First program

This lesson will teach you how to write your first simple C++ program that can display text and perform simple calculations.

Application code

We finished the previous lesson with the following code:

main.cpp
#include <iostream>

int main()
{
    std::cout << "Hello, World!";
}

Result (console)

Hello, World!

It displays the "Hello, World" text in the console.

Explanation

The main function

Let's start with the heart of our application, the main function

Part of main.cpp
int main()
{

}

The first line starts the function definition. It consists of a return type int, the name main, and its parameters inside the () (none in our case). While most of these terms aren't important now, all that you should pay attention to is the name main. It will always look like this, int main(). Your program must always contain the main function specified in this format.

The computer runs the program line by line. When it starts the application, it executes the instructions in the main code block sequentially.

A code block in C++, is a set of instructions contained inside curly braces:

{

}

The main function has a block of code, and everything between its curly braces will be executed top to bottom.

Remember

The content of the main function is always the first code run by the program.

The `iostream` header file

At the very beginning of the code there is a line:

#include <iostream>

This allows us to use the standard input and output tools provided by C++. The name iostream is short for input/output stream. In practice, this means that we the programmer can write a message in the console; then, our program can intercept the response from the user.

iostream is a header file. We will use more of them in the near future. In most cases we put #include <header_file> at the beginning of the file. We'll tell more about #include itself in the future. Enough for the moment.

The console

The "console" or "terminal" is the most basic form of communication for a program. It allows for a simple, two-way text interface between the program and the user. You're going to become very familiar with this tool, as you will be using it every step of the way on your journey to becoming an expert programmer.

Below are some examples of running the same code

in multiple environments. While not every terminal is the same, they will look very similar by only displaying simple text with little to no color.

🖼 Overview:

As you can see, many code editors like Visual Studio, VS Code, Replit.com, CLion, Vim, etc., have a console window integrated directly into the interface.

Printing text to the console

To display text in the console we used the following statement inside the main block:

std::cout << "Hello, World!";

The name cout stands for a character output. It is used to send text to the console. We will mention the std:: prefix in a minute.

For those who are curious 😉

Strictly speaking, cout is a so-called character output stream that writes the content to a standard output, but you don't need to remember that for the moment.

I can now duplicate the line above a couple of times:

std::cout << "Hello, World!1";
std::cout << "Hello, World!2";
std::cout << "Hello, World!3";

to get the following effect:

Hello, World!1Hello, World!2Hello, World!3

Semicolon at the end of instructions!

A semicolon (;) separates instructions. The compiler doesn't care about instructions being written on separate lines. The semicolon is treated as the end of the statement. For this reason, such code as below is acceptable but very illegible.

std::cout << "Hello, World!1"; std::cout << "Hello, World!2"; std::cout << "Hello, World!3";

Do not write code like this!

Presentation of the above-created program:

Note that the text was written to the console without any spaces, in a single line, and wrapped when it reached the end of the available space.

The program does exactly what it says in the code - no more, no less. We told it that it should display the given characters... and it does. We have to explicitly "break" the line in the right place. This will cause the following characters to be displayed from the very beginning of the next line. To do this, we put a special newline character inside the text: \n, like this:

std::cout << "Hello, World!\n";

Backslash

Note that we used the backslash (\n), and not the forward slash (/n).

This is how the program is being executed when we add newline characters:

Newline Character

The \n sequence represents a single character for the program. More on that later.

Prefix of the standard namespace

std:: is the prefix of the standard library namespace from which cout comes.

Beginners often find typing std:: to be unwieldy. It can be omitted if we give the appropriate line of code beforehand:

The 'using' statement
#include <iostream>int main(){    using std::cout;    cout << "Hello, World!";}

After the line that contains using std::cout;, the std:: prefix is no longer required for the cout object. It lasts until the end of the current code block (the main function in this case).

using namespace std

In many other online tutorials and sources, it is extremely common to find usage of

using namespace std;

at the top of your file. While this is legal code in C++, it is strongly discouraged by nearly all experienced C++ programmers. We will cover reasons for this later. For the time being, prefer the using std::cout; approach we mentioned before.

Performing calculations

Programming is used to automate certain activities. For example, we can have a computer do a mathematical calculation for us.

Use the following code:

main.cpp
#include <iostream>
int main()
{
    std::cout << "120 * 120 + 540";
}

Result (console)

120 * 120 + 540

Pay attention to the result of the above program.

Text vs Instructions

Everything in quotation marks is treated as text, not instructions.

Therefore, instead of displaying the result of the calculation, we got what we entered in the quotation marks. To say that we want to perform some calculation, we need to remove the quotes:

main.cpp
#include <iostream>
int main()
{
    std::cout << 120 * 120 + 540;
}

Result (console)

After starting the program, we will get the correct result of the calculation in the code.

Mathematical operators

+ adds two numbers together
- subtracts two numbers from each other
* multiplies two numbers by itself
/ divides two numbers by itself
() parentheses order operations

C++ adheres to the "PEMDAS" order you may have learned in school.

First Parentheses
Then Multiplication/Division from left to right
Finally Addition/Subtraction from left to right

Summary

You've learned about the structure of the first program's code.

tip

For exercises visit this page.

First program

Application code​

Explanation​

The main function​

The iostream header file​

The console​

🖼 Overview:​

Printing text to the console​

Prefix of the standard namespace​

Performing calculations​

Mathematical operators​

Summary​

Application code

Explanation

The main function

The `iostream` header file

The console

🖼 Overview:

Printing text to the console

Prefix of the standard namespace

Performing calculations

Mathematical operators

Summary