[R] Conditional Random selection

[Ashta]

Hi  Rui ,

I tried that one  before I send out my original message.
it gave me only this,

tapply(tab$S1, tab$time, function(x) length(unique(x)))
1 2 3
2 1 3

I am expecting an output of like this

 time  S1
    1    2
    2    1
    3    3






On Sat, Nov 21, 2015 at 2:38 PM,  <ruipbarradas at sapo.pt> wrote:
> Hello,
>
> Try
>
> tapply(tab$S1, tab$time, function(x) length(unique(x)))
>
> Hope this helps,
>
> Rui Barradas
>
>
> Citando Ashta <sewashm at gmail.com>:
>
> Hi  Bert  and all,
> I have related question.  In each  time period there were different
> locations where the samples were collected (S1).   I  want count  the
> number of unique locations (S1)  for each unique time period . So in
> time 1 the samples were collected from two locations and time 2 only
> from one location and time 3  from  three locations..
>
> tab  <- read.table(textConnection(" time   S1  rep
> 1      1       1
> 1      2       1
> 1      2       2
> 2      1       1
> 2      1       2
> 2      1       3
> 2      1       4
> 3      1       1
> 3      2       1
> 3      3       1   "),header = TRUE)
>
> what I want is
>
> time  S1
>    1    2
>    2    1
>    3    3
>
> Thank you again.
>
>
>
> On Sat, Nov 21, 2015 at 1:30 PM, Ashta <sewashm at gmail.com> wrote:
>
> 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.
>
> ______________________________________________
> 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.htmland provide commented, minimal,
> self-contained, reproducible code.
>
>
>