You could avoid the loop to run for ever by introducing a stop() check.
Here is an example using Dr. Savicky's code:

# function to sample B pairs of
# fishes from different families
# -- d has columns fam, born, spawn
foo <- function(d, B){

    # internal function
    foo <- function(d){
        if(length(unique(d[, 'fam'])) < 2) stop('only one family!')
        while (1) {
            ran <- sample(NROW(d), size = 2)
            if (d[ran[1], 1] != d[ran[2], 1]) break
          }
      d[ran, ]
      }

      # sampling B pairs of fishes
      lapply(1:B, function(i) foo(d))
      }

# example:  2 pairs of fishes from different families
foo(fish, 2)

#  data with only one family
ff <- fish[1,]
foo(ff, 2)  # Error in foo(d) : only one family!

HTH,
Jorge.-


On Thu, Mar 22, 2012 at 5:27 PM, Petr Savicky <> wrote:

> On Thu, Mar 22, 2012 at 11:42:53AM -0700, aly wrote:
> > Hi,
> >
> > I want to randomly pick 2 fish born the same day but I need those
> > individuals to be from different families. My table includes 1787 fish
> > distributed in 948 families. An example of a subset of fish born in one
> > specific day would look like:
> >
> > >fish
> >
> > fam   born  spawn
> > 25    46      43
> > 25    46      56
> > 26    46      50
> > 43    46      43
> > 131   46      43
> > 133   46      64
> > 136   46      43
> > 136   46      42
> > 136   46      50
> > 136   46      85
> > 137   46      64
> > 142   46      85
> > 144   46      56
> > 144   46      64
> > 144   46      78
> > 144   46      85
> > 145   46      64
> > 146   46      64
> > 147   46      64
> > 148   46      78
> > 149   46      43
> > 149   46      98
> > 149   46      85
> > 150   46      64
> > 150   46      78
> > 150   46      85
> > 151   46      43
> > 152   46      78
> > 153   46      43
> > 156   46      43
> > 157   46      91
> > 158   46      42
> >
> > Where "fam" is the family that fish belongs to, "born" is the day it was
> > born (in this case day 46), and "spawn" is the day it was spawned. I
> want to
> > know if there is a correlation in the day of spawn between fish born the
> > same day but that are unrelated (not from the same family).
> > I want to randomly select two rows but they have to be from different
> fam.
> > The fist part (random selection), I got it by doing:
> >
> > > ran <- sample(nrow (fish), size=2); ran
> >
> > [1]  9 12
> >
> > > newfish <- fish [ran,];  newfish
> >
> >     fam born spawn
> > 103 136   46    50
> > 106 142   46    85
> >
> > In this example I got two individuals from different families (good) but
> I
> > will repeat the process many times and there's a chance that I get two
> fish
> > from the same family (bad):
> >
> > > ran<-sample (nrow(fish), size=2);ran
> >
> > [1] 26 25
> >
> > > newfish <-fish [ran,]; newfish
> >
> >     fam born spawn
> > 127 150   46    85
> > 126 150   46    78
> >
> > I need a conditional but I have no clue on how to include it in the code.
>
> Hi.
>
> Try the following.
>
>  while (1) {
>    ran <- sample(nrow(fish), size=2)
>    if (fish[ran[1], 1] != fish[ran[2], 1]) break
>  }
>  fish[ran, ]
>
> This will generate only pairs from different families. However,
> note that the loop will run forever, if the data contain only
> fish from one family.
>
> Hope this helps.
>
> Petr Savicky.
>
> ______________________________________________
> R-help@r-project.org mailing list
> 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.
>

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