38  Returning a List of Vectors

TipObjective

Work with an iterator of tuples and turn each into a small character vector.

The query string of a url is a set of key=value pairs. In https://example.com/?a=1&b=2 the pairs are a=1 and b=2. We want to pull these out for each url.

query_pairs() returns tuples

The query_pairs() method gives an iterator of tuples. Each tuple is a (key, value) where both are string-like. We covered tuples this morning. We access the parts by position, or destructure them in a closure.

For each pair we build a two element character vector c(key, value). Each url then becomes a list of those pairs, and the whole thing is a list of lists.

The key and value come back as a Cow<str>, which is a borrowed-or-owned string. We call .to_string() on each to get an owned String we can hand to R.

Exercise

• Write url_query_pairs(url: List) -> List
• For each url, get the pointer with get_extptr
• Map over query_pairs(), destructuring each tuple as (key, value)
• Build a Strings of c(key, value) for each pair
• Collect each url’s pairs into a List
• A failed pointer returns NULL

View solution

#[extendr]
fn url_query_pairs(url: List) -> List {
    url.into_iter()
        .map(|(_, robj)| match RUrl::get_extptr(robj) {
            Ok(rurl) => rurl
                .0
                .query_pairs()
                .map(|(key, value)| {
                    Strings::from_values([
                        Rstr::from(key.to_string()),
                        Rstr::from(value.to_string()),
                    ])
                    .into_robj()
                })
                .collect::<List>()
                .into_robj(),
            Err(_) => ().into_robj(),
        })
        .collect()
}