[R] Simulating data, loop

[Sarah]

Hello,

I would like to run a script in which a loop is included. Since I'm new to
R, I cannot manage the following problem. I really hope someone could help
me out.

Data in the variable Y should be removed from the simulated data set with
probability 0.50 if the variable X has a value below zero, and with
probability 0.10 if X has a value above zero (see script).
However, the total number of removed values from Y should be 20 when X < 0,
and 4 when X>0. Whenever the total number of removed values is less than 20
(when X<0), R should remove more values from Y at random (until a total of
20 values has been removed). When R has removed more than 20 values from Y,
R should restore some removed values such that a maximum of 20 values has
been removed from variable Y. 

How can I tell R to put some removed values back in the data set, or to
remove more values until a maximum number of removed values has been
reached?

y <- rnorm(40,1,3)
x <- 1+2*y1+ rnorm(40,0,5)
#Remove values in Y dependent on X:
ynew <- rep(NA,40)
for (j in 1:40)
{
if (x[j] < 0){ynew[j] <- rbinom(1,1,0.50)}
if (x[j] > 0){ynew[j] <- rbinom(1,1,0.10)}
}
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