unordered_map operator[] in C++ STL (original) (raw)

Last Updated : 14 Dec, 2018

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

CPP `

// 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

CPP `

// 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.