Traversing a Map and unordered_map in C++ STL (original) (raw)

The maps are described as mapped associative containers for elements where each element has a key and value assigned to it. Another form of map container seen in the C++ STL is the unordered map. It is the same as map containers just that they don't store the data in sorted order.

We can traverse map and unordered_map using 4 different ways which are as follows:

1. Using a **ranged based **for loop
2. Using **begin() and **end()
3. Using **Iterators
4. Using **std::for_each and **lambda function

**1. Using a Range Based **for **Loop

We can use a range-based _for loop to iterate over a map or an unordered_map in C++.

**Example:

map `

// C++ program to traverse a map using range // based for loop #include <bits/stdc++.h> using namespace std;

int main() { int arr[] = { 1, 1, 2, 1, 1, 3, 4, 3 }; int n = sizeof(arr) / sizeof(arr[0]);

  // inserting elements
map<int, int> m;
for (int i = 0; i < n; i++)
    m[arr[i]]++;

// Printing of MAP
cout << "Element  Frequency" << endl;
for (auto i : m)
    cout << i.first << " \t\t\t " << i.second << endl;

return 0;

}

unordered\_map

// C++ program to traverse a unordered_map using // range based for loop #include <bits/stdc++.h> using namespace std;

int main() { int arr[] = { 1, 1, 2, 1, 1, 3, 4, 3 }; int n = sizeof(arr) / sizeof(arr[0]);

unordered_map<int, int> m;
for (int i = 0; i < n; i++)
    m[arr[i]]++;

// Printing of Unordered_MAP
cout << "Element Frequency" << endl;
for (auto i : m)
    cout << i.first << "    " << i.second << endl;

return 0;

}

`

Output

Element Frequency 1 4 2 1 3 2 4 1

**Note: The above output is for map only. For unordered_map, the output rows can be in any order.

**2. Traversing using **begin() **and **end()

The begin() and end() are the member functions of container classes that return the iterator to the first and the last key-value pair in a map or unordered_map respectively. We can use them to traverse over a map or an unordered_map.

**Example:

map `

// C++ program to traverse a map using begin() and end() #include <bits/stdc++.h> using namespace std;

int main() { int arr[] = { 1, 1, 2, 1, 1, 3, 4, 3 }; int n = sizeof(arr) / sizeof(arr[0]);

// inserting data to map map<int, int> m; for (int i = 0; i < n; i++) m[arr[i]]++;

// Printing of MAP
cout << "Element  Frequency" << endl;
for (auto i = m.begin(); i != m.end(); i++)
    cout << i->first << " \t\t\t" << i->second << endl;

return 0;

}

unordered\_map

// C++ program to traverse a unordered_map // using begin() and end() #include <bits/stdc++.h> using namespace std;

int main() { int arr[] = { 1, 1, 2, 1, 1, 3, 4, 3 }; int n = sizeof(arr) / sizeof(arr[0]);

unordered_map<int, int> m;
for (int i = 0; i < n; i++)
    m[arr[i]]++;

// Printing of Unordered_MAP
cout << " Element Frequency" << endl;
for (auto i = m.begin(); i != m.end(); i++)
    cout << i->first << "       " << i->second << endl;

return 0;

}

`

Output

Element Frequency 1 4 2 1 3 2 4 1

**3. Iterating over a map by using an STL Iterator

We can create an iterator of std::map (or std::unordered_map), initialize it to the start of the map, and increment it till the end to traverse the map.

**Example:

C++ `

// C++ program to traverse a map using Iterators() #include #include #include #include #include

int main() { // Map created std::map<std::string, int> ExampleMap;

// elements are inserted into map
ExampleMap.insert(
    std::pair<std::string, int>("Sunday", 1));
ExampleMap.insert(
    std::pair<std::string, int>("Monday", 2));
ExampleMap.insert(
    std::pair<std::string, int>("Tuesday", 3));
ExampleMap.insert(
    std::pair<std::string, int>("Wednesday", 4));
ExampleMap.insert(
    std::pair<std::string, int>("Thursday", 5));
ExampleMap.insert(
    std::pair<std::string, int>("Friday", 6));
ExampleMap.insert(
    std::pair<std::string, int>("Saturday", 7));

// map iterator created
// iterator pointing to start of map
std::map<std::string, int>::iterator it
    = ExampleMap.begin();

// Iterating over the map using Iterator till map end.
while (it != ExampleMap.end()) {
    // Accessing the key
    std::string word = it->first;
    // Accessing the value
    int count = it->second;
    std::cout << word << " :: " << count << std::endl;
    // iterator incremented to point next item
    it++;
}
return 0;

}

`

Output

Friday :: 6 Monday :: 2 Saturday :: 7 Sunday :: 1 Thursday :: 5 Tuesday :: 3 Wednesday :: 4

**4. Iterating over a map by using std::for_each and lambda function

By using the std::for_each algorithm and lambda functions we can easily iterate over all the elements of the map. Here the lambda function will be used as a call-back function and will receive each map entry.

**Example:

map `

// C++ program to traverse a map using // std::for_each() and lambda() #include #include #include #include #include

int main() { // Map created std::map<std::string, int> ExampleMap;

// elements are inserted into map
ExampleMap.insert(
    std::pair<std::string, int>("Sunday", 1));
ExampleMap.insert(
    std::pair<std::string, int>("Monday", 2));
ExampleMap.insert(
    std::pair<std::string, int>("Tuesday", 3));
ExampleMap.insert(
    std::pair<std::string, int>("Wednesday", 4));
ExampleMap.insert(
    std::pair<std::string, int>("Thursday", 5));
ExampleMap.insert(
    std::pair<std::string, int>("Friday", 6));
ExampleMap.insert(
    std::pair<std::string, int>("Saturday", 7));

// map iterator created
// iterator pointing to start of map
std::map<std::string, int>::iterator it
    = ExampleMap.begin();

// Iterating over the map till map end.
std::for_each(
    ExampleMap.begin(), ExampleMap.end(),
    [](std::pair<std::string, int> p) {
      
        std::cout << p.first << " :: " << p.second
                  << std::endl;
    });
return 0;

}

unordered\_map

// C++ program to traverse a unordered_map using // std::for_each() and lambda() #include #include #include #include #include

int main() { // Map created std::unordered_map<std::string, int> ExampleMap;

// elements are inserted into unordered_map
ExampleMap.insert(
    std::pair<std::string, int>("Sunday", 1));
ExampleMap.insert(
    std::pair<std::string, int>("Monday", 2));
ExampleMap.insert(
    std::pair<std::string, int>("Tuesday", 3));
ExampleMap.insert(
    std::pair<std::string, int>("Wednesday", 4));
ExampleMap.insert(
    std::pair<std::string, int>("Thursday", 5));
ExampleMap.insert(
    std::pair<std::string, int>("Friday", 6));
ExampleMap.insert(
    std::pair<std::string, int>("Saturday", 7));

// unordered_map iterator created
// iterator pointing to start of unordered_map
std::unordered_map<std::string, int>::iterator it
    = ExampleMap.begin();

// Iterating over the unordered_map till unordered_map
// end.
std::for_each(ExampleMap.begin(), ExampleMap.end(),
              [](std::pair<std::string, int> p) {
                  std::cout << p.first
                            << " :: " << p.second
                            << std::endl;
              });
return 0;

}

`

Output

Friday :: 6 Monday :: 2 Saturday :: 7 Sunday :: 1 Thursday :: 5 Tuesday :: 3 Wednesday :: 4