Consider adding a drop_na verb

[jankatins]

[forwarded from https://github.com/tidyverse/dplyr/issues/1797 as per @hadley: "This feels more like a tidyr verb to me."]

pandas has the pandas.DataFrame.dropna() which lets you drop rows which have NAs in it but also let you specify subset of columns (-> subset) to be looked at for NAs.

A similar dplyr verb would be

drop_na <- function(data, ...){
    if (missing(...)){
        f = complete.cases(data)
    } else {
        f <- complete.cases(select_(data, .dots = lazyeval::lazy_dots(...)))
    }
    filter(data, f)
}

Examples:

> df <- data.frame(a=c(1,2,3,4,NA), b=c(NA,1,2,3,4), ac=c(1,2,NA,3,4))
> df %>% drop_na(a,b)
  a b ac
1 2 1  2
2 3 2 NA
3 4 3  3
> df %>% drop_na(starts_with("a"))
  a  b ac
1 1 NA  1
2 2  1  2
3 4  3  3
> df %>% drop_na()
  a b ac
1 2 1  2
2 4 3  3

I can clean the above up and submit a PR if such a verb would be considered.

[hadley]

Yeah, I'd be happy to review a PR.