問題描述
既然 std 在 unordered_map 中有一個真正的哈希映射，為什么(或何時)我還想使用舊的 mapunordered_map 在它實際存在的系統(tǒng)上?是否有任何我無法立即看到的明顯情況?
Now that std has a real hash map in unordered_map, why (or when) would I still want to use the good old map over unordered_map on systems where it actually exists? Are there any obvious situations that I cannot immediately see?
推薦答案
As 已經(jīng)提到，map 允許以排序的方式迭代元素，但unordered_map 不允許.這在許多情況下非常重要，例如顯示集合(例如地址簿).這也體現(xiàn)在其他間接方式上，例如:(1) 從 find() 返回的迭代器開始迭代，或 (2) 存在像 lower_bound() 這樣的成員函數(shù).
As already mentioned, map allows to iterate over the elements in a sorted way, but unordered_map does not. This is very important in many situations, for example displaying a collection (e.g. address book). This also manifests in other indirect ways like: (1) Start iterating from the iterator returned by find(), or (2) existence of member functions like lower_bound().
此外，我認為在最壞情況 搜索復雜性方面存在一些差異.
Also, I think there is some difference in the worst case search complexity.
對于
map，是O(lg N)
對于unordered_map，它是O( N ) [當哈希函數(shù)不好導致太多哈希沖突時，可能發(fā)生這種情況.]
For unordered_map, it is O( N ) [This may happen when the hash function is not good leading to too many hash collisions.]
同樣適用于最壞情況 刪除復雜性.
這篇關于在 std::map 和 std::unordered_map 之間進行選擇的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！