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

// 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