We start the tutorial series with one of the simplest programs that can be written in the C++ language.
- How 'Hello, World!' Program works? The #include is a preprocessor command that tells the compiler to include the contents of stdio.h (standard input and output) file in the program. The stdio.h file contains functions such as scanf and printf to take input and display output respectively.
- C Hello World. Now, we will see that how to write C hello world program with Dev C IDE. We can use Dev C for C programming very easily and Dev C makes creating C programs and applications simple for us.
Hello World Program in C. To print Hello World in C programming, use coutHello World after coutC Programming Code to Print Hello World. Following C program prints 'Hello World' on the output screen. Working of C 'Hello World!' Program // Your First C Program In C, any line starting with // is a comment. Comments are intended for the person reading.
The hello world program is one of the simplest programs, but it already contains the fundamental components that every C++ program has. Don’t be overwhelmed, we will take a look at the code line by line.
Type the code in your favorite editor (always type, don’t use cut/paste. This is better for learning purposes).
Save the program with the name: hello.cpp.
Save the program with the name: hello.cpp.
After saving the source code you can compile it. It should compile without any errors.
Note: Remember, the examples included in the C and C++ tutorials are all console programs. That means they use text to communicate. All compilers support the compilation of console programs. Check the user manual of you compiler for more information on how to compile them.
(It is not doable for us to write this down for every compiler).
(It is not doable for us to write this down for every compiler).
// A hello world program in C++
The first line in our program is a comment line. Every line that starts with two slash signs ( // ) are considered comments and will have no effect on the behavior or outcome of the program. (Words between /* and */ will also be considered as comments (old style comments)). Use comments in your programs to explain difficult sections, but don’t overdo it. It is also common practice to start every program with a brief description on what the program will do.
#include<iostream>
Lines beginning with a pound sign (#) are used by the compilers pre-processor. In this case the directive #include tells the pre-processor to include the iostream standard file. This file iostream includes the declarations of the basic standard input/output library in C++. (See it as including extra lines of code that add functionality to your program).
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using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace. In this case the namespace with the name std. We put this line in to declare that we will make use of the functionality offered in the namespace std. This line of code is used very frequent in C++ programs that use the standard library. You will see that we will make use of it in most of the source code included in these tutorials.
int main()
![Hello Hello](/uploads/1/2/6/1/126157086/159867667.jpg)
int is what is called the return value (in this case of the type integer. Integer is a whole number). What is used for will be explained further down.
Every program must have a main() function. The main function is the point where all C++ programs start their execution. The word main is followed by a pair of round brackets. That is because it is a function declaration (Functions will be explained in more detail in a later tutorial). It is possible to enclose a list of parameters within the round brackets.
{}
The two curly brackets (one in the beginning and one at the end) are used to indicate the beginning and the end of the function main. (Also called the body of a function). Everything contained within these curly brackets is what the function does when it is called and executed. In the coming tutorials you will see that many other statements make use of curly brackets.
cout << “Hello World”;
This line is a C++ statement. A statement is a simple expression that can produce an effect. In this case the statement will print something to our screen.
cout represents the standard output stream in C++.
(There is also a standard input stream that will be explained in another tutorial). In this a sequence of characters (Hello World) will be send to the standard output stream (in most cases this will be your screen).
The words Hello World have to be between ” “, but the ” ” will not be printed on the screen. They indicate that the sentence begins and where it will end.
(There is also a standard input stream that will be explained in another tutorial). In this a sequence of characters (Hello World) will be send to the standard output stream (in most cases this will be your screen).
The words Hello World have to be between ” “, but the ” ” will not be printed on the screen. They indicate that the sentence begins and where it will end.
cout is declared in the iostream standard file within the std namespace. This is the reason why we needed to include that specific file. This is also the reason why we had to declare this specific namespace.
Dev C++ Hello World Problem
As you can see the statement ends with a semicolon (;). The semicolon is used to mark the end of the statement. The semicolon must be placed behind all statements in C++ programs. So, remember this. One of the common errors is to forget to include a semicolon after a statement.
return 0;
The return statement causes the main function to finish. You may return a return code (in this case a zero) but it depends on how you start your function main( ). We said that main ( ) will return an int (integer). So we have to return something (in this case a zero). A zero normally indicates that everything went ok and a one normally indicates that something has gone wrong. (This is standard practice. After running an UNIX/Linux program there is often a check on the return code).
Indentations
As you can see the cout and the return statement have been indented or moved to the right side. This is done to make the code more readable. In a program as Hello World, it seems a stupid thing to do. But as the programs become more complex, you will see that it makes the code more readable. (Also you will make fewer errors, like forgetting a curly bracket on the end of a function).
So, always use indentations and comments to make the code more readable. It will make your life (programming life at least) much easier if the code becomes more complex.
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The left panel above shows the C++ code for this program. The right panel shows the result when the program is executed by a computer. The grey numbers to the left of the panels are line numbers to make discussing programs and researching errors easier. They are not part of the program.
Let's examine this program line by line:
- Line 1:
// my first program in C++
- Two slash signs indicate that the rest of the line is a comment inserted by the programmer but which has no effect on the behavior of the program. Programmers use them to include short explanations or observations concerning the code or program. In this case, it is a brief introductory description of the program.
- Line 2:
#include <iostream>
- Lines beginning with a hash sign (
#
) are directives read and interpreted by what is known as the preprocessor. They are special lines interpreted before the compilation of the program itself begins. In this case, the directive#include <iostream>
, instructs the preprocessor to include a section of standard C++ code, known as header iostream, that allows to perform standard input and output operations, such as writing the output of this program (Hello World) to the screen. - Line 3: A blank line.
- Blank lines have no effect on a program. They simply improve readability of the code.
- Line 4:
int main ()
- This line initiates the declaration of a function. Essentially, a function is a group of code statements which are given a name: in this case, this gives the name 'main' to the group of code statements that follow. Functions will be discussed in detail in a later chapter, but essentially, their definition is introduced with a succession of a type (
int
), a name (main
) and a pair of parentheses (()
), optionally including parameters.
The function namedmain
is a special function in all C++ programs; it is the function called when the program is run. The execution of all C++ programs begins with themain
function, regardless of where the function is actually located within the code. - Lines 5 and 7:
{
and}
- The open brace (
{
) at line 5 indicates the beginning ofmain
's function definition, and the closing brace (}
) at line 7, indicates its end. Everything between these braces is the function's body that defines what happens whenmain
is called. All functions use braces to indicate the beginning and end of their definitions. - Line 6:
std::cout << 'Hello World!';
- This line is a C++ statement. A statement is an expression that can actually produce some effect. It is the meat of a program, specifying its actual behavior. Statements are executed in the same order that they appear within a function's body.
This statement has three parts: First,std::cout
, which identifies the standardcharacter output device (usually, this is the computer screen). Second, the insertion operator (<<
), which indicates that what follows is inserted intostd::cout
. Finally, a sentence within quotes ('Hello world!'), is the content inserted into the standard output.
Notice that the statement ends with a semicolon (;
). This character marks the end of the statement, just as the period ends a sentence in English. All C++ statements must end with a semicolon character. One of the most common syntax errors in C++ is forgetting to end a statement with a semicolon.
You may have noticed that not all the lines of this program perform actions when the code is executed. There is a line containing a comment (beginning with
//
). There is a line with a directive for the preprocessor (beginning with #
). There is a line that defines a function (in this case, the main
function). And, finally, a line with a statements ending with a semicolon (the insertion into cout
), which was within the block delimited by the braces ( { }
) of the main
function. The program has been structured in different lines and properly indented, in order to make it easier to understand for the humans reading it. But C++ does not have strict rules on indentation or on how to split instructions in different lines. For example, instead of
We could have written:
all in a single line, and this would have had exactly the same meaning as the preceding code.
In C++, the separation between statements is specified with an ending semicolon (
;
), with the separation into different lines not mattering at all for this purpose. Many statements can be written in a single line, or each statement can be in its own line. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it, but has no effect on the actual behavior of the program.Now, let's add an additional statement to our first program:
In this case, the program performed two insertions into
std::cout
in two different statements. Once again, the separation in different lines of code simply gives greater readability to the program, since main
could have been perfectly valid defined in this way:The source code could have also been divided into more code lines instead:
And the result would again have been exactly the same as in the previous examples.
Preprocessor directives (those that begin by
#
) are out of this general rule since they are not statements. They are lines read and processed by the preprocessor before proper compilation begins. Preprocessor directives must be specified in their own line and, because they are not statements, do not have to end with a semicolon (;
).Dev C++ Hello World Code
C# Hello World Example
Using namespace std
If you have seen C++ code before, you may have seencout
being used instead of std::cout
. Both name the same object: the first one uses its unqualified name (cout
), while the second qualifies it directly within the namespacestd
(as std::cout
).cout
is part of the standard library, and all the elements in the standard C++ library are declared within what is called a namespace: the namespace std
.In order to refer to the elements in the
std
namespace a program shall either qualify each and every use of elements of the library (as we have done by prefixing cout
with std::
), or introduce visibility of its components. The most typical way to introduce visibility of these components is by means of using declarations:The above declaration allows all elements in the
std
namespace to be accessed in an unqualified manner (without the std::
prefix).With this in mind, the last example can be rewritten to make unqualified uses of
cout
as:Both ways of accessing the elements of the
std
namespace (explicit qualification and using declarations) are valid in C++ and produce the exact same behavior. For simplicity, and to improve readability, the examples in these tutorials will more often use this latter approach with using declarations, although note that explicit qualification is the only way to guarantee that name collisions never happen.Namespaces are explained in more detail in a later chapter.
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