Home > Custom Function > A quick way to do row repeat and col repeat (rep.row, rep.col)

A quick way to do row repeat and col repeat (rep.row, rep.col)

Today I worked on a simulation program which require me to create a matrix by repeating the vector n times (both by row and by col).

Even the task is extremely simple and only take 1 line to finish(10sec), I have to think about should the argument in rep be each or times and should the argument in matrix is nrow or ncol. It distracted me from the original task i am working on.

Just now, I wrote a function rep.row and rep.col to do what I really want to do. Next time, i don’t have to worry about how to use the matrix and rep command to repeat an vector to form a matrix!

Code

rep.row<-function(x,n){
   matrix(rep(x,each=n),nrow=n)
}
rep.col<-function(x,n){
   matrix(rep(x,each=n), ncol=n, byrow=TRUE)
}

x is the vector to be repeated and n is the number of replication. Example:

> rep.row(1:3,5)
      [,1] [,2] [,3]
[1,]    1    2    3
[2,]    1    2    3
[3,]    1    2    3
[4,]    1    2    3
[5,]    1    2    3
> rep.col(1:3,5)
      [,1] [,2] [,3] [,4] [,5]
[1,]    1    1    2    2    3
[2,]    1    1    2    3    3
[3,]    1    2    2    3    3

I am sure it should appear in some packages, but it would be faster for me to write it out than find it out!

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Categories: Custom Function
  1. March 12, 2012 at 13:01

    It looks like rep.col doesn’t give quite what you are looking for. I assume that what you want is given by this:

    matrix(rep(x,each=n), ncol=n, byrow=TRUE)

    Cheers,
    Derek

  2. March 12, 2012 at 15:00

    Right! I should do a test before i put it here!
    Sorry for that.

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