s<-c("y","n")

da<-sample(s,20,replace=T)

n<-rep(0,length(da))

for(i in seq_along(da)) {

ifelse(da[1]=="y",n[1]<-1,n[1]<-0)

if(i==1) next

ifelse(da[i]=="y",n[i]<-n[i-1]+1,n[i]<-n[i-1])

}

n