Programming Rust, 3rd Edition

Chapter 14. Closures

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Save the environment! Create a closure today!

Cormac Flanagan

Sorting a vector of integers is easy:

integers.sort();

It is, therefore, a sad fact that when we want some data sorted, it’s hardly ever a vector of integers. We typically have records of some kind, and the built-in sort method typically does not work:

struct City {
    name: String,
    population: i64,
    country: String,
    ...
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort();  // error: how do you want them sorted?
}

Rust complains that City does not implement std::cmp::Ord. We need to specify the sort order, like this:

/// Helper function for sorting cities by latitude.
fn city_latitude(city: &City) -> i64 {
    city.latitude
}

fn sort_cities(cities: &mut Vec<City>) {
    cities.sort_by_key(city_latitude);  // ok
}

The helper function, city_latitude, takes a City record and extracts the key, the field by which we want to sort our data. The sort_by_key method takes this key-function as a parameter.