It can be useful to not have to worry about missing keys in a map. If a key is requested that doesn't have a value a default value is simply returned. This is exactly what this library provides.
A clear use case of this is when counting the unique elements in a list. Here you want to add one to the existing value in the map for that key. This is a problem for the first addition when there's no value for the key yet. With this library you can specify when creating the map that the default value should be zero.
# use defaultmap::*;
let nums = [1, 4, 3, 3, 4, 2, 4];
let mut counts: DefaultHashMap<i32, i32> = DefaultHashMap::new(0);
// DefaultHashMap::default() is equivalent.
for num in nums.into_iter() {
counts[*num] += 1;
}
println!("{:?}", counts);
// DefaultHashMap { map: {1: 1, 3: 2, 2: 1, 4: 3}, default: 0 }
# assert_eq!(1, counts[1]);
# assert_eq!(1, counts[2]);
# assert_eq!(2, counts[3]);
# assert_eq!(3, counts[4]);Another way the default map can be used is using a map filled with other collections, such as a
a Vec, a HashMap or even another default map.
Next follows some code to create a map where we start with tuples of synonyms and we end with a
map that contains the list of synonyms for each word.
# use defaultmap::*;
let synonym_tuples = [
("nice", "sweet"),
("sweet", "candy"),
("nice", "entertaining"),
("nice", "good"),
("entertaining", "absorbing"),
];
let mut synonym_map: DefaultHashMap<&str, Vec<&str>> = DefaultHashMap::new(vec![]);
// DefaultHashMap::default() is equivalent.
for &(l, r) in synonym_tuples.into_iter() {
synonym_map[l].push(r);
synonym_map[r].push(l);
}
assert_eq!(synonym_map["good"], vec!["nice"]);
assert_eq!(synonym_map["nice"], vec!["sweet", "entertaining", "good"]);
assert_eq!(synonym_map["evil"], Vec::<&str>::new());
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