R help - Jun 2011 - Random Selection of numbers

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.

Thank you

Best regards
Ogbos
1 703116
2 243714
3 297060
4 307697
5 296588
6 255266
7 297116
8 291530
9 239259
10 239126
11 212396
12 202471
13 227833
14 212977
15 207408
16 228564
1 230414
2 15372
3 19647
4 29523
5 26234
6 34766
7 16738
8 25215
9 20757
10 31250
11 27993
12 24441
13 19853
14 20751
15 7658
16 5934

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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.