Overview of C++ STL std::map

std::map is an associative container in the C++ Standard Template Library (STL) that stores elements in key-value pairs. It maintains the keys in sorted order using a binary search tree (typically a red-black tree).

Key Features:

  1. Key-Value Pair: Each element is a combination of a unique key and a corresponding value.
  2. Sorted Order: Keys are always sorted in ascending order by default.
  3. Unique Keys: Each key must be unique.
  4. Efficient Operations:
    • Insertion, deletion, and search have O(log n) complexity.
  5. Custom Comparator: You can define a custom comparator for sorting keys.

Basic Syntax

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    map<string, int> ageMap;

    // Insert elements
    ageMap["Alice"] = 25;
    ageMap["Bob"] = 30;
    ageMap["Charlie"] = 20;

    // Iterate and print
    for (const auto &pair : ageMap) {
        cout << pair.first << ": " << pair.second << endl;
    }

    return 0;
}

Output:

Alice: 25
Bob: 30
Charlie: 20

Example 1: Mapping Strings to Integers

Problem: Store the frequency of words in a sentence.

#include <iostream>
#include <map>
#include <sstream>
using namespace std;

int main() {
    string sentence = "this is a test this is only a test";
    map<string, int> wordFrequency;

    stringstream ss(sentence);
    string word;
    while (ss >> word) {
        wordFrequency[word]++;
    }

    // Print word frequencies
    for (const auto &pair : wordFrequency) {
        cout << pair.first << ": " << pair.second << endl;
    }

    return 0;
}

Output:

a: 2
is: 2
only: 1
test: 2
this: 2

Example 2: Mapping Integers to Strings

Problem: Map student IDs to their names.

#include <iostream>
#include <map>
using namespace std;

int main() {
    map<int, string> studentMap;

    // Insert student records
    studentMap[101] = "Alice";
    studentMap[102] = "Bob";
    studentMap[103] = "Charlie";

    // Access by key
    cout << "Student with ID 102: " << studentMap[102] << endl;

    // Print all records
    for (const auto &pair : studentMap) {
        cout << "ID: " << pair.first << ", Name: " << pair.second << endl;
    }

    return 0;
}

Output:

Student with ID 102: Bob
ID: 101, Name: Alice
ID: 102, Name: Bob
ID: 103, Name: Charlie

Example 3: Mapping Characters to Integers

Problem: Count the frequency of characters in a string.

#include <iostream>
#include <map>
using namespace std;

int main() {
    string text = "hello world";
    map<char, int> charFrequency;

    for (char c : text) {
        if (c != ' ') { // Skip spaces
            charFrequency[c]++;
        }
    }

    // Print character frequencies
    for (const auto &pair : charFrequency) {
        cout << pair.first << ": " << pair.second << endl;
    }

    return 0;
}

Output:

: 1
e: 1
h: 1
l: 3
o: 2
r: 1
w: 1

Example 4: Mapping Pairs

Problem: Map pairs of integers to strings.

#include <iostream>
#include <map>
using namespace std;

int main() {
    map<pair<int, int>, string> pairMap;

    // Insert pairs
    pairMap[{1, 2}] = "Alice and Bob";
    pairMap[{2, 3}] = "Charlie and Dave";

    // Access by pair
    cout << "Pair (1, 2): " << pairMap[{1, 2}] << endl;

    // Print all pairs
    for (const auto &pair : pairMap) {
        cout << "(" << pair.first.first << ", " << pair.first.second << "): " << pair.second << endl;
    }

    return 0;
}

Output:

Pair (1, 2): Alice and Bob
(1, 2): Alice and Bob
(2, 3): Charlie and Dave

Example 5: Custom Comparator

Problem: Sort keys in descending order.

#include <iostream>
#include <map>
using namespace std;

// Custom comparator
struct Descending {
    bool operator()(int a, int b) const {
        return a > b;
    }
};

int main() {
    map<int, string, Descending> customMap;

    // Insert elements
    customMap[1] = "One";
    customMap[3] = "Three";
    customMap[2] = "Two";

    // Print elements
    for (const auto &pair : customMap) {
        cout << pair.first << ": " << pair.second << endl;
    }

    return 0;
}

Output:

3: Three
2: Two
1: One

Example 6: Mapping Multitypes

Problem: Map a string to a vector of integers.

#include <iostream>
#include <map>
#include <vector> 
using namespace std;

int main() {
    map<string, vector<int>> studentGrades;

    // Insert grades
    studentGrades["Alice"] = {90, 85, 88};
    studentGrades["Bob"] = {75, 80, 72};

    // Print grades
    for (const auto &pair : studentGrades) {
        cout << pair.first << ": ";
        for (int grade : pair.second) {
            cout << grade << " ";
        }
        cout << endl;
    }

    return 0;
}

Output:

Alice: 90 85 88 
Bob: 75 80 72

Key Methods in std::map

  1. insert: Add key-value pairs.
  2. find: Search for a key.
  3. erase: Remove a key-value pair.
  4. size: Return the number of elements.
  5. clear: Remove all elements.
  6. count: Check if a key exists.
  7. begin, end: Iterators for traversal.
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