1.3. C++ First Example

1.3. C++ First Example
	Chapter 1. C++ Introduction

[ fromfile: cppintro.xml id: example1 ]

Example 1.1. src/early-examples/example0/fac.cpp

/* Computes and prints n! for a given n.
   Uses several basic elements of C++. */

#include <iostream>                         
int main() {                                
    using namespace std;                    
    // Declarations of variables
    int factArg = 0 ;                       
    int fact(1) ;                           
    do {                                    
        cout << "Factorial of: ";           
        cin >> factArg;                     
        if ( factArg < 0 ) {
            cout << "No negative values, please!" << endl;
        }                                   
    } while (factArg < 0) ;                 
    int i = 2;
    while ( i <= factArg ) {                
        fact = fact * i;
        i = i + 1;
    }                                       
    cout << "The Factorial of " << factArg << " is: " << fact << endl;
    return 0;                               
}

	Standard C++ library - In older versions of C++, you might find <iostream.h> instead, but that version is regarded as "deprecated"; i.e. its use is discouraged.
	Start of function "main" which returns an int
	Permits us to use the symbols cin, cout, and endl without prefixing each name with std::
	C style initialization syntax
	C++ style initialization syntax
	Start of "do..while" loop
	Write to standard output
	Read from standard input and convert to int
	End of if block
	if false, break out of do loop
	Start of while loop
	End of while block
	When main returns 0, that normally means "no errors"
	End of main block

You can compile and run this program using the GNU C compiler, gcc.

src/early-examples/example0> g++ -Wall fac.cpp
src/early-examples/example0> g++ -Wall -o execFile fac.cpp

-Wall enables all possible warnings about constructions that might be considered questionable even if they conform to the standard.

On a *nix system you can view the manual page, a summary of command options and how they are used, by typing the command
```
     man g++
 or
     info g++
```
For more complete gcc documentation, visit the GNU online document center.

After successful compilation, you can run the program on a *nix system by typing:

src/early-examples/example0> ./a.out
Factorial of: -3
No negative values, please!
Factorial of: 5
The Factorial of 5 is: 120
src/early-examples/example0>

Note the two kinds of comment delimiters.

The pre-compiler directive #include enables you to reuse library code.

A namespace (Section 20.4) is a collection of classes, functions, and objects that can be addressed with a named prefix.
The using declaration tells the compiler to add all symbols from the specified namespace (std) into the global namespace.


1.2. A Brief History of C++		1.4. Standard Input and Output