[R] Conditional Random selection

[David L Carlson]

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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