[R] Conditional Random selection

[Ashta]

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?
>
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