[R] Adding NAs to data.frame

[soeren.vogel at eawag.ch]

Hello, after the creation of a data.frame I like to add NAs as follows:

n <- 743;
x <- runif(n, 1, 7);
Y <- runif(n, 1, 7);
Ag6 <- runif(n, 1, 7);
df <- data.frame(x, Y, Ag6);
# a list with positions:
v <- apply(df, 2, function(x) sample(n, sample(1:ceiling(5*n/100), 1), repl=F));
# a loop too much?
for (i in 1:length(df)){
  df[unlist(v[i]), i] <- NA;
}
summary(df);

This works fine but it appears to me that there is a more elegant and simple way -- which one?

Thanks
Sören