The std::unordered_map::operator[] is a built in function in C++ STL which returns the reference of value if key matches in the container. If no key is found then it inserts that key into container.
Syntax:
mapped_type& operator[](key_type&& k);
Parameter: It takes parameter as key whose mapped value is accessed.
Return type: Returns a reference associated to that key.
Example 1
// C++ code to illustrate the method // unordered_map operator[] #include <bits/stdc++.h> using namespace std;
int main()
{ unordered_map< int , int > sample;
// Map initialization
sample = { { 1, 2 }, { 3, 4 }, { 5, 6 } };
// print element before doing
// any operations
for ( auto & it : sample)
cout << it.first << " : " << it.second << endl;
// existing element is read
int m = sample[1];
// existing element is written
sample[3] = m;
// existing elements are accessed
sample[5] = sample[1];
// non existing element
// new element 25 will be inserted
m = sample[25];
// new element 10 will be inserted
sample[5] = sample[10];
// print element after doing
// operations
for ( auto & it : sample)
cout << it.first << " : " << it.second << endl;
return 0;
} |
Output:
5 : 6 3 : 4 1 : 2 10 : 0 1 : 2 5 : 0 3 : 2 25 : 0
Example 2
// C++ code to illustrate the method // unordered_map operator[] #include <bits/stdc++.h> using namespace std;
int main()
{ unordered_map< char , int > sample;
// Map initialization
sample = { { 'a' , 2 }, { 'b' , 4 }, { 'c' , 6 } };
// print element before doing
// any operations
for ( auto & it : sample)
cout << it.first << " : " << it.second << endl;
// existing element is read
int m = sample[ 'a' ];
// existing element is written
sample[ 'b' ] = m;
// existing elements are accessed
sample[ 'c' ] = sample[ 'a' ];
// non existing element
// new element 'd' will be inserted
m = sample[ 'd' ];
// new element 'f' will be inserted
sample[ 'c' ] = sample[ 'f' ];
// print element after doing
// operations
for ( auto & it : sample)
cout << it.first << " : " << it.second << endl;
return 0;
} |
Output:
c : 6 b : 4 a : 2 f : 0 a : 2 b : 2 c : 0 d : 0
Time Complexity O(n) in worst case.