使用整数键和向量的映射之间的差异

Question

Is vector the specialized form of unordered_map with integer key? It seems so because a vector has integer keys, too.

If not, what are the differences?

Answer 1

Vector is a container around an array.

An unordered map is a container around a binary tree.

That means they have different performance characteristics. Some examples:

• Accessing an element by index is a constant-time operation in a vector/array, but a logarithmic time operation in a binary tree. Both are cheap, but on a vector it's even faster.
• Increasing the capacity of a vector is expensive (linear time), but cheap for a binary tree (logarithmic time).

Answer 2

The main difference is in how the data is stored.

A vector stores the data in an internal array that it resizes and you add more elements. An unordered_map uses a hash table internally.

Practically, a vector gives you amortized constant time insertions at the back (it needs to resize and copy/move everything once in a while), constant time access by index, and up to linear time insertion and deletion (all the subsequent elements have to be shifted). Also, since a vector is contiguous, you can pass it into functions expecting a c-style array.

unordered_map gives you amortized constant time lookup by key (because hashing is not perfect, and collisions force the lookup to traverse through internal linked lists), amortized constant time inserts and deletes.

See: http://en.cppreference.com/w/cpp/container/unordered_map and: http://en.cppreference.com/w/cpp/container/vector

Answer 3

Nope, indexes in a vector are continuous, in a map they don't have to be.

Also, values in a vector are guaranteed to be in continuous memory, not in a map .

These two imply different complexities for most operations on the two.