[R] Conditional Random selection

[Bert Gunter]

David's "solution" is incorrect. It can also fail to give you times
with a total of 500 items to sample from in the time periods.

It is not entirely clear what you want. The solution below gives you a
random sample of time periods in which X1>0 and the total number of
samples among them is >= 500. It does not give you the fewest number
of periods that can do this. Is this what you want?

tab[with(tab,{
  rownums<- sample(seq_len(nrow(tab))[X1>0])
  sz <- cumsum(X2[rownums])
  rownums[c(TRUE,sz<500)]
}),]

Cheers,
Bert


Bert Gunter

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
   -- Clifford Stoll


On Sat, Nov 21, 2015 at 10:56 AM, Ashta <sewashm at gmail.com> wrote:
> Thank you  David!
>
> I rerun the your script and it is giving me the first three time periods
> is it doing random sampling?
>
>       tab.fan
>   time X1  X2
> 2    2  5 230
> 3    3  1 300
> 5    5  2  10
>
>
>
> On Sat, Nov 21, 2015 at 12:20 PM, David L Carlson <dcarlson at tamu.edu> wrote:
>> Use dput() to send data to the list as it is more compact:
>>
>>> dput(tab)
>> structure(list(time = 1:8, X1 = c(0L, 5L, 1L, 0L, 2L, 3L, 1L,
>> 4L), X2 = c(251L, 230L, 300L, 25L, 10L, 101L, 300L, 185L)), .Names = c("time",
>> "X1", "X2"), class = "data.frame", row.names = c(NA, -8L))
>>
>> You can just remove the lines with X1 = 0 since you don't want to use them.
>>
>>> tab.sub <- tab[tab$X1>0, ]
>>
>> Then the following gives you a sample:
>>
>>> tab.sub[cumsum(sample(tab.sub$X2))<=500, ]
>>
>> Note, that your "solution" of times 6, 7, and 8 will never appear because the sum of the values is 586.
>>
>>
>> David L. Carlson
>> Department of Anthropology
>> Texas A&M University
>>
>> -----Original Message-----
>> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Ashta
>> Sent: Saturday, November 21, 2015 11:53 AM
>> To: R help <r-help at r-project.org>
>> Subject: [R] Conditional Random selection
>>
>> Hi all,
>>
>> I have a data set that contains samples collected over time.   In
>> each time period the total number of samples are given (X2)   The goal
>> is to  select 500  random samples.    The selection should be based on
>> time  (select time periods until I reach 500 samples). Also the time
>> period should have greater than 0 for  X1 variable. X1 is an indicator
>> variable.
>>
>> Select "time" until reaching the  sum of X2  is > 500 and if   X1 is  >  0
>>
>> tab  <- read.table(textConnection(" time   X1 X2
>> 1      0        251
>> 2      5        230
>> 3      1        300
>> 4      0         25
>> 5      2         10
>> 6      3         101
>> 7      1         300
>>  8     4         185   "),header = TRUE)
>>
>> In the above example,  samples from time 1 and 4  will not be selected
>> ( X1 is zero)
>> So I could reach my target by selecting time 6,7, and 8 or  time 2 and
>> 3 and so on.
>>
>> Can any one help to do that?
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.