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());