Working with capacity(), push_back(), and size()

This file is a C++ program that demonstrates the use of the standard library's `vector` container. Here’s a breakdown of what it does: 1. **Includes Necessary Libraries**: It begins by including the `iostream` and `vector` libraries, which are essential for input/output operations and the use of dynamic arrays, respectively. 2. **Using Namespace**: The `using namespace std;` line allows the program to use standard library names without needing to prefix them with `std::`. 3. **Main Function**: The program starts execution in the `main()` function. 4. **Vector Declaration**: It declares a vector of integers named `numbers` and an iterator `it` for traversing the vector. 5. **User Input Loop**: The program enters an infinite loop where it prompts the user to enter positive integers. The loop continues until the user inputs `-1`, which serves as a signal to exit. 6. **Dynamic Size Management**: After each valid number is input, the program uses `push_back()` to add the number to the `numbers` vector. It then outputs the current size and capacity of the vector. The `capacity()` function returns the size of the allocated storage used for the vector, which may be greater than or equal to its current size. 7. **Accessing Vector Element**: After the loop, the program accesses and prints the third element (index 2) of the vector. 8. **Modifying Vector Element**: The program then modifies the third element of the vector (index 2) to a value of `99` and outputs the changed value. Overall, this program demonstrates basic C++ programming concepts, including loops, user input handling, the dynamic array capabilities of `vector`, and element accessing/modification. It's a good example for beginners to understand how vectors work in C++.