r - Repeat vector to fill down column in data frame

Question

Seems like this very simple maneuver used to work for me, and now it simply doesn't. A dummy version of the problem:

df <- data.frame(x = 1:5) # create simple dataframe
df
  x
1 1
2 2
3 3
4 4
5 5

df$y <- c(1:5) # adding a new column with a vector of the exact same length. Works out like it should
df
 x y
1 1 1
2 2 2
3 3 3
4 4 4
5 5 5

df$z <- c(1:4) # trying to add a new colum, this time with a vector with less elements than there are rows in the dataframe.

Error in `$<-.data.frame`(`*tmp*`, "z", value = 1:4) : 
  replacement has 4 rows, data has 5

I was expecting this to work with the following result:

 x y z
1 1 1 1
2 2 2 2
3 3 3 3
4 4 4 4
5 5 5 1

I.e. the shorter vector should just start repeating itself automatically. I'm pretty certain this used to work for me (it's in a script that I've been running a hundred times before without problems). Now I can't even get the above dummy example to work like I want to. What am I missing?

Answer 1

If the vector can be evenly recycled, into the data.frame, you do not get and error or a warning:

df <- data.frame(x = 1:10)
df$z <- 1:5

This may be what you were experiencing before.

You can get your vector to fit as you mention with rep_len:

df$y <- rep_len(1:3, length.out=10)

This results in

df
    x z y
1   1 1 1
2   2 2 2
3   3 3 3
4   4 4 1
5   5 5 2
6   6 1 3
7   7 2 1
8   8 3 2
9   9 4 3
10 10 5 1

Note that in place of rep_len, you could use the more common rep function:

df$y <- rep(1:3,len=10)

From the help file for rep:

rep.int and rep_len are faster simplified versions for two common cases. They are not generic.