switch case in C and C++
Switch case statements are a substitute for long if statements that compare a variable to several "integral" values ("integral" values are simply values that can be expressed as an integer, such as the value of a char). The basic format for using switch case is outlined below. The value of the variable given into switch is compared to the value following each of the cases, and when one value matches the value of the variable, the computer continues executing the program from that point.
#include
using namespace std;
void playgame()
{
cout << "Play game called";
}
void loadgame()
{
cout << "Load game called";
}
void playmultiplayer()
{
cout << "Play multiplayer game called";
}
int main()
{
int input;
cout<<"1. Play game\n";
cout<<"2. Load game\n";
cout<<"3. Play multiplayer\n";
cout<<"4. Exit\n";
cout<<"Selection: ";
cin>> input;
switch ( input ) {
case 1: // Note the colon, not a semicolon
playgame();
break;
case 2: // Note the colon, not a semicolon
loadgame();
break;
case 3: // Note the colon, not a semicolon
playmultiplayer();
break;
case 4: // Note the colon, not a semicolon
cout<<"Thank you for playing!\n";
break;
default: // Note the colon, not a semicolon
cout<<"Error, bad input, quitting\n";
break;
}
cin.get();
}
This program will compile, but cannot be run until the undefined functions are given bodies, but it serves as a model (albeit simple) for processing input. If you do not understand this then try mentally putting in if statements for the case statements. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. You could easily make a few small functions if you wish to test the code.
Pointers
Pointers are an extremely powerful programming tool. They can make some things much easier, help improve your program's efficiency, and even allow you to handle unlimited amounts of data. For example, using pointers is one way to have a function modify a variable passed to it. It is also possible to use pointers to dynamically allocate memory, which means that you can write programs that can handle nearly unlimited amounts of data on the fly--you don't need to know, when you write the program, how much memory you need. Wow, that's kind of cool. Actually, it's very cool, as we'll see in some of the next tutorials. For now, let's just get a basic handle on what pointers are and how you use them.
Pointers are aptly named: they "point" to locations in memory. Think of a row of safety deposit boxes of various sizes at a local bank. Each safety deposit box will have a number associated with it so that the teller can quickly look it up. These numbers are like the memory addresses of variables. A pointer in the world of safety deposit boxes would simply be anything that stored the number of another safety deposit box. Perhaps you have a rich uncle who stored valuables in his safety deposit box, but decided to put the real location in another, smaller, safety deposit box that only stored a card with the number of the large box with the real jewelry. The safety deposit box with the card would be storing the location of another box; it would be equivalent to a pointer. In the computer, pointers are just variables that store memory addresses, usually the addresses of other variables.
Pointing to Something: Retrieving an Address
In order to have a pointer actually point to another variable it is necessary to have the memory address of that variable also. To get the memory address of a variable (its location in memory), put the & sign in front of the variable name. This makes it give its address. This is called the address-of operator, because it returns the memory address. Conveniently, both ampersand and address-of start with a; that's a useful way to remember that you use & to get the address of a variable.
For example:
#include
using namespace std;
int main()
{
int x; // A normal integer
int *p; // A pointer to an integer
p = &x; // Read it, "assign the address of x to p"
cin>> x; // Put a value in x, we could also use *p here
cin.ignore();
cout<< *p <<"\n"; // Note the use of the * to get the value
cin.get();
}
The cout outputs the value stored in x. Why is that? Well, let's look at the code. The integer is called x. A pointer to an integer is then defined as p. Then it stores the memory location of x in pointer by using the address-of operator (&) to get the address of the variable. Using the ampersand is a bit like looking at the label on the safety deposit box to see its number rather than looking inside the box, to get what it stores. The user then inputs a number that is stored in the variable x; remember, this is the same location that is pointed to by p.
The next line then passes *p into cout. *p performs the "dereferencing" operation on p; it looks at the address stored in p, and goes to that address and returns the value. This is akin to looking inside a safety deposit box only to find the number of (and, presumably, the key to ) another box, which you then open.
Structures
Before discussing classes, this lesson will be an introduction to data structures similar to classes. Structures are a way of storing many different values in variables of potentially different types under the same name. This makes it a more modular program, which is easier to modify because its design makes things more compact. Structs are generally useful whenever a lot of data needs to be grouped together--for instance, they can be used to hold records from a database or to store information about contacts in an address book. In the contacts example, a struct could be used that would hold all of the information about a single contact--name, address, phone number, and so forth.
The format for defining a structure is
struct Tag {
Members
};
Where Tag is the name of the entire type of structure and Members are the variables within the struct. To actually create a single structure the syntax is
struct Tag name_of_single_structure;
I will talk only a little bit about unions as well. Unions are like structures except that all the variables share the same memory. When a union is declared the compiler allocates enough memory for the largest data-type in the union. It's like a giant storage chest where you can store one large item, or a small item, but never the both at the same time.
The '.' operator is used to access different variables inside a union also.
As a final note, if you wish to have a pointer to a structure, to actually access the information stored inside the structure that is pointed to, you use the -> operator in place of the . operator. All points about pointers still apply.
A quick example:
#include
using namespace std;
struct xampl {
int x;
};
int main()
{
xampl structure;
xampl *ptr;
structure.x = 12;
ptr = &structure; // Yes, you need the & when dealing with structures
// and using pointers to them
cout<< ptr->x; // The -> acts somewhat like the * when used with pointers
// It says, get whatever is at that memory address
// Not "get what that memory address is"
cin.get();
}
Array
Arrays are useful critters because they can be used in many ways to store large amounts of data in a structured way. For example, a tic-tac-toe board can be held in an array. Arrays are essentially a way to store many values under the same name. You can make an array out of any data-type including structures and classes.
Inline Functions in C++
Although you've already learned about basic functions in c++, there is more: the inline function. Inline functions are not always important, but it is good to understand them. The basic idea is to save time at a cost in space. Inline functions are a lot like a placeholder. Once you define an inline function, using the 'inline' keyword, whenever you call that function the compiler will replace the function call with the actual code from the function.
How does this make the program go faster? Simple, function calls are simply more time consuming than writing all of the code without functions. To go through your program and replace a function you have used 100 times with the code from the function would be time consuming not too bright. Of course, by using the inline function to replace the function calls with code you will also greatly increase the size of your program.
Using the inline keyword is simple, just put it before the name of a function. Then, when you use that function, pretend it is a non-inline function.
#include
using namespace std;
inline void hello()
{
cout<<"hello";
}
int main()
{
hello(); //Call it like a normal function...
cin.get();
}
Typecasting in C and C++
Typecasting is making a variable of one type, such as an int, act like another type, a char, for one single operation. To typecast something, simply put the type of variable you want the actual variable to act as inside parentheses in front of the actual variable. (char)a will make 'a' function as a char.
#include
using namespace std;
int main()
{
cout<< (char)65 <<"\n";
// The (char) is a typecast, telling the computer to interpret the 65 as a
// character, not as a number. It is going to give the character output of
// the equivalent of the number 65 (It should be the letter A for ASCII).
cin.get();
}
One use for typecasting for is when you want to use the ASCII characters. For example, what if you want to create your own chart of all 256 ASCII characters. To do this, you will need to use to typecast to allow you to print out the integer as its character equivalent.
#include
using namespace std;
int main()
{
for ( int x = 0; x < 256; x++ ) {
cout<< x <<". "<< (char)x <<" ";
//Note the use of the int version of x to
// output a number and the use of (char) to
// typecast the x into a character
// which outputs the ASCII character that
// corresponds to the current number
}
cin.get();
}
Introduction to Classes in C++
C++ is a bunch of small additions to C, with a few major additions. One major addition is the object-oriented approach (the other addition is support for generic programming, which we'll cover later). As the name object-oriented programming suggests, this approach deals with objects. Of course, these are not real-life objects themselves. Instead, these objects are the essential definitions of real world objects. Classes are collections of data related to a single object type. Classes not only include information regarding the real world object, but also functions to access the data, and classes possess the ability to inherit from other classes. (Inheritance is covered in a later lesson.)
If a class is a house, then the functions will be the doors and the variables will be the items inside the house. The functions usually will be the only way to modify the variables in this structure, and they are usually the only way even to access the variables in this structure. This might seem silly at first, but the idea to make programs more modular - the principle itself is called "encapsulation". The key idea is that the outside world doesn't need to know exactly what data is stored inside the class--it just needs to know which functions it can use to access that data. This allows the implementation to change more easily because nobody should have to rely on it except the class itself.
The syntax for these classes is simple. First, you put the keyword 'class' then the name of the class. Our example will use the name Computer. Then you put an open bracket. Before putting down the different variables, it is necessary to put the degree of restriction on the variable. There are three levels of restriction. The first is public, the second protected, and the third private. For now, all you need to know is that the public restriction allows any part of the program, including parts outside the class, to access the functions and variables specified as public. The protected restriction prevents functions outside the class to access the variable. The private restriction is similar to protected (we'll see the difference later when we look at inheritance. The syntax for declaring these access restrictions is merely the restriction keyword (public, private, protected) and then a colon. Finally, you put the different variables and functions (You usually will only put the function prototype[s]) you want to be part of the class. Then you put a closing bracket and semicolon. Keep in mind that you still must end the function prototype(s) with a semi-colon.
Classes must always contain two functions: a constructor and a destructor. The syntax for them is simple: the class name denotes a constructor, a ~ before the class name is a destructor. The basic idea is to have the constructor initialize variables, and to have the destructor clean up after the class, which includes freeing any memory allocated. If it turns out that you don't need to actually perform any initialization, then you can allow the compiler to create a "default constructor" for you. Similarly, if you don't need to do anything special in the destructor, the compiler can write it for you too!
When the programmer declares an instance of the class, the constructor will be automatically called. The only time the destructor is called is when the instance of the class is no longer needed--either when the program ends, the class reaches the end of scope, or when its memory is deallocated using delete (if you don't understand all of that, don't worry; the key idea is that destructors are always called when the class is no longer usable). Keep in mind that neither constructors nor destructors return arguments! This means you do not want to (and cannot) return a value in them.
#include
using namespace std;
class Computer // Standard way of defining the class
{
public:
// This means that all of the functions below this(and any variables)
// are accessible to the rest of the program.
// NOTE: That is a colon, NOT a semicolon...
Computer();
// Constructor
~Computer();
// Destructor
void setspeed ( int p );
int readspeed();
protected:
// This means that all the variables under this, until a new type of
// restriction is placed, will only be accessible to other functions in the
// class. NOTE: That is a colon, NOT a semicolon...
int processorspeed;
};
// Do Not forget the trailing semi-colon
Computer::Computer()
{
//Constructors can accept arguments, but this one does not
processorspeed = 0;
}
Computer::~Computer()
{
//Destructors do not accept arguments
}
void Computer::setspeed ( int p )
{
// To define a function outside put the name of the class
// after the return type and then two colons, and then the name
// of the function.
processorspeed = p;
}
int Computer::readspeed()
{
// The two colons simply tell the compiler that the function is part
// of the class
return processorspeed;
}
int main()
{
Computer compute;
// To create an 'instance' of the class, simply treat it like you would
// a structure. (An instance is simply when you create an actual object
// from the class, as opposed to having the definition of the class)
compute.setspeed ( 100 );
// To call functions in the class, you put the name of the instance,
// a period, and then the function name.
cout<< compute.readspeed();
// See above note.
}
command line arguments in C++ using argc and argv
In C++ it is possible to accept command line arguments. Command-line arguments are given after the name of a program in command-line operating systems like DOS or Linux, and are passed in to the program from the operating system. To use command line arguments in your program, you must first understand the full declaration of the main function, which previously has accepted no arguments. In fact, main can actually accept two arguments: one argument is number of command line arguments, and the other argument is a full list of all of the command line arguments.
int main ( int argc, char *argv[] )
The integer, argc is the ARGument Count (hence argc). It is the number of arguments passed into the program from the command line, including the name of the program.
The array of character pointers is the listing of all the arguments. argv[0] is the name of the program, or an empty string if the name is not available. After that, every element number less than argc is a command line argument. You can use each argv element just like a string, or use argv as a two dimensional array. argv[argc] is a null pointer.
How could this be used? Almost any program that wants its parameters to be set when it is executed would use this. One common use is to write a function that takes the name of a file and outputs the entire text of it onto the screen.
#include
#include
using namespace std;
int main ( int argc, char *argv[] )
{
if ( argc != 2 ) // argc should be 2 for correct execution
// We print argv[0] assuming it is the program name
cout<<"usage: "<< argv[0] <<"\n";
else {
// We assume argv[1] is a filename to open
ifstream the_file ( argv[1] );
// Always check to see if file opening succeeded
if ( !the_file.is_open() )
cout<<"Could not open file\n";
else {
char x;
// the_file.get ( x ) returns false if the end of the file
// is reached or an error occurs
while ( the_file.get ( x ) )
cout<< x;
}
// the_file is closed implicitly here
}
}
This program is fairly simple. It incorporates the full version of main. Then it first checks to ensure the user added the second argument, theoretically a file name. The program then checks to see if the file is valid by trying to open it. This is a standard operation that is effective and easy. If the file is valid, it gets opened in the process. The code is self-explanatory, but is littered with comments, you should have no trouble understanding its operation this far into the tutorial.
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