[R] Split data frame into 250-row chunks

[David Winsemius]

On Jun 10, 2015, at 5:39 AM, Liz Hare wrote:

> 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).

split( map, trunc( 0:(nrow(map)-1 )/nrow(map) ) )

Untested. Designed to return a list with indices starting at "0".

> trunc( 0:19/5)
 [1] 0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3



> 
> 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]]
> 
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David Winsemius
Alameda, CA, USA