[R] Conditional Random selection

[Ashta]

 Thank you Bert!

What I want is at least 500 samples based on random  sampling of time
period. This allows samples  collected at the same time period are
included together.

Your script is doing what I wanted to do!!

Many thanks




On Sat, Nov 21, 2015 at 1:15 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
> 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.