R help - Jun 2011 - Missing completely at random

Hello!
I would like to sample 5 % of cases and from 1 to 3 variables within selected 
cases and set them as NA (MCAR-Missing completely at random). I managed to 
sample cases and variables, but I don’t know how to set them as NA.
R code:
N <- 1000      ####number of cases 
n <- 12           ####number of variables
X <- matrix(rnorm(N * n), N, n)    ####matrix
pMiss <- 0.05     ####percent of missing values
idMiss <- sample(1:N, N * pMiss)    ####sample cases
nMiss <- length(idMiss)
m <- 3    ####maximum number of missing variables within selected cases
howmanyMiss <- sapply(idMiss, function(x) sample(1:m, 1))
howmanyMiss
lapply(howmanyMiss, function(x) sample(1:n, x))
 
Any suggestion?
Thanks,
Blaž
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This works, but there might be a better way:

misscols<-lapply(howmanyMiss, function(x) sample(1:n, x))

for (i in 1:nMiss){
	for (j in misscols[[i]]){
		X[idMiss[i],j]<-NA
	}
}