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Showing content from https://cplusplus.com/reference/unordered_set/unordered_set/find/ below:

public member function

<unordered_set>

std::unordered_set::find 
      iterator find ( const key_type& k );const_iterator find ( const key_type& k ) const;

Get iterator to element

Searches the container for an element with k as value and returns an iterator to it if found, otherwise it returns an iterator to unordered_set::end (the element past the end of the container).

Another member function, unordered_set::count, can be used to just check whether a particular element exists.

All iterators in an unordered_set have const access to the elements (even those whose type is not prefixed with const_): Elements can be inserted or removed, but not modified while in the container.



Parameters
k
Key to be searched for.
Member type key_type is the type of the elements in the container. In unordered_set containers it is the same as value_type, defined as an alias of the class's first template parameter (Key).

Return value An iterator to the element, if the specified value is found, or unordered_set::end if it is not found in the container.

Member types iterator and const_iterator are forward iterator types. Both may be aliases of the same iterator type.



Example 
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// unordered_set::find
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_set<std::string> myset = { "red","green","blue" };

  std::string input;
  std::cout << "color? ";
  getline (std::cin,input);

  std::unordered_set<std::string>::const_iterator got = myset.find (input);

  if ( got == myset.end() )
    std::cout << "not found in myset";
  else
    std::cout << *got << " is in myset";

  std::cout << std::endl;

  return 0;
}

Possible output: 
color? blue
blue is in myset


Complexity Average case: constant.
Worst case: linear in container size.

Iterator validity No changes.

See also
unordered_set::count
Count elements with a specific key (public member function)
unordered_set::equal_range
Get range of elements with a specific key (public member function)
unordered_set::begin
Return iterator to beginning (public member type)

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