Question

How can I use replication function in the following situation:

mydata <- data.frame (var1 = 1:4, replication = c(3, 5, 3, 7))

mydata
  var1 replication
1    1           3
2    2           5
3    3           3
4    4           7

I want a result like this:

1,1,1, 2,2,2,2,2,  3,3,3,  4,4,4,4,4,4,4 

1 is repeated 3 times, 2 for 5 times and so on

I tried apply function, do not do any thing good.

apply (mydata,2,rep )

Answer 1

You were on the right track: rep is the function for the job. But try this instead:

with(mydata, rep(var1, replication))

The way you’re using apply here, rep is being called on each of the columns individually. It would be like calling rep(1:4), then rep(c(3,5,3,7)), and combining the results into a matrix.

apply is a great function to be familiar with, but it isn’t the tool for this job. In fact, a solution to this using apply would be pretty ugly:

unlist(apply(mydata, 1, function(var1.rep) do.call(rep, unname(as.list(var1.rep)))))

@MatthewLundberg demonstrates the appropriate way to do this with apply in the comments.