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I'm wondering in which case I should use unordered_map instead of std::map.
I have to use unorderd_map each time I don't pay attention of order of element in the map ?
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map is used to store elements as key,value pairs in sorted order. unordered_map is used to store elements as key,value pairs in non-sorted order.
std::map Internally store elements in a balanced BST. Therefore, elements will be stored in sorted order of keys. std::unordered_map store elements using hash table. Therefore, elements will not be stored in any sorted order.
Therefore, the optimized access with std::map is about 20% faster, but the access time of std::unordered_map about 6 times faster.
Generally, an unordered_map in C++ is faster than map in C++ because the average time complexity for insertion, deletion, and updation is O(1) while in the case of map, the average time complexity for all the operations is O(log(n)) where n is the number of elements present inside the map.
mapunordered_map
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Compare hash table (undorded_map) vs. binary tree (map), remember your CS classes and adjust accordingly.
The hash map usually has O(1) on lookups, the map has O(logN). It can be a real difference if you need many fast lookups.
The map keeps the order of the elements, which is also useful sometimes.
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