sample within a loop

Dear R users,
                My data frame has four "groups" namely A1, B2, C3, & D4.  Each group has 12 rows (variable "plotno).  I like to randomly sample one "plotno" within each "groups" variable and label it as "CONTROL" and label others as "TEST" in a new variable called "entry".  I am trying to do this by looping over the group variable and then sample "plotno" within a given group.  I am ending up with four "CONTROL" plots but they are generated by sampling over all the groups instead of each group.  I need one random "plotno" assigned as a "CONTROL" per group (A1, B2, C3, D4).  I would appreciate any help in modifying my function "funa" or suggest any alternative and better way to do this task.  Below is the dataset and function I am working with.

# dataset (df)
structure(list(plotno = 1:48, groups = c("A1", "A1", "A1", "A1",
"A1", "A1", "A1", "A1", "A1", "A1", "A1", "A1", "B2", "B2", "B2",
"B2", "B2", "B2", "B2", "B2", "B2", "B2", "B2", "B2", "C3", "C3",
"C3", "C3", "C3", "C3", "C3", "C3", "C3", "C3", "C3", "C3", "D4",
"D4", "D4", "D4", "D4", "D4", "D4", "D4", "D4", "D4", "D4", "D4"
)), .Names = c("plotno", "groups"), row.names = c(NA, -48L), class = "data.frame")

# function (funa)

function (dataset)

{

    set.seed(1)

    bay <- unique(dataset$groups)

    IND <- c()

    df2 <- dataset

    for (i in bay) {

        IND[i] <- which(plotno %in% sample(plotno, 1))

        df2$entry <- ifelse(df2$plotno %in% IND, "CONTROL", "TEST")

    }

    df2

}


# session info

R version 3.2.1 (2015-06-18)

Platform: i386-w64-mingw32/i386 (32-bit)

Running under: Windows 7 x64 (build 7601) Service Pack 1



locale:

[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252

[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252



attached base packages:

[1] stats     graphics  grDevices utils     datasets  methods   base



loaded via a namespace (and not attached):

[1] tools_3.2.1

Thanks.
Nilesh


Nilesh Dighe
(806)-252-7492 (Cell)
(806)-741-2019 (Office)


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