make_nas<-function(data, number, keep_n=2){
new.data<-data
no.na<-which(!is.na(data))
indexes<-matrix(data=c(1:(length(data)*nrow(data))), nrow=nrow(data))
keepers<-character()
for(j in 1:ncol(data)){
search<-no.na[no.na %in% indexes[,j]]
keep<-sample(search, 2)
keepers<-append(keepers, keep)
}
to.change<-sample(no.na[! no.na %in% keepers], number)
to.change.ind<-lapply(to.change, function(x) {which(indexes==x, arr.ind=TRUE)})
to.change.ind<-matrix(unlist(to.change.ind), ncol=2, byrow=TRUE)
for(i in 1:nrow(to.change.ind)){
rownum<-to.change.ind[i,1]
colnum<-to.change.ind[i,2]
new.data[rownum, colnum]<-NA
}
return(new.data)
}
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