Function count_by 

pub fn count_by<T, K, F>(items: &[T], key_of: F) -> HashMap<K, usize>
where
    K: Hash + Eq,
    F: Fn(&T) -> K,

按指定键函数统计元素出现次数/Count elements by key function

参数/Arguments

• items - 要统计的数组/The array to count
• key_of - 生成统计键的函数/Function to generate key

返回值/Returns

HashMap<K, usize> 键为统计键，值为出现次数/Key is the grouping key, value is count

示例/Examples

use slice_reducer::count_by;
let words = ["hello", "world"];
let counts = count_by(&words, |s| s.len());
assert_eq!(counts[&5], 2);