public member function
<unordered_map>

std::unordered_map::at

mapped_type& at ( const key_type& k );
const mapped_type& at ( const key_type& k ) const;
Access element
Returns a reference to the mapped value of the element with key k in the unordered_map.

If k does not match the key of any element in the container, the function throws an out_of_range exception.

Parameters

k
Key value of the element whose mapped value is accessed.
Member type key_type is the keys for the elements in the container. defined in unordered_map as an alias of its first template parameter (Key).

Return value

A reference to the mapped value of the element with a key value equivalent to k.

Member type mapped_type is the type of the mapped values in the container, defined in unordered_map as an alias of its second template parameter (T).

Example

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// unordered_map::at
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_map<std::string,int> mymap = {
                { "Mars", 3000},
                { "Saturn", 60000},
                { "Jupiter", 70000 } };

  mymap.at("Mars") = 3396;
  mymap.at("Saturn") += 272;
  mymap.at("Jupiter") = mymap.at("Saturn") + 9638;

  for (auto& x: mymap) {
    std::cout << x.first << ": " << x.second << std::endl;
  }

  return 0;
}


Possible output:
Saturn: 60272
Mars: 3396
Jupiter: 69910

Complexity

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

Iterator validity

No changes.

See also

unordered_map::operator[]
Access element (public member function )
unordered_map::find
Get iterator to element (public member function)

C++