, which looks like "tester", without looping? (i can use rbind in a for loop, but is rather slow)
> 


If you're always trying to get the unique rows, then just use unique():

 > tester
   groups one two
1      A   1   6
2      A   1   6
3      B   2   7
4      B   2   7
5      C   3   8
6      C   3   8
 > unique(tester)
   groups one two
1      A   1   6
3      B   2   7
5      C   3   8

Or use do.call("rbind", ...)

 > do.call("rbind", lapply(split(tester, tester$group),
+  function(x) x[1, ]))
   groups one two
A      1   1   6
B      2   2   7
C      3   3   8
 >

Or just for fun:

 > tester[which(!duplicated(tester$groups)), ]
   groups one two
1      A   1   6
3      B   2   7
5      C   3   8


Regards,
Sundar