Here is my example

```
mydf<-data.frame('col_1' = c('A','A','B','B'), 'col_2' = c(100,NA, 90,30))

```

I would like to group by `col_1` and count non-`NA` elements in `col_2`

I would like to do it with `dplyr`. Here is what I tried:

```
mydf %>% group_by(col_1) %>% summarise_each(funs(!is.na(col_2)))
mydf %>% group_by(col_1) %>% mutate(non_na_count = length(col_2, na.rm=TRUE))
mydf %>% group_by(col_1) %>% mutate(non_na_count = count(col_2, na.rm=TRUE))

```

Nothing worked. Any suggestions?