C++ unordered_map - at() Function

The C++ unordered_map::at function returns a reference to the mapped value of element identified with specified key. If the specified key does not match with key of any element of the unordered_map, the function throws an out_of_range exception.

Note: The unordered_map::at function produces the same result as operator[] function, except operator[] does not throw an out_of_range exception if the specified key does not match with key of any element of the unordered_map.

Syntax

• C++11

mapped_type& at (const key_type& k);
const mapped_type& at (const key_type& k) const;

Parameters

Time Complexity

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

Example:

In the example below, the unordered_map::at function is used to get the mapped value of the specified key in uMap.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap;
  unordered_map<int, string>::iterator it;

  //populating uMap
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";

  //changing the mapped value for key=103
  uMap.at(103) = "Ramesh";

  //accessing mapped value of specified key
  cout<<"uMap.at(101) = "<<uMap.at(101)<<"\n";
  cout<<"uMap.at(102) = "<<uMap.at(102)<<"\n";
  cout<<"uMap.at(103) = "<<uMap.at(103)<<"\n";

  return 0;
}

The output of the above code will be:

uMap.at(101) = John
uMap.at(102) = Marry
uMap.at(103) = Ramesh