[R] Split data frame into 250-row chunks

[Liz Hare]

Hi R-Experts,

I have a data.frame like this:

> head(map)
  chr snp   poscm   posbp    dist
1   1  M1 2.99043 3249189      NA
2   1  M2 3.06457 3273096 0.07414
3   1  M3 3.17018 3307151 0.10561
4   1  M4 3.20892 3319643 0.03874
5   1  M5 3.28120 3342947 0.07228
6   1  M6 3.29624 3347798 0.01504

I need to split this into chunks of 250 rows (there will usually be a last chunk with < 250 rows).

If I only had to extract one 250-line chunk, it would be easy:

map1 <- map[1:250, ]

or using subset().

I tried to make it a loop iterating through num and using beg and nd for starting and ending indices, but I couldn’t figure out how to reference all the variables I needed in this:

> chunks
    beg   nd let num
1     1  250   a   1
2   251  500   b   2
3   501  750   c   3
4   751 1000   d   4
5  1001 1250   e   5
6  1251 1500   f   6
7  1501 1750   g   7
8  1751 2000   h   8
9  2001 2250   i   9
10 2251 2500   j  10
…

Remembering that loops are not always the best answer in R, I looked at other options like split, following this example but not being able to adapt it from a vector to a data.frame version
http://stackoverflow.com/questions/3318333/split-a-vector-into-chunks-in-r <http://stackoverflow.com/questions/3318333/split-a-vector-into-chunks-in-r> (Yes, I’ve reviewed the language documentation). I checked out ddply and data.table, but couldn’t find a way to use them with index positions instead of column values.

Thanks,
Liz



	[[alternative HTML version deleted]]