How to choose between map and unordered_map?

Question

Suppose I wanted to map data with a string as the key. What container should I have chosen, map or unordered_map? unordered_map takes

Answer 1

What container should I have chosen, map or unordered_map? unordered_map takes up more memory so let's suppose memory isn't an issue, and the concern is speed.

Profile and then decide. unordered_map is generally faster, but it varies per case.

In what cases would it get to O(n)?

When the hashing isn't good and a bunch of elements are being assigned to the same bins.

When does a map get more time efficient than unordered_map? Does it happaen when n is small?

Probably not, but profile it if you really care. Having a container with a small size be the bottleneck of your program seems extremely unlikely. Anyway, a simple vector with linear search may be faster for such cases.

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The most important thing when deciding is the requirements of ordering and lack of iterator invalidation. If you need either, you pretty much have to use map. Otherwise, unordered_map.