[R] dataframe, simulating data

[Sarah]

Dear all,

I'm having trouble with my dataframe, and I hope someone can help me out...
I have data from 40 subjects, displayed in a dataframe. I have randomly
assigned subjects to group 1 or 0 (mar.y==0 or mar.y==1, with probabilities
used).
In the end, I want 34 cases assigned to group 0, with the rest of the
subjects assigned to group 1. However, if there are more than 34 cases
assigned to group 0 due to the randomness, I would like to keep 34 cases in
group 0 (this is already written in my script below), but with the rest of
the cases assigned to group 1. (Vice versa, if there are less than 34 cases
assigned to group 0, I would like to sample cases from group 1 and put them
in group 0, while retaining the rest of group 1 in my dataframe.)
I can't figure out how to keep 34 cases in group 0, WHILE assigning the rest
of the cases a value 1 (mar.y==1)... 

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)))), ]
}

(I'm aware that using this script is not the most elegant way to solve the
problem, but because this script is part of a larger design, I have to stick
to this example.)

Hope someone has an answer. 
Wishing you a very happy 2011,
Sarah.
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