36  Combining iterators

TipObjective

Combine two vectors element-by-element using .zip().

Previously we only used a scalar string value for our methods that modified the url struct.

Typically we want to be able to take EITHER a scalar string and apply it to every element of the first argument in a vectorized manner OR a vector that is the same length as the first argument.

.zip()

To do this effectively we can combine iterators using .zip(). It pairs up the elements of two iterators, one at a time, into a tuple.

url.into_iter().zip(host.into_iter())

Each step of the combined iterator gives us a tuple of ((name, url), host). We destructure it in the closure and ignore the name with _.

.zip() works best when both vectors are the same length. For now we’ll take a host that is the same length as url. In the next chapter we’ll handle the case where they aren’t.

Exercise

• Write url_set_host(url: List, host: Strings) -> List
• Zip the urls together with the hosts
• If the url is NULL or the host is NA, return NULL for that element
• Otherwise clone the url with clone_from_robj, set the host, and return the new object
• Collect into a List

For now, assume url and host are the same length.

View solution

#[extendr]
fn url_set_host(url: List, host: Strings) -> List {
    url.into_iter()
        .zip(host.into_iter())
        .map(|((_, url), host)| {
            if url.is_null() || host.is_na() {
                return ().into_robj();
            }

            match RUrl::clone_from_robj(url) {
                Ok(mut rurl) => match rurl.0.set_host(Some(host)) {
                    Ok(_) => rurl.into_robj(),
                    Err(_) => ().into_robj(),
                },
                Err(_) => ().into_robj(),
            }
        })
        .collect()
}