[R] dataframe, simulating data

[Sarah]

I'm sorry, I don't think I've made myself clear enough.

Cases have been randomly assigned to one of the two groups, with certain
probabilities (based on other variables).
So, if there are too many people (i.e., more than 34) assigned to group 0, I
would like to sample 34 cases from group 0, and give the rest of the cases a
value 1. My dataframe would contain 40 cases; 34 with mar.y==0 and the rest
given (or some already had) a value mar.y==1.
If, however, too few cases have been assigned to group 0, I need to randomly
select cases from group 1 and put them in group 0 (i.e., give them a value
0). My dataframe would contain the previous selected cases (mar.y==0), PLUS
cases from group 1 who are now assigned to group 0 (mar.y==0), PLUS the
remaining cases who stayed in group 1 (mar.y==1). 
(In other words, how can I change the value for df$mar.y from 1 to 0 or vice
versa for some cases)?

With the script I've designed, only 34 cases would remain in the dataframe
(the cases assigned to group 0)...

if (length(which(df$mar.y==0))>34) { 
df <- df[sample(which(df$mar.y==0),34), ] 
} else { 
df <- df[c(which(df$mar.y==0), 
sample(which(df$mar.y==1),34-length(which(df$mar.y==0)))), ] 
} 

....while 40 cases are needed. 
Thanks for your replies.
Sarah.
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