[R] Randomly select elements based on criteria

[aly]

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.
Thanks in advance for any suggestions,

Aly

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