[R] split the data.frame

[Robert Citek]

On May 15, 2006, at 10:45 PM, YIHSU CHEN wrote:
> I wonder anyone has a elegent way of doing what I need to do.
>
> I have a data frame called with four columns: V1, V2, A1 and A2:
>
> V1  V2  A1   A2
> A    B    1.2  2.0
> A    D    1.2  4.0
> A    C    2.4  2.2
>
> What I need to do is to convert it into the following data frame  
> with a new column x, where x is just the stacked up of A1 and A2  
> placed with respective V1 and V2 in the first two columns:
>
> V1  V2   x
> A    B    1.2
> A    B    2.0
> A    D    1.2
> A    D    4.0
> A    C    2.4
> A    C    2.2
>
> I wonder whether there is an efficient way to do it since I have  
> huge dataset.

How big is huge?  Also, what operating system are you using?

If your data set is really big, i.e. bigger than R can handle in  
memory, then you might want to write the data frame to disk,  
manipulate it there, and then read it back in.

For example:

myDF <- data.frame(V1=rep("A",3), V2=c("B","D","C"), A1=c 
(1.2,1.2,2.4), A2=c(2,4,2.2) )
write.table(subset(myDF,select=c(V1,V2,A1)), file="foo.txt",
   row.name=FALSE, col.names = FALSE)
write.table(subset(myDF,select=c(V1,V2,A2)), file="foo.txt",
   row.name=FALSE, col.names = FALSE, append= TRUE)
newDF <- read.table("foo.txt", col.names=c("V1","V2","x"))
newDF[1:10,]

There's also the operating system solution if using Linux or Cywin/ 
Windows:

myDF <- data.frame(V1=rep("A",3), V2=c("B","D","C"), A1=c 
(1.2,1.2,2.4), A2=c(2,4,2.2) )
write.table(myDF, file="foo.txt", sep="\t", na="",
   quote=FALSE, row.names = FALSE, col.names=FALSE)
system("{ cut -f1,2,3 foo.txt ; cut -f1,2,4 foo.txt ; } > bar.txt")
newDF <- read.table("bar.txt", col.names=c("V1","V2","x"))
newDF[1:10,]

Please post back letting us know what worked for you.

Regards,
- Robert
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