On Thu, Jun 02, 2011 at 09:56:51AM +0200, ogbos okike wrote:
> Hello,
> I am attempting to randomly select a data of equal length from my dataset.
> My dataset is of equal length each ranging from 1 to 16 rows. Since they are
> of equal length, I can form a matrix of equal length and rows or concatenate
> them into a data of 16n x 2 matrix where n is number of samples. I have
> reproduced small part of the data below.
>
> Now the problem is how to select as many samples of the same length (i.e.
> 16 rows) as possible from the two dataset below. If the first is taken as
> X1 and the second as X2, manually selecting from the 4th row of X1 to 3rd
> row of X2 gives a data of length 16, from 5th row of X1 to 4th row of X2
> gives a data of length 16, etc. This implies choosing any row from X1 and
> counting 15 rows down from that to get 16 rows. I can then concatenate these
> new samples to the original sample and sort them out to do my work.
>
> Doing this random selection manually when my dataset becomes larger may not
> be good. I will be obliged should anyone suggests how I can do this in R.
Hello.
Let me put your data as an R command for simplicity.
x <- c(703116, 243714, 297060, 307697, 296588, 255266, 297116,
291530, 239259, 239126, 212396, 202471, 227833, 212977, 207408,
228564, 230414, 15372, 19647, 29523, 26234, 34766, 16738, 25215,
20757, 31250, 27993, 24441, 19853, 20751, 7658, 5934)
a <- cbind(rep(1:16, times=2), x)
If i is the starting index, is the following, what you ask for?
i <- 4
a[i:(i+15), ]
x
[1,] 4 307697
[2,] 5 296588
[3,] 6 255266
[4,] 7 297116
[5,] 8 291530
[6,] 9 239259
[7,] 10 239126
[8,] 11 212396
[9,] 12 202471
[10,] 13 227833
[11,] 14 212977
[12,] 15 207408
[13,] 16 228564
[14,] 1 230414
[15,] 2 15372
[16,] 3 19647
The index i may be chosen at random for example as
i <- sample(1:16, 1)
This allows to get 16 different samples or, perhaps, 17
if we can start at the first row of the second datset.
I am not sure, whether you can consider also other types
of subsets to increase the number of different samples.
For example, the following selects 16 rows at random
a[sort(sample(1:32, 16)), ]
Hope this helps.
Petr Savicky.
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