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Standard Template Library (STL)

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The Standard Template Library (STL) is a powerful set of C++ template classes to provide general-purpose classes and functions with templates that implement many popular and commonly used algorithms and data structures like vectors, lists, queues, and stacks. STL has four components:

  1. Algorithms
  2. Containers
  3. Functions
  4. Iterators

  1. Algorithms

STL provides a rich set of algorithms that can be used to perform various operations on data structures. Some common algorithms include:

  • Sorting: sort(), stable_sort()
  • Searching: find(), binary_search()
  • Modifying: copy(), swap()
  • Removing: remove(), remove_if()

Example: Sorting a Vector

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> vec = {4, 2, 5, 1, 3};
    
    // Sorting the vector
    std::sort(vec.begin(), vec.end());
    
    // Displaying the sorted vector
    for (int num : vec) {
        std::cout << num << " ";
    }
    return 0;
}

Explanation:

  • #include <algorithm>: Includes the algorithm library.
  • std::sort(vec.begin(), vec.end()): Sorts the elements in the vector from the beginning to the end.
  • The for loop prints the sorted vector.

  1. Containers

Containers are objects that store data. STL provides several types of containers:

  • Sequence Containers: vector, deque, list
  • Associative Containers: set, map, multiset, multimap
  • Unordered Associative Containers: unordered_set, unordered_map
  • Container Adapters: stack, queue, priority_queue

Example: Using a Map

#include <iostream>
#include <map>

int main() {
    std::map<std::string, int> ageMap;
    
    // Inserting elements into the map
    ageMap["Alice"] = 30;
    ageMap["Bob"] = 25;
    ageMap["Charlie"] = 35;
    
    // Accessing elements
    std::cout << "Alice's age: " << ageMap["Alice"] << std::endl;
    std::cout << "Bob's age: " << ageMap["Bob"] << std::endl;
    
    // Iterating through the map
    for (const auto& pair : ageMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }
    return 0;
}

Explanation:

  • #include <map>: Includes the map library.
  • std::map<std::string, int> ageMap: Declares a map with string keys and integer values.
  • Elements are inserted using the [] operator.
  • The for loop iterates through the map and prints each key-value pair.

  1. Functions

STL provides function objects, also known as functors, which are objects that can be called as if they are ordinary functions. They are used to customize the behavior of algorithms.

Example: Using a Functor

#include <iostream>
#include <vector>
#include <algorithm>

struct MultiplyByTwo {
    void operator()(int& n) const {
        n *= 2;
    }
};

int main() {
    std::vector<int> vec = {1, 2, 3, 4, 5};
    
    // Applying the functor to each element
    std::for_each(vec.begin(), vec.end(), MultiplyByTwo());
    
    // Displaying the modified vector
    for (int num : vec) {
        std::cout << num << " ";
    }
    return 0;
}

Explanation:

  • struct MultiplyByTwo: Defines a functor that multiplies a number by two.
  • std::for_each(vec.begin(), vec.end(), MultiplyByTwo()): Applies the functor to each element in the vector.

  1. Iterators

Iterators are used to point to the elements of a container. They are similar to pointers and can be used to traverse through the elements of a container.

Example: Using Iterators with a Vector

#include <iostream>
#include <vector>

int main() {
    std::vector<int> vec = {10, 20, 30, 40, 50};
    
    // Declaring an iterator
    std::vector<int>::iterator it;
    
    // Using the iterator to traverse the vector
    for (it = vec.begin(); it != vec.end(); ++it) {
        std::cout << *it << " ";
    }
    return 0;
}

Explanation:

  • std::vector<int>::iterator it: Declares an iterator for a vector of integers.
  • The for loop uses the iterator to traverse the vector and print each element.

Practical Exercises

Exercise 1: Sorting and Searching

Task:

  1. Create a vector of integers.
  2. Sort the vector in ascending order.
  3. Search for a specific element in the sorted vector.

Solution:

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> vec = {7, 2, 9, 4, 1, 5};
    
    // Sorting the vector
    std::sort(vec.begin(), vec.end());
    
    // Searching for an element
    int target = 4;
    if (std::binary_search(vec.begin(), vec.end(), target)) {
        std::cout << "Element " << target << " found in the vector." << std::endl;
    } else {
        std::cout << "Element " << target << " not found in the vector." << std::endl;
    }
    
    return 0;
}

Exercise 2: Using a Set

Task:

  1. Create a set of strings.
  2. Insert some elements into the set.
  3. Check if a specific element exists in the set.

Solution:

#include <iostream>
#include <set>

int main() {
    std::set<std::string> nameSet;
    
    // Inserting elements into the set
    nameSet.insert("Alice");
    nameSet.insert("Bob");
    nameSet.insert("Charlie");
    
    // Checking if an element exists
    std::string target = "Bob";
    if (nameSet.find(target) != nameSet.end()) {
        std::cout << target << " is in the set." << std::endl;
    } else {
        std::cout << target << " is not in the set." << std::endl;
    }
    
    return 0;
}

Summary

In this section, we covered the basics of the Standard Template Library (STL) in C++. We explored its four main components: algorithms, containers, functions, and iterators. We also provided practical examples and exercises to reinforce the concepts. Understanding STL is crucial for writing efficient and maintainable C++ code, and it provides a solid foundation for tackling more advanced topics in C++ programming.

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