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];letmut 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"),];letmut 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());