how to input a vector when size is unknown

To input a vector in C++ when the size is unknown, you can follow the steps below:

1. Include the necessary header files:

#include <iostream>
#include <vector>

The iostream header file is required for input and output operations, while the vector header file is needed to work with vectors.

1. Declare an empty vector:

std::vector<int> vec;

Here, std::vector<int> declares a vector of integers named vec.

1. Read input values using a loop:

int num;
while (std::cin >> num) {
    vec.push_back(num);
}

This loop reads input values from the user until the input stream (std::cin) is valid. The input values are then added to the vector using the push_back function.

1. Access the vector elements: You can access the elements of the vector using the square bracket notation ([]):

for (int i = 0; i < vec.size(); i++) {
    std::cout << vec[i] << " ";
}

This loop iterates over the vector and prints each element.

Putting it all together, here's a complete example:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> vec;
    int num;
    while (std::cin >> num) {
        vec.push_back(num);
    }

    for (int i = 0; i < vec.size(); i++) {
        std::cout << vec[i] << " ";
    }

    return 0;
}

This code will read integers from the user until the input stream is valid, store them in the vector, and then print the vector elements.