# Basic Syntax & I/O Let's kick things off by familiarising ourselves with the basic structure of a C++ program. ### Anatomy of a C++ Program Recall the source code you previously saw: {% code title="main.cpp" %} ```cpp #include int main() { std::cout << "Hello, world!"; return 0; } ``` {% endcode %} Breaking it down:

Code	What it does
`#include <iostream>`	Brings in the standard I/O library so that we can use things like `std::cout`
`int main() {`	The entry point of every C++ program. `int` indicates the return type of the function `main` is the function name.
`std::cout << "Hel...`	Prints the text to the console. 👉 You can remember `<<` as pushing the text into the output stream (i.e `std::cout` in this case)
`return 0;`	Recall that `int` is the return type of the `main` function. Hence, at the end of our `main` function, we should^[1] return an int. You can return anything, really. But usually returning 0 indicates our program has successfully executed.

{% hint style="info" %} Try augmenting the `Hello, world!` string to something else like your name!

{% endhint %} _{\[1] Your code will still compile even if you don't return anything from the main function. This is because C++ implicilty returns 0 at the end of} _`main()`_{. For more info, refer} [_here](https://stackoverflow.com/questions/19293642/why-does-the-main-function-work-with-no-return-value)_. ### Newlines If you try printing multiple times, you will notice that the output gets concatenated on the same line. You can fix this by adding a newline using `std::endl` .

Code Output

Code	Output
`#include <iostream> int main() { std::cout << "benn"; std::cout << "tan"; std::cout << "jia"; return 0; }`
`#include <iostream> int main() { std::cout << "benn" << std::endl; std::cout << "tan" << std::endl; std::cout << "jia"; return 0; }`

#include <iostream>
int main() {
    std::cout << "benn";
    std::cout << "tan";
    std::cout << "jia";
    return 0;
}

#include <iostream>
int main() {
    std::cout << "benn" << std::endl;
    std::cout << "tan" << std::endl;
    std::cout << "jia";
    return 0;
}

### Input It's not quite fun if we hardcode everything into program. Let's augment our program to accept user input externally. To read input from the user, we can use `std::cin >>` {% code title="main.cpp" %} ```cpp #include int main() { int x; std::cin >> x; std::cout << "You entered: " << x << std::endl; } ``` {% endcode %} {% hint style="info" %} Note that the direction of the arrows (`>>`) for `std::cin` is now reversed compared to `<<`. 👉 You can remember as **data flowing into `x`**. {% endhint %} ### Namespaces If you find prefixing `std::` before the function calls cumbersome, you can omit it with `using namespace std` {% code title="main.cpp" %} ```cpp #include using namespace std; int main() { int x; cin >> x; cout << "You entered: " << x << endl; } ``` {% endcode %} However, this is generally discouraged in larger projects as it does lead to naming conflicts. 👉 It's usually best practice to be explicit and use `std::` ### Exercise Build a program that takes in and prints the result of the summation of two numbers.