Question

I have an object of type data.frame like this, but much bigger:

> head(mydf)  
   id1     id2   n  
1    0 1032142   3  
2    0 1072163   1  
3    0  119323   2  

I need to print to a file columns a1 and a1, each of them n times. So that I could get a file like that:

0 1032142  
0 1032142  
0 1032142  
0 1072163  
0  119323  
0  119323  

I tried the following solutions, but they make use of explicit for loops and are incredibly slow (it take few days to finish them with my data…):

for (j in 1:(nrow(mydf))) for (i in 1:(mydf[j,"n"])) write.table( mydf[j,c("id1","id2")], file="trials", append=T, row.names= F, col.names=F )

The other tries to build a new data.frame with multiplied rows, but it is even slower to run.

towrite=data.frame(); for (j in 1:(nrow(mydf))) for (i in 1:(mydf[j,"n"])) towrite=rbind(towrite,mydf[j,c("id1","id2")])

What is the simplest and fastest way of resolving this under R?

Answer 1

Try subsetting your data and save in one batch:

mydf[rep(1:nrow(mydf), mydf$n), ]

If your data is numeric, then manipulating the matrix is much faster:

mymat <- as.matrix(mydf)
reps <- as.integer(mydf$n)
mymat[rep(1:nrow(mymat), reps), ]

  id1     id2 n
1   0 1032142 3
1   0 1032142 3
1   0 1032142 3
2   0 1072163 1
3   0  119323 2
3   0  119323 2

If you managed to manipulate your original data.frame, then you will probably be able to handle the above matrix.