Ok first off I would like to say that I DID NOT WRITE THIS However I found it in some OLD backups of disks I've found and thought I would share this. This is a GREAT beginner tutorial and it can also be used as a reference for experts... This is VERY long 20 lessons (I have them in text files which I will upload on the last post after the tutorial, but I do want the reference here on EZ (I know that I still haven't finished my batch file tut, but that one I'm writing from hand and its VERY extensive) Anyways without further ado:
Lesson 1 - The Basics
This tutorial is designed for everyone: even if you've never programmed before or if you have extensive experience programming in other languages and want to expand into C++! It is for everyone who wants the feeling of accomplishment from a working program.
Getting set up
C++ is a programming language of many different dialects, similar to the way that each spoken language has many different dialects. In C++, dialects are not because the speakers live in the North or South. Instead, it is because there are many different compilers that support slightly different features. There are several common compilers: in particular, Borland C++, Microsoft C++, and GNU C++. There are also many front-end environments for the different compilers--the most common is Dev-C++ around GNU's G++ compiler. Some, such as G++, are free, while others are not. Please see the compiler listing for more information on how to get a compiler and set it up.
Each of these compilers is slightly different. Each one should support the ANSI/ISO standard C++ functions, but each compiler will also have nonstandard functions (these functions are similar to slang spoken in different parts of a country). Sometimes the use of nonstandard functions will cause problems when you attempt to compile source code (the actual C++ written by a programmer and saved as a text file) with a different compiler. These tutorials use ANSI/ISO standard C++ and should not suffer from this problem (with sufficiently modern compilers). Note that if you are using an older compiler, such as TCLite, you should read check out some compatability issues.
If you don't have a compiler, I strongly suggest that you get one. A simple compiler is sufficient for our use, but make sure that you do get one in order to get the most from these tutorials. The page linked above, compilers, lists compilers by operating system.
C++ is a different breed of programming language. A C++ program begins with a function, a collection of commands that do "something". The function that begins a C++ program is called main; this function is always called when the program first executes. From main, we can also call other functions whether they be written by us or by others. To access a standard function that comes with the compiler, you include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program:
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
cin.get();
}
Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called iostream into our program before actually creating the executable. By including header files, you an gain access to many different functions. For example, the cout function requires iostream. Following the include is the statement, "using namespace std;". This line tells the compiler to use a group of functions that are part of the standard library (std). By including this line at the top of a file, you allow the program to use functions such as cout. The semicolon is part of the syntax of C and C++. It tells the compiler that you're at the end of a command. You will see later that the semicolon is used to end most commands in C++.
The next imporant line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ({ and }) signal the beginning and end of functions and other code blocks. If you have programmed in Pascal, you will know them as BEGIN and END. Even if you haven't programmed in Pascal, this is a good way to think about their meaning.
The next line of the program may seem strange. If you have programmed in another language, you might expect that print would be the function used to display text. In C++, however, the cout object is used to display text. It uses the << symbols, known as "insertion operators", to indicate what to output. cout<< results in a function call with the ensuing text as an argument to the function. The quotes tell the compiler that you want to output the literal string as-is. The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail). It moves the cursor on your screen to the next line. Again, notice the semicolon: it is added onto the end of all, such as function calls, in C++.
The next command is cin.get(). This is another function call: it reads in input and expects the user to hit the return key. Many compiler environments will open a new console window, run the program, and then close the window. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run.
Upon reaching the end of main, the closing brace, our program will return the value of 0 (and integer, hence why we told main to return an int) to the operating system. This return value is important as it can be used to tell the OS whether our program succeeded or not. A return value of 0 means success and is returned automatically (but only for main, other functions require you to manually return a value), but if we wanted to return something else, such as 1, we would have to do it with a return statement:
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
cin.get();
return 1;
}
The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .cpp (or whatever extension your compiler requires) file. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse.
You might start playing around with the cout function and get used to writing C++.
Comments are critical for all but the most trivial programs and this tutorial will often use them to explain sections of code. When you tell the compiler a section of text is a comment, it will ignore it when running the code, allowing you to use any text you want to describe the real code. To create a comment use either //, which tells the compiler that the rest of the line is a comment, or /* and then */ to block off everything between as a comment. Certain compiler environments will change the color of a commented area, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program. When you are learning to program, it is useful to be able to comment out sections of code in order to see how the output is affected.
So far you should be able to write a simple program to display information typed in by you, the programmer and to describe your program with comments. That's great, but what about interacting with your user? Fortunately, it is also possible for your program to accept input. The function you use is known as cin, and is followed by the insertion operator >>.
Of course, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float.
A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable.
Sometimes it can be confusing to have multiple variable types when it seems like some variable types are redundant. Using the right variable size can be important for making your code readable and for efficiency--some variables require more memory than others. For now, suffice it to say that the different variable types will almost all be used!
To declare a variable you use the syntax type <name>. It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. The declaration of a variable or set of variables should be followed by a semicolon (Note that this is the same procedure used when you call a function). If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake.
Here are some variable declaration examples:
Code: [Select]
int x;
int a, b, c, d;
char letter;
float the_float;
While you can have multiple variables of the same type, you cannot have multiple variables with the same name. Moreover, you cannot have variables and functions with the same name.
Here is a sample program demonstrating the use a a variable:
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
int thisisanumber;
cout<<"Please enter a number: ";
cin>> thisisanumber;
cin.ignore();
cout<<"You entered: "<< thisisanumber <<"\n";
cin.get();
}
Let's break apart this program and examine it line by line. The keyword int declares thisisanumber to be an integer. The function cin>> reads a value into thisisanumber; the user must press enter before the number is read by the program. cin.ignore() is another function that reads and discards a character. Remember that when you type intput into a program, it takes the enter key too. We don't need this, so we throw it away. Keep in mind that the variable was declared an integer; if the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored). Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Notice that when printing out a variable quotation marks are not used. Were there quotation marks, the output would be "You Entered: thisisanumber." The lack of quotation marks informs the compiler that there is a variable, and therefore that the program should check the value of the variable in order to replace the variable name with the variable when executing the output function. Do not be confused by the inclusion of two separate insertion operators on one line. Including multiple insertion operators on one line is perfectly acceptable and all of the output will go to the same place. In fact, you must separate string literals (strings enclosed in quotation marks) and variables by giving each its own insertion operators (<<). Trying to put two variables together with only one << will give you an error message, do not try it. Do not forget to end functions and declarations with a semicolon. If you forget the semicolon, the compiler will give you an error message when you attempt to compile the program.
Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C++ == is used for that task. The equal sign is still extremely useful. It sets the left input to the equal sign, which must be one, and only one, variable equal to the value on the right side of the equal sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.
Here are a few examples:
Code: [Select]
a = 4 * 6; // (Note use of comments and of semicolon) a is 24
a = a + 5; // a equals the original value of a with five added to it
a == 5 // Does NOT assign five to a. Rather, it checks to see if a equals 5.
The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is useful in other areas of C++; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators.
For example:
Code: [Select]
a < 5 // Checks to see if a is less than five
a > 5 // Checks to see if a is greater than five
a == 5 // Checks to see if a equals five, for good measure
Lesson 2 - IF
The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user entered password, your program can decide whether a user is allowed access to the program. Without a conditional statement such as the if statement, programs would run almost the exact same way every time. If statements allow the flow of the program to be changed, and so they allow algorithms and more interesting code.
Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison with the relational operators, the operator will return 1 if the comparison is true, or 0 if the comparison is false. For example, the check 0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use a cout statement to output the result of those various comparisons (for example cout<< ( 2 == 1 );)
When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other.
There are a number of operators that allow these checks.
Here are the relational operators, as they are known, along with examples:
Code: [Select]
> greater than 5 > 4 is TRUE
< less than 4 < 5 is TRUE
>= greater than or equal 4 >= 4 is TRUE
<= less than or equal 3 <= 4 is TRUE
== equal to 5 == 5 is TRUE
!= not equal to 5 != 4 is TRUE
It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding. Now that you understand TRUE and FALSE in computer terminology as well as the comparison operators, let us look at the actual structure of if statements.
The structure of an if statement is as follows:
Code: [Select]
if ( TRUE )
Execute the next statement
To have more than one statement execute after an if statement that evaluates to true, use braces, like we did with the body of a function. Anything inside braces is called a compound statement, or a block.
For example:
Code: [Select]
if ( TRUE ) {
Execute all statements inside the braces
}
There is also the else statement. The code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.
It can look like this:
Code: [Select]
if ( TRUE ) {
// Execute these statements if TRUE
}
else {
// Execute these statements if FALSE
}
One use for else is if there are two conditional statements that may both evaluate to true, yet you wish only one of the two to have the code block following it to be executed. You can use an else if after the if statement; that way, if the first statement is true, the else if will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements.
Let's look at a simple program for you to try out on your own.
Code: [Select]
#include <iostream>
using namespace std;
int main() // Most important part of the program!
{
int age; // Need a variable...
cout<<"Please input your age: "; // Asks for age
cin>> age; // The input is put in age
cin.ignore(); // Throw away enter
if ( age < 100 ) { // If the age is less than 100
cout<<"You are pretty young!\n"; // Just to show you it works...
}
else if ( age == 100 ) { // I use else just to show an example
cout<<"You are old\n"; // Just to show you it works...
}
else {
cout<<"You are really old\n"; // Executed if no other statement is
}
cin.get();
}
Boolean operators allow you to create more complex conditional statements. For example, if you wish to check if a variable is both greater than five and less than ten, you could use the boolean AND to ensure both var > 5 and var < 10 are true. In the following discussion of boolean operators, I will capitalize the boolean operators in order to distinguish them from normal english. The actual C++ operators of equivalent function will be described further into the tutorial - the C++ symbols are not: OR, AND, NOT, although they are of equivalent function.
When using if statements, you will often wish to check multiple different conditions. You must understand the Boolean operators OR, NOT, and AND. The boolean operators function in a similar way to the comparison operators: each returns 0 if evaluates to FALSE or 1 if it evaluates to TRUE.
NOT: The NOT operator accepts one input. If that input is TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1) evalutes to 0, and NOT (0) evalutes to 1. NOT (any number but zero) evaluates to 0. In C and C++ NOT is written as !. NOT is evaluated prior to both AND and OR.
AND: This is another important command. AND returns TRUE if both inputs are TRUE (if 'this' AND 'that' are true). (1) AND (0) would evaluate to zero because one of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND (1) evaluates to 1. (any number but 0) AND (0) evaluates to 0. The AND operator is written && in C++. Do not be confused by thinking it checks equality between numbers: it does not. Keep in mind that the AND operator is evaluated before the OR operator.
OR: Very useful is the OR statement! If either (or both) of the two values it checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. The OR is written as || in C++. Those are the pipe characters. On your keyboard, they may look like a stretched colon. On my computer the pipe shares its key with \. Keep in mind that OR will be evaluated after AND.
It is possible to combine several boolean operators in a single statement; often you will find doing so to be of great value when creating complex expressions for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is because 1 && 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).
Try some of these - they're not too hard.
Code: [Select]
A. !( 1 || 0 ) ANSWER: 0
B. !( 1 || 1 && 0 ) ANSWER: 0 (AND is evaluated before OR)
C. !( ( 1 || 0 ) && 0 ) ANSWER: 1 (Parenthesis are useful)
Lesson 3 - Loops
Loops are used to repeat a block of code. You should understand the concept of C++'s true and false, because it will be necessary when working with loops (the conditions are the same as with if statements). There are three types of loops: for, while, and do..while. Each of them has their specific uses. They are all outlined below.
FOR - for loops are the most useful type. The layout is
Code: [Select]
for ( variable initialization; condition; variable update ) {
Code to execute while the condition is true
}
The variable initialization allows you to either declare a variable and give it a value or give a value to an already existing variable. Second, the condition tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable update section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you could call other functions that do nothing to the variable but still have a useful effect on the code. Notice that a semicolon separates each of these sections, that is important. Also note that every single one of the sections may be empty, though the semicolons still have to be there. If the condition is empty, it is evaluated as true and the loop will repeat until something else stops it.
Example:
Code: [Select]
#include <iostream>
using namespace std; // So the program can see cout and endl
int main()
{
// The loop goes while x < 10, and x increases by one every loop
for ( int x = 0; x < 10; x++ ) {
// Keep in mind that the loop condition checks
// the conditional statement before it loops again.
// consequently, when x equals 10 the loop breaks.
// x is updated before the condition is checked.
cout<< x <<endl;
}
cin.get();
}
This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls cout<< x <<endl; and it adds 1 to x until the condition is met. Keep in mind also that the variable is incremented after the code in the loop is run for the first time.
WHILE - WHILE loops are very simple. The basic structure is
while ( condition ) { Code to execute while the condition is true } The true represents a boolean expression which could be x == 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x = =5 || v == 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is the same as a for loop without the initialization and update sections. However, an empty condition is not legal for a while loop as it is with a for loop.
Example:
Code: [Select]
#include <iostream>
using namespace std; // So we can see cout and endl
int main()
{
int x = 0; // Don't forget to declare variables
while ( x < 10 ) { // While x is less than 10
cout<< x <<endl;
x++; // Update x so the condition can be met eventually
}
cin.get();
}
This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is that when it reaches the brace at the end it jumps back up to the beginning of the loop, which checks the condition again and decides whether to repeat the block another time, or stop and move to the next statement after the block.
DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure is
Code: [Select]
do {
} while ( condition );
Notice that the condition is tested at the end of the block instead of the beginning, so the block will be executed at least once. If the condition is true, we jump back to the beginning of the block and execute it again. A do..while loop is basically a reversed while loop. A while loop says "Loop while the condition is true, and execute this block of code", a do..while loop says "Execute this block of code, and loop while the condition is true".
Example:
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
int x;
x = 0;
do {
// "Hello, world!" is printed at least one time
// even though the condition is false
cout<<"Hello, world!\n";
} while ( x != 0 );
cin.get();
}
Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.
Lesson 4 - Functions
Now that you should have learned about variables, loops, and conditional statements it is time to learn about functions. You should have an idea of their uses as we have already use them and defined one in the guise of main. cin.get() is an example of a function. In general, functions are blocks of code that perform a number of pre-defined commands to accomplish something productive.
Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four.
For example:
Code: [Select]
#include <cstdlib> // Include rand()
using namespace std; // Make rand() visible
int a = rand(); // rand is a standard function that all compilers have
Do not think that 'a' will change at random, it will be set to the value returned when the function is called, but it will not change again.
The general format for a prototype is simple:
Code: [Select]
return-type function_name ( arg_type arg1, ..., arg_type argN );
There can be more than one argument passed to a function or none at all (where the parentheses are empty), and it does not have to return a value. Functions that do not return values have a return type of void. Lets look at a function prototype:
Code: [Select]
int mult ( int x, int y );
This prototype specifies that the function mult will accept two arguments, both integers, and that it will return an integer. Do not forget the trailing semi-colon. Without it, the compiler will probably think that you are trying to write the actual definition of the function.
When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a block with the code that the function is to execute, just as you would write it for the main function. Any of the arguments passed to the function can be used as if they were declared in the block. Finally, end it all with a cherry and a closing brace. Okay, maybe not a cherry.
Lets look at an example program:
Code: [Select]
#include <iostream>
using namespace std;
int mult ( int x, int y );
int main()
{
int x;
int y;
cout<<"Please input two numbers to be multiplied: ";
cin>> x >> y;
cin.ignore();
cout<<"The product of your two numbers is "<< mult ( x, y ) <<"\n";
cin.get();
}
int mult ( int x, int y )
{
return x * y;
}
This program begins with the only necessary include file and a directive to make the std namespace visible. Everything in the standard headers is inside of the std namespace and not visible to our programs unless we make them so. Next is the prototype of the function. Notice that it has the final semi-colon! The main function returns an integer, which you should always have to conform to the standard. You should not have trouble understanding the input and output functions. It is fine to use cin to input to variables as the program does. But when typing in the numbers, be sure to separate them by a space so that cin can tell them apart and put them in the right variables.
Notice how cout actually outputs what appears to be the mult function. What is really happening is cout is printing the value returned by mult, not mult itself. The result would be the same as if we had use this print instead
Code: [Select]
cout<<"The product of your two numbers is "<< x * y <<"\n";
The mult function is actually defined below main. Due to its prototype being above main, the compiler still recognizes it as being defined, and so the compiler will not give an error about mult being undefined. As long as the prototype is present, a function can be used even if there is no definition. However, the code cannot be run without a definition even though it will compile. The prototype and definition can be combined into one also. If mult were defined before it is used, we could do away with the prototype because the definition can act as a prototype as well.
Return is the keyword used to force the function to return a value. Note that it is possible to have a function that returns no value. If a function returns void, the retun statement is valid, but only if it does not have an expression. In otherwords, for a function that returns void, the statement "return;" is legal, but redundant.
The most important functional (Pun semi-intended) question is why do we need a function? Functions have many uses. For example, a programmer may have a block of code that he has repeated forty times throughout the program. A function to execute that code would save a great deal of space, and it would also make the program more readable. Also, having only one copy of the code makes it easier to make changes. Would you rather make forty little changes scattered all throughout a potentially large program, or one change to the function body? So would I.
Another reason for functions is to break down a complex program into logical parts. For example, take a menu program that runs complex code when a menu choice is selected. The program would probably best be served by making functions for each of the actual menu choices, and then breaking down the complex tasks into smaller, more manageable tasks, which could be in their own functions. In this way, a program can be designed that makes sense when read. And has a structure that is easier to understand quickly. The worst programs usually only have the required function, main, and fill it with pages of jumbled code.
Lesson 5 - Switch Case
Switch case statements are a substitute for long if statements that compare to an integral value. The basic format for using switch case is outlined below.
Code: [Select]
switch ( value ) {
case this:
Code to execute if value == this
break;
case that:
Code to execute if value == that
break;
...
default:
Code to execute if value != this or that
break;
}
The condition of a switch statement is a value. The case says that if it has the value of whatever is after that case then do whatever follows the colon. The break is used to break out of the case statements. Break is a keyword that breaks out of the code block, usually surrounded by braces, which it is in. In this case, break prevents the program from falling through and executing the code in all the other case statements. An important thing to note about the switch statement is that the case values may only be constant integral expressions. Sadly, it isn't legal to use case like this:
Code: [Select]
int a = 10;
int b = 10;
int c = 20;
switch ( a ) {
case b:
// Code
break;
case c:
// Code
break;
default:
// Code
break;
}
The default case is optional, but it is wise to include it as it handles any unexpected cases. Switch statements serves as a simple way to write long if statements when the requirements are met. Often it can be used to process input from a user.
Below is a sample program, in which not all of the proper functions are actually declared, but which shows how one would use switch in a program.
Code: [Select]
#include <iostream>
using namespace std;
void playgame();
void loadgame();
void playmultiplayer();
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.Lesson 6 - Pointers
Pointers can be confusing, and at times, you may wonder why you would ever want to use them. The truth is, they can make some things much easier. 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 allowing certain programming techniques, such as linked lists and resizable arrays. Pointers are what they sound like...pointers. They point to locations in memory. Picture a big jar that holds the location of another jar. In the other jar holds a piece of paper with the number 12 written on it. The jar with the 12 is an integer, and the jar with the memory address of the 12 is a pointer.
Pointer syntax can also be confusing, because pointers can both give the memory location and give the actual value stored in that same location. When a pointer is declared, the syntax is this: variable_type *name; Notice the *. This is the key to declaring a pointer, if you use it before the variable name, it will declare the variable to be a pointer.
As I have said, there are two ways to use the pointer to access information about the memory address it points to. It is possible to have it give the actual address to another variable, or to pass it into a function. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing.
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 the variable, put the & sign in front of the variable name. This makes it give its address. This is called the address operator, because it returns the memory address.
For example:
Code: [Select]
#include <iostream>
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 in x. Why is that? Well, 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 operator (&). If you wish, you can think of it as if the jar that had the integer had a ampersand in it then it would output its name (in pointers, the memory address) Then the user inputs the value for x. Then the cout uses the * to put the value stored in the memory location of pointer. If the jar with the name of the other jar in it had a * in front of it would give the value stored in the jar with the same name as the one in the jar with the name. It is not too hard, the * gives the value in the location. The unasterisked gives the memory location.
Notice that in the above example, pointer is initialized to point to a specific memory address before it is used. If this was not the case, it could be pointing to anything. This can lead to extremely unpleasant consequences to the computer. You should always initialize pointers before you use them.
It is also possible to initialize pointers using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists. This difficult topic is too complex for this text. An understanding of the keywords new and delete will, however, be tremendously helpful in the future.
The keyword new is used to initialize pointers with memory from free store (a section of memory available to all programs). The syntax looks like the example:
Code: [Select]
int *ptr = new int;
It initializes ptr to point to a memory address of size int (because variables have different sizes, number of bytes, this is necessary). The memory that is pointed to becomes unavailable to other programs. This means that the careful coder should free this memory at the end of its usage.
The delete operator frees up the memory allocated through new. To do so, the syntax is as in the example.
Code: [Select]
delete ptr;
After deleting a pointer, it is a good idea to reset it to point to 0. When 0 is assigned to a pointer, the pointer becomes a null pointer, in other words, it points to nothing. By doing this, when you do something foolish with the pointer (it happens a lot, even with experienced programmers), you find out immediately instead of later, when you have done considerable damage.
Lesson 7 - Structures
Before discussing classes, this lesson will be an introduction to data structures similar to classes. Structures are a way of storing many different variables of 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. It is also useful for databases.
The format for declaring a structure (in C++, it is different in C) is
Code: [Select]
struct Tag {
Members
};
Where Tag is the name of the entire type of structure. To actually create a single structure the syntax is
Code: [Select]
Tag name_of_single_structure;
To access a variable of the structure it goes
Code: [Select]
name_of_single_structure.name_of_variable;
For example:
Code: [Select]
struct example {
int x;
};
example an_example; //Treating it like a normal variable type
an_example.x = 33; //How to access it's members
Here is an example program:
Code: [Select]
struct database {
int id_number;
int age;
float salary;
};
int main()
{
database employee; //There is now an employee variable that has modifiable
// variables inside it.
employee.age = 22;
employee.id_number = 1;
employee.salary = 12000.21;
}
The struct database declares that database has three variables in it, age, id_number, and salary. You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. For instance:
Code: [Select]
database fn();
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. Its 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:
Code: [Select]
#include <iostream>
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();
}
Lesson 8 - Arrays
Arrays are useful critters because they can be used in many ways. 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.
Think about arrays like this:
Code: [Select]
[][][][][][]
Each of the bracket pairs is a slot(element) in the array, and you can put information into each one of them. It is almost like having a group of variables side by side. Lets look at the syntax for declaring an array.
Code: [Select]
int examplearray[100]; // This declares an array
This would make an integer array with 100 slots, or places to store values(also called elements). To access a specific part element of the array, you merely put the array name and, in brackets, an index number. This corresponds to a specific element of the array. The one trick is that the first index number, and thus the first element, is zero, and the last is the number of elements minus one. 0-99 in a 100 element array, for example.
What can you do with this simple knowledge? Lets say you want to store a string, because C++ has no built-in datatype for strings, you can make an array of characters.
For example:
Code: [Select]
char astring[100];
will allow you to declare a char array of 100 elements, or slots. Then you can receive input into it it from the user, and if the user types in a long string, it will go in the array. The neat thing is that it is very easy to work with strings in this way, and there is even a header file called cstring. There is another lesson on the uses of strings, so its not necessary to discuss here.
The most useful aspect of arrays is multidimensional arrays. How I think about multi-dimensional arrays:
Code: [Select]
[][][][][]
[][][][][]
[][][][][]
[][][][][]
[][][][][]
This is a graphic of what a two-dimensional array looks like when I visualize it.
For example:
Code: [Select]
int twodimensionalarray[8][8];
declares an array that has two dimensions. Think of it as a chessboard. You can easily use this to store information about some kind of game or to write something like tic-tac-toe. To access it, all you need are two variables, one that goes in the first slot and one that goes in the second slot. You can even make a three dimensional array, though you probably won't need to. In fact, you could make a four-hundred dimensional array. It would be confusing to visualize, however. Arrays are treated like any other variable in most ways. You can modify one value in it by putting:
Code: [Select]
arrayname[arrayindexnumber] = whatever;
or, for two dimensional arrays
Code: [Select]
arrayname[arrayindexnumber1][arrayindexnumber2] = whatever;
However, you should never attempt to write data past the last element of the array, such as when you have a 10 element array, and you try to write to the [10] element. The memory for the array that was allocated for it will only be ten locations in memory, but the next location could be anything, which could crash your computer.
You will find lots of useful things to do with arrays, from storing information about certain things under one name, to making games like tic-tac-toe. One suggestion I have is to use for loops when access arrays.
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
int x;
int y;
int array[8][8]; // Declares an array like a chessboard
for ( x = 0; x < 8; x++ ) {
for ( y = 0; y < 8; y++ )
array[x][y] = x * y; // Set each element to a value
}
cout<<"Array Indices:\n";
for ( x = 0; x < 8;x++ ) {
for ( y = 0; y < 8; y++ )
cout<<"["<<x<<"]["<<y<<"]="<< array[x][y] <<" ";
cout<<"\n";
}
cin.get();
}
Here you see that the loops work well because they increment the variable for you, and you only need to increment by one. Its the easiest loop to read, and you access the entire array.
One thing that arrays don't require that other variables do, is a reference operator when you want to have a pointer to the string. For example:
Code: [Select]
char *ptr;
char str[40];
ptr = str; // Gives the memory address without a reference operator(&)
As opposed to
Code: [Select]
int *ptr;
int num;
ptr = # // Requires & to give the memory address to the ptr
The reason for this is that when an array name is used as an expression, it refers to a pointer to the first element, not the entire array. This rule causes a great deal of confusion.
Lesson 9 - Strings
In C++ there are two types of strings, C-style strings, and C++-style strings. This lesson will discuss C-style strings. C-style strings are really arrays, but there are some different functions that are used for strings, like adding to strings, finding the length of strings, and also of checking to see if strings match. The definition of a string would be anything that contains more than one character strung together. For example, "This" is a string. However, single characters will not be strings, though they can be used as strings.
Strings are arrays of chars. String literals are words surrounded by double quotation marks.
Code: [Select]
"This is a static string"
To declare a string of 49 letters, you would want to say:
Code: [Select]
char string[50];
This would declare a string with a length of 50 characters. Do not forget that arrays begin at zero, not 1 for the index number. In addition, a string ends with a null character, literally a '\0' character. However, just remember that there will be an extra character on the end on a string. It is like a period at the end of a sentence, it is not counted as a letter, but it still takes up a space. Technically, in a fifty char array you could only hold 49 letters and one null character at the end to terminate the string.
TAKE NOTE: char *arry; Can also be used as a string. If you have read the tutorial on pointers, you can do something such as:
Code: [Select]
arry = new char[256];
which allows you to access arry just as if it were an array. Keep in mind that to use delete you must put [] between delete and arry to tell it to free all 256 bytes of memory allocated.
For example:
Code: [Select]
delete [] arry.
Strings are useful for holding all types of long input. If you want the user to input his or her name, you must use a string. Using cin>> to input a string works, but it will terminate the string after it reads the first space. The best way to handle this situation is to use the function cin.getline. Technically cin is a class (a beast similar to a structure), and you are calling one of its member functions. The most important thing is to understand how to use the function however.
The prototype for that function is:
Code: [Select]
istream& getline(char *buffer, int length, char terminal_char);
The char *buffer is a pointer to the first element of the character array, so that it can actually be used to access the array. The int length is simply how long the string to be input can be at its maximum (how big the array is). The char terminal_char means that the string will terminate if the user inputs whatever that character is. Keep in mind that it will discard whatever the terminal character is.
It is possible to make a function call of cin.getline(arry, 50); without the terminal character. Note that '\n' is the way of actually telling the compiler you mean a new line, i.e. someone hitting the enter key.
For a example:
Code: [Select]
#include <iostream>
using namespace std;
int main()
{
char string[256]; // A nice long string
cout<<"Please enter a long string: ";
cin.getline ( string, 256, '\n' ); // Input goes into string
cout<<"Your long string was: "<< string <<endl;
cin.get();
}
Remember that you are actually passing the address of the array when you pass string because arrays do not require an address operator (&) to be used to pass their address. Other than that, you could make '\n' any character you want (make sure to enclose it with single quotes to inform the compiler of its character status) to have the getline terminate on that character.
cstring is a header file that contains many functions for manipulating strings. One of these is the string comparison function.
Code: [Select]
int strcmp ( const char *s1, const char *s2 );
strcmp will accept two strings. It will return an integer. This integer will either be:
Code: [Select]
Negative if s1 is less than s2.
Zero if s1 and s2 are equal.
Positive if s1 is greater than s2.
Strcmp is case sensitive. Strcmp also passes the address of the character array to the function to allow it to be accessed.
Code: [Select]
char *strcat ( char *dest, const char *src );
strcat is short for string concatenate, which means to add to the end, or append. It adds the second string to the first string. It returns a pointer to the concatenated string. Beware this function, it assumes that dest is large enough to hold the entire contents of src as well as its own contents.
Code: [Select]
char *strcpy ( char *dest, const char *src );
strcpy is short for string copy, which means it copies the entire contents of src into dest. The contents of dest after strcpy will be exactly the same as src such that strcmp ( dest, src ) will return 0.
Code: [Select]
size_t strlen ( const char *s );
strlen will return the length of a string, minus the termating character ('\0'). The size_t is nothing to worry about. Just treat it as an integer that cannot be negative, which it is.
Here is a small program using many of the previously described functions:
Code: [Select]
#include <iostream> //For cout
#include <cstring> //For the string functions
using namespace std;
int main()
{
char name[50];
char lastname[50];
char fullname[100]; // Big enough to hold both name and lastname
cout<<"Please enter your name: ";
cin.getline ( name, 50 );
if ( strcmp ( name, "Julienne" ) == 0 ) // Equal strings
cout<<"That's my name too.\n";
else // Not equal
cout<<"That's not my name.\n";
// Find the length of your name
cout<<"Your name is "<< strlen ( name ) <<" letters long\n";
cout<<"Enter your last name: ";
cin.getline ( lastname, 50 );
fullname[0] = '\0'; // strcat searches for '\0' to cat after
strcat ( fullname, name ); // Copy name into full name
strcat ( fullname, " " ); // We want to separate the names by a space
strcat ( fullname, lastname ); // Copy lastname onto the end of fullname
cout<<"Your full name is "<< fullname <<"\n";
cin.get();
}
Lesson 10 - File I-O
This is a slightly more advanced topic than what I have covered so far, but I think that it is useful. File I/O is reading from and writing to files. This lesson will only cover text files, that is, files that are composed only of ASCII text.
C++ has two basic classes to handle files, ifstream and ofstream. To use them, include the header file fstream. Ifstream handles file input (reading from files), and ofstream handles file output (writing to files). The way to declare an instance of the ifstream or ofstream class is:
Code: [Select]
ifstream a_file;
or
Code: [Select]
ifstream a_file ( "filename" );
The constructor for both classes will actually open the file if you pass the name as an argument. As well, both classes have an open command (a_file.open()) and a close command (a_file.close()). You aren't required to use the close command as it will automatically be called when the program terminates, but if you need to close the file long before the program ends, it is useful.
The beauty of the C++ method of handling files rests in the simplicity of the actual functions used in basic input and output operations. Because C++ supports overloading operators, it is possible to use << and >> in front of the instance of the class as if it were cout or cin. In fact, file streams can be used exactly the same as cout and cin after they are opened.
For example:
Code: [Select]
#include <fstream>
#include <iostream>
using namespace std;
int main()
{
char str[10];
//Creates an instance of ofstream, and opens example.txt
ofstream a_file ( "example.txt" );
// Outputs to example.txt through a_file
a_file<<"This text will now be inside of example.txt";
// Close the file stream explicitly
a_file.close();
//Opens for reading the file
ifstream b_file ( "example.txt" );
//Reads one string from the file
b_file>> str;
//Should output 'this'
cout<< str <<"\n";
// b_file is closed implicitly here
}
The default mode for opening a file with ofstream's constructor is to create it if it does not exist, or delete everything in it if something does exist in it. If necessary, you can give a second argument that specifies how the file should be handled. They are listed below:
Code: [Select]
ios::app -- Append to the file
ios::ate -- Set the current position to the end
ios::trunc -- Delete everything in the file
For example:
Code: [Select]
ofstream a_file ( "test.txt", ios::app );
This will open the file without destroying the current contents and allow you to append new data. When opening files, be very careful not to use them if the file could not be opened. This can be tested for very easily:
Code: [Select]
ifstream a_file ( "example.txt" );
if ( !a_file.is_open() ) {
// The file could not be opened
}
else {
// Safely use the file stream
}Lesson 11 - Typecasting
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.
For example:
Code: [Select]
#include <iostream>
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.
Code: [Select]
#include <iostream>
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();
}
The typecast described above is a C-style cast, C++ supports two other types. First is the function-style cast:
Code: [Select]
int main()
{
cout<< char ( 65 ) <<"\n";
cin.get();
}
This is more like a function call than a cast as the type to be cast to is like the name of the function and the value to be cast is like the argument to the function. Next is the named cast, of which there are four:
Code: [Select]
int main()
{
cout<< static_cast<char> ( 65 ) <<"\n";
cin.get();
}
static_cast is similar in function to the other casts described above, but the name makes it easier to spot and less tempting to use since it tends to be ugly. Typecasting should be avoided whenever possible. The other three types of named casts are const_cast, reinterpret_cast, and dynamic_cast. They are of no use to us at this time.
Lesson 12 - Classes
C++ is a bunch of small additions to C, and one major addition. This one addition is the object-oriented approach. As its name suggests, this deals with objects. Of course, these are not real-life objects. 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.
What is this mysterious beast? Well, it is not only a collection of variables under one heading, but it is a collection of functions under that same heading. If a class is a house, then the functions will be the doors and the variables will be the items inside the house. They usually will be the only way to modify the variables in this structure, and they are usually the only to access the variables in this structure. The idea to make programs more modular - encapsulation. A section of code will have its own functions and variables that control what it can do, and it does not require anything outside of itself to function. While the class may require initialization with data, it does not require outside variables or functions to manipulate the data. That allows programs to reuse the same code more easily.
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 that which is not part of the class, access the variables specified as public. The protected restriction prevents functions outside the class to access the variable. The syntax for that 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 prototypes) 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 should always contain two functions: the constructor and 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. The only time the constructor is called is when the programmer declares an instance of the class, which will automatically call the constructor. The only time the destructor is called is when the instance of the class is no longer needed. When the program ends, or when its memory is deallocated (if you do not understand the deallocation part, do not worry). Keeps in mind this: NEITHER constructors NOR destructors RETURN AN ARGUMENT! This means you do not want to try to return a value in them.
The syntax for defining a function that is a member of a class outside of the actual class definition is to put the return type, then put the class name, two colons, and then the function name. This tells the compiler that the function is a member of that class.
For example:
Code: [Select]
#include <iostream>
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 funct