The best way to start learning a programming language is probably by learning to write a program. Writing a program is the most basic and also crucial part of programming. Why don't we start by learning about the way a program works, it's structure and basics.
Therefore, here is our first program:
This will be the input This will be output
#include <iostream> Hello World!
using namespace std;
int main ()
{
cout << "Hello World!";
return 0;
}
The input is the very first program a programmer will ever see and same can be said for the output. Now lets start with the learning.
#include <iostream>
The "#include" is a pre-processor function that tells the compiler to include the <iostream> library. They are no regular code lines. This specific file "iostream" includes the declarations of the basic standard input-output library in C++. Keep it in mind.
using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace, the
namespace with the name std (standard) . So in order to access its functionality we declare with this expression that we will be using these entities (std) . This line is very frequent in C++ programs that use the standard library. Most programs do use the std library so it is an important point to remember.
int main ()
This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function.
The word main is followed in the code by a pair of parentheses (()). That is because it is a function
declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them.
Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.
cout << "Hello World!";
This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program.
cout represents the standard output stream in C++, and the meaning of the entire statement is to insert
a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen).
cout is declared in the iostream standard file within the std namespace, so that's why we needed to
include that specific file and to declare that we were going to use this specific namespace earlier in our code.
Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).
return 0;
The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ console program.
(FIN)
This code could also be written on a single line keep in mind like this :
int main () { cout << "Hello World!"; return 0; }
int main () { cout << "Hello World!"; return 0; }
This would mean exactly the same as the above code and would have same meaning.
See you on the next lesson and feel free to ask me any questions you might want.
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