why there is no find for vector in C++

Who told you that? There's is "find" algorithm for vector in C++. Ironically Coincidentally, it is called std::find. Or maybe std::binary_search. Or something else, depending on the properties of the data stored in your vector.

Containers get their own specific versions of generic algorithms (implemented as container methods) only when the effective implementation of the algorithm is somehow tied to the internal details of the container. std::list<>::sort would be one example.

In all other cases, the algorithms are implemented by standalone functions.

Having a 'find' functionality in the container class violates 'SRP' (Single Responsibility Principle). A container's core functionality is to provide interfaces for storage, retrieval of elements in the container. 'Finding', 'Sorting', 'Iterating' etc are not core functionality of any container and hence not part of it's direct interface.

However as 'Herb' states in Namespace Principle, 'find' is a part of the interface by being defined in the same namespace as 'vector' namely 'std'.

There is the std::find() algorithm, which performs a linear search over an iterator range, e.g.,

std::vector<int> v;

// Finds the first element in the vector that has the value 42:
// If there is no such value, it == v.end()
std::vector<int>::const_iterator it = std::find(v.begin(), v.end(), 42);

If your vector is sorted, you can use std::binary_search() to test whether a value is present in the vector, and std::equal_range() to get begin and end iterators to the range of elements in the vector that have that value.

The reason there is no vector::find is because there is no algorithmic advantage over std::find (std::find is O(N) and in general, you can't do better for vectors).

But the reason you have map::find is because it can be more efficient (map::find is O(log N) so you would always want to use that over std::find for maps).