Your first program: #include, main and std::cout

The small program you ran in the previous lesson is the skeleton of every C++ program. Now let's take it apart piece by piece and print something a little more interesting.

Anatomy of a program

#include <iostream>

int main() {
    std::cout << "Byte Akademi\n";
    std::cout << "V = 12 V, R = 4 ohm" << '\n';
    return 0;
}

Byte Akademi
V = 12 V, R = 4 ohm

Let's go through the lines in order:

• #include <iostream>: Tells the preprocessor "paste the header file of the input-output library here". std::cout is defined in this header. Lines starting with # are commands for the preprocessor, not C++ code, so they don't end with a semicolon.
• int main(): The program's entry point. When the operating system runs the program, this is the first function it calls. The int in front says that the function returns an integer.
• The part between { and } is the body of the function.
• std::cout: The standard output stream, i.e. the screen. The std:: prefix shows that this name comes from the "standard library" namespace.
• <<: The stream operator. It sends the value on its right into the stream. It can be chained.
• return 0;: Tells the operating system "everything went fine". If you leave this line out of main, the compiler adds it for you; still, writing it out of habit makes the code easier to read.

Line endings: \n or std::endl?

There are two ways to move to a new line. '\n' sends only the newline character. std::endl sends a newline and also empties (flushes) the buffer. Flushing the buffer every time is slow; if you don't need it, use '\n'. Single quotes mark a single character and double quotes mark a piece of text; both work here.

using namespace std

Writing std:: everywhere may look tiring, and many beginner books recommend the line using namespace std;. It is harmless in small experiments, but in large projects the same names from different libraries can clash. In this course we will prefer to write std::; that way you always see when you are using something from the standard library.

Comments

// Single-line comment: the compiler ignores this line.

/* A comment that spans
   several lines */

int main() {
    // Ohm's law: V = I * R
    return 0;
}

A good comment explains why the code does something, not "what" it does. Lines like i++; // increment i are noise; // divide by 1023 because the ADC is 10-bit is valuable.

Common mistakes

Try it yourself

1. Change the program to print your name and your department on two separate lines.
2. Print an engineering formula, for example P = V * I.
3. Use std::endl instead of '\n' and see that the output does not change.
4. Add a comment line and confirm that the compiler ignores it.