Introduction to C
This tutorial is designed to be a stand-alone introduction to C, even if you've never programmed before. However, because C++ is a more modern language, if you're not sure if you should learn C or C++, I recommend the C++ tutorial instead, which is also designed for people who have never programmed before. Nevertheless, if you do not desire some of C++'s advanced features or simply wish to learn C instead of C++, then this tutorial is for you.
Getting set up - finding a C compiler
The very first thing you need to do, before starting out in C, is to make sure that you have a compiler. What is a compiler, you ask? A compiler turns the program that you write into an executable that your computer can actually understand and run. If you're taking a course, you probably have one provided through your school. If you're starting out on your own, your best bet is to use Code::Blocks with MinGW. If you're on Linux, you can use gcc, and if you're on Mac OS X, you can use XCode. If you haven't yet done so, go ahead and get a compiler set up--you'll need it for the rest of the tutorial.
Intro to C
Every full C program begins inside a function called "main". A function is simply a collection of commands that do "something". The main function is always called when the program first executes. From main, we can call other functions, whether they be written by us or by others or use built-in language features. To access the standard functions that comes with your compiler, you need to 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:
#include
int main()
{
printf( "I am alive! Beware.\n" );
getchar();
return 0;
}
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 stdio.h into our program before actually creating the executable. By including header files, you can gain access to many different functions--both the printf and getchar functions are included in stdio.h.
The next important 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.
Explaining your Code
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 in C, you surround the text with /* and then */ to block off everything between as a comment. Certain compiler environments or text editors will change the color of a commented area to make it easier to spot, 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 also useful to comment out sections of code in order to see how the output is affected.
Using Variables
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.
But first, 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. Each type can store different types of data.
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. Some variables also use more of the computer's memory to store their values.
It may seem strange to have multiple variable types when it seems like some variable types are redundant. But using the right variable size can be important for making your program efficient because some variables require more memory than others. For now, suffice it to say that the different variable types will almost all be used!
Before you can use a variable, you must tell the compiler about it by declaring it and telling the compiler about what its "type" is. To declare a variable you use the syntax ;. (The brackets here indicate that your replace the expression with text described within the brackets.) For instance, a basic variable declaration might look like this:
int myVariable;
Note once again the use of a semicolon at the end of the line. Even though we're not calling a function, a semicolon is still required at the end of the "expression". This code would create a variable called myVariable; now we are free to use myVariable later in the program.
It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. 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:
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.
A final restriction on variables is that variable declarations must come before other types of statements in the given "code block" (a code block is just a segment of code surrounded by { and }). So in C you must declare all of your variables before you do anything else:
Wrong
#include
int main()
{
/* wrong! The variable declaration must appear first */
printf( "Declare x next" );
int x;
return 0;
}
Fixed
#include
int main()
{
int x;
printf( "Declare x first" );
return 0;
}
Reading input
Using variables in C for input or output can be a bit of a hassle at first, but bear with it and it will make sense. We'll be using the scanf function to read in a value and then printf to read it back out. Let's look at the program and then pick apart exactly what's going on. You can even compile this and run it if it helps you follow along.
#include
int main()
{
int this_is_a_number;
printf( "Please enter a number: " );
scanf( "%d", &this_is_a_number );
printf( "You entered %d", this_is_a_number );
getchar();
return 0;
}
So what does all of this mean? We've seen the #include and main function before; main must appear in every program you intend to run, and the #include gives us access to printf (as well as scanf). (As you might have guessed, the io in stdio.h stands for "input/output"; std just stands for "standard.") The keyword int declares this_is_a_number to be an integer.
This is where things start to get interesting: the scanf function works by taking a string and some variables modified with &. The string tells scanf what variables to look for: notice that we have a string containing only "%d" -- this tells the scanf function to read in an integer. The second argument of scanf is the variable, sort of. We'll learn more about what is going on later, but the gist of it is that scanf needs to know where the variable is stored in order to change its value. Using & in front of a variable allows you to get its location and give that to scanf instead of the value of the variable. Think of it like giving someone directions to the soda aisle and letting them go get a coca-cola instead of fetching the coke for that person. The & gives the scanf function directions to the variable.
Basic If Syntax
The structure of an if statement is as follows:
if ( statement is TRUE )
Execute this line of code
Here is a simple example that shows the syntax:
if ( 5 < 10 )
printf( "Five is now less than ten, that's a big surprise" );
Here, we're just evaluating the statement, "is five less than ten", to see if it is true or not; with any luck, it is! If you want, you can write your own full program including stdio.h and put this in the main function and run it to test.
To have more than one statement execute after an if statement that evaluates to true, use braces, like we did with the body of the main function. Anything inside braces is called a compound statement, or a block. When using if statements, the code that depends on the if statement is called the "body" of the if statement.
For example:
if ( TRUE ) {
/* between the braces is the body of the if statement */
Execute all statements inside the body
}
I recommend always putting braces following if statements. If you do this, you never have to remember to put them in when you want more than one statement to be executed, and you make the body of the if statement more visually clear.
Else
Sometimes when the condition in an if statement evaluates to false, it would be nice to execute some code instead of the code executed when the statement evaluates to true. The "else" statement effectively says that whatever code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.
It can look like this:
if ( TRUE ) {
/* Execute these statements if TRUE */
}
else {
/* Execute these statements if FALSE */
}
Else if
Another use of else is when there are multiple conditional statements that may all evaluate to true, yet you want only one if statement's body to execute. You can use an "else if" statement following an if statement and its body; 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 to ensure that only one block of code is executed.
Let's look at a simple program for you to try out on your own.
#include
int main() /* Most important part of the program! */
{
int age; /* Need a variable... */
printf( "Please enter your age" ); /* Asks for age */
scanf( "%d", &age ); /* The input is put in age */
if ( age < 100 ) { /* If the age is less than 100 */
printf ("You are pretty young!\n" ); /* Just to show you it works... */
}
else if ( age == 100 ) { /* I use else just to show an example */
printf( "You are old\n" );
}
else {
printf( "You are really old\n" ); /* Executed if no other statement is */
}
return 0;
}
Lesson 3: Loops
Loops are used to repeat a block of code. Being able to have your program repeatedly execute a block of code is one of the most basic but useful tasks in programming -- many programs or websites that produce extremely complex output (such as a message board) are really only executing a single task many times. (They may be executing a small number of tasks, but in principle, to produce a list of messages only requires repeating the operation of reading in some data and displaying it.) Now, think about what this means: a loop lets you write a very simple statement to produce a significantly greater result simply by repetition.
One caveat: before going further, 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). This concept is covered in the previous tutorial. 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 syntax for a for loop is
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:
#include
int main()
{
int x;
/* The loop goes while x < 10, and x increases by one every loop*/
for ( 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. */
printf( "%d\n", x );
}
getchar();
}
This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls printf to display the value of the variable x, 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 like a stripped-down version of a for loop-- it has no initialization or update section. However, an empty condition is not legal for a while loop as it is with a for loop.
Example:
#include
int main()
{
int x = 0; /* Don't forget to declare variables */
while ( x < 10 ) { /* While x is less than 10 */
printf( "%d\n", x );
x++; /* Update x so the condition can be met eventually */
}
getchar();
}
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
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 almost the same as a while loop except that the loop body is guaranteed to execute at least once. 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 then continue to loop while the condition is true".
Example:
#include
int main()
{
int x;
x = 0;
do {
/* "Hello, world!" is printed at least one time
even though the condition is false */
printf( "Hello, world!\n" );
} while ( x != 0 );
getchar();
}
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.
Break and Continue
Two keywords that are very important to looping are break and continue. The break command will exit the most immediately surrounding loop regardless of what the conditions of the loop are. Break is useful if we want to exit a loop under special circumstances. For example, let's say the program we're working on is a two-person checkers game. The basic structure of the program might look like this:
while (true)
{
take_turn(player1);
take_turn(player2);
}
This will make the game alternate between having player 1 and player 2 take turns. The only problem with this logic is that there's no way to exit the game; the loop will run forever! Let's try something like this instead:
while(true)
{
if (someone_has_won() || someone_wants_to_quit() == TRUE)
{break;}
take_turn(player1);
if (someone_has_won() || someone_wants_to_quit() == TRUE)
{break;}
take_turn(player2);
}
This code accomplishes what we want--the primary loop of the game will continue under normal circumstances, but under a special condition (winning or exiting) the flow will stop and our program will do something else.
Continue is another keyword that controls the flow of loops. If you are executing a loop and hit a continue statement, the loop will stop its current iteration, update itself (in the case of for loops) and begin to execute again from the top. Essentially, the continue statement is saying "this iteration of the loop is done, let's continue with the loop without executing whatever code comes after me." Let's say we're implementing a game of Monopoly. Like above, we want to use a loop to control whose turn it is, but controlling turns is a bit more complicated in Monopoly than in checkers. The basic structure of our code might then look something like this:
for (player = 1; someone_has_won == FALSE; player++)
{
if (player > total_number_of_players)
{player = 1;}
if (is_bankrupt(player))
{continue;}
take_turn(player);
}
Lesson 6: Pointers in C
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.
What are pointers? Why should you care?
Pointers are aptly name: 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 you can quickly look it up. These numbers are like the memory addresses of variables. A pointer in the world of safety deposit box 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.
The cool thing is that once you can talk about the address of a variable, you'll then be able to go to that address and retrieve the data stored in it. If you happen to have a huge piece of data that you want to pass into a function, it's a lot easier to pass its location to the function than to copy every element of the data! Moreover, if you need more memory for your program, you can request more memory from the system--how do you get "back" that memory? The system tells you where it is located in memory; that is to say, you get a memory address back. And you need pointers to store the memory address.
A note about terms: the word pointer can refer either to a memory address itself, or to a variable that stores a memory address. Usually, the distinction isn't really that important: if you pass a pointer variable into a function, you're passing the value stored in the pointer--the memory address. When I want to talk about a memory address, I'll refer to it as a memory address; when I want a variable that stores a memory address, I'll call it a pointer. When a variable stores the address of another variable, I'll say that it is "pointing to" that variable.
C Pointer Syntax
Pointers require a bit of new syntax because when you have a pointer, you need the ability to both request the memory location it stores and the value stored at that memory location. Moreover, since pointers are somewhat special, you need to tell the compiler when you declare your pointer variable that the variable is a pointer, and tell the compiler what type of memory it points to.
The pointer declaration looks like this:
*;
For example, you could declare a pointer that stores the address of an integer with the following syntax:
int *points_to_integer;
Notice the use of the *. This is the key to declaring a pointer; if you add it directly before the variable name, it will declare the variable to be a pointer. Minor gotcha: if you declare multiple pointers on the same line, you must precede each of them with an asterisk:
/* one pointer, one regular int */ int *pointer1, nonpointer1; /* two pointers */ int *pointer1, *pointer2;
As I mentioned, there are two ways to use the pointer to access information: it is possible to have it give the actual address to another variable. 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 the pointer; in essence, you're taking the reference to some memory address and following it, to retrieve the actual value. It can be tricky to keep track of when you should add the asterisk. Remember that the pointer's natural use is to store a memory address; so when you use the pointer:
call_to_function_expecting_memory_address(pointer);
then it evaluates to the address. You have to add something extra, the asterisk, in order to retrieve the value stored at the address. You'll probably do that an awful lot. Nevertheless, the pointer itself is supposed to store an address, so when you use the bare pointer, you get that address back.
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
int main()
{
int x; /* A normal integer*/
int *p; /* A pointer to an integer ("*p" is an integer, so p
must be a pointer to an integer) */
p = &x; /* Read it, "assign the address of x to p" */
scanf( "%d", &x ); /* Put a value in x, we could also use p here */
printf( "%d\n", *p ); /* Note the use of the * to get the value */
getchar();
}
The printf 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 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. In fact, since we use an ampersand to pass the value to scanf, it should be clear that scanf is putting the value in the address pointed to by p. (In fact, scanf works because of pointers!)
The next line then passes *p into printf. *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.
Notice that in the above example, the 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 program. For instance, the operating system will probably prevent you from accessing memory that it knows your program doesn't own: this will cause your program to crash. If it let you use the memory, you could mess with the memory of any running program--for instance, if you had a document opened in Word, you could change the text! Fortunately, Windows and other modern operating systems will stop you from accessing that memory and cause your program to crash. To avoid crashing your program, you should always initialize pointers before you use them.
It is also possible to initialize pointers using free memory. This allows dynamic allocation of memory. It is useful for setting up structures such as linked lists or data trees where you don't know exactly how much memory will be needed at compile time, so you have to get memory during the program's execution. We'll look at these structures later, but for now, we'll simply examine how to request memory from and return memory to the operating system.
The function malloc, residing in the stdlib.h header file, is used to initialize pointers with memory from free store (a section of memory available to all programs). malloc works just like any other function call. The argument to malloc is the amount of memory requested (in bytes), and malloc gets a block of memory of that size and then returns a pointer to the block of memory allocated.
Since different variable types have different memory requirements, we need to get a size for the amount of memory malloc should return. So we need to know how to get the size of different variable types. This can be done using the keyword sizeof, which takes an expression and returns its size. For example, sizeof(int) would return the number of bytes required to store an integer.
#include int *ptr = malloc( sizeof(int) );
This code set ptr to point to a memory address of size int. 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 lest the memory be lost to the operating system for the duration of the program (this is often called a memory leak because the program is not keeping track of all of its memory).
Note that it is slightly cleaner to write malloc statements by taking the size of the variable pointed to by using the pointer directly:
int *ptr = malloc( sizeof(*ptr) );
What's going on here? sizeof(*ptr) will evaluate the size of whatever we would get back from dereferencing ptr; since ptr is a pointer to an int, *ptr would give us an int, so sizeof(*ptr) will return the size of an integer. So why do this? Well, if we later rewrite the declaration of ptr the following, then we would only have to rewrite the first part of it:
float *ptr = malloc( sizeof(*ptr) );
We don't have to go back and correct the malloc call to use sizeof(float). Since ptr would be pointing to a float, *ptr would be a float, so sizeof(*ptr) would still give the right size!
This becomes even more useful when you end up allocating memory for a variable far after the point you declare it:
float *ptr; /* hundreds of lines of code */ ptr = malloc( sizeof(*ptr) );
The free function returns memory to the operating system.
free( ptr );
After freeing 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.
The concept of the null pointer is frequently used as a way of indicating a problem--for instance, malloc returns 0 when it cannot correctly allocate memory. You want to be sure to handle this correctly--sometimes your operating system might actually run out of memory and give you this value
Lesson 7: Structures in C
When programming, it is often convenient to have a single name with which to refer to a group of a related values. Structures provide 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;
To access a variable of the structure it goes
name_of_single_structure.name_of_variable;
For example:
struct example {
int x;
};
struct example an_example; /* Treating it like a normal variable type
except with the addition of struct*/
an_example.x = 33; /*How to access its members */
Here is an example program:
struct database {
int id_number;
int age;
float salary;
};
int main()
{
struct 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 it 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:
struct 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. 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
struct xampl {
int x;
};
int main()
{
struct xampl structure;
struct xampl *ptr;
structure.x = 12;
ptr = &structure; /* Yes, you need the & when dealing with
structures and using pointers to them*/
printf( "%d\n", ptr->x ); /* The -> acts somewhat like the * when
does when it is used with pointers
It says, get whatever is at that memory
address Not "get what that memory address
is"*/
getchar();
}
Lesson 8: Arrays in C
Arrays are useful critters that often show up when it would be convenient to have one name for a group of variables of the same type that can be accessed by a numerical index. For example, a tic-tac-toe board can be held in an array and each element of the tic-tac-toe board can easily be accessed by its position (the upper left might be position 0 and the lower right position 8). At heart, 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.
One way to visualize an array is like this:
[][][][][][]
Each of the bracket pairs is a slot in the array, and you can store information in slot--the information stored in the array is called an element of the array. It is very much as though you have a group of variables lined up side by side.
Let's look at the syntax for declaring an array.
int examplearray[100]; /* This declares an array */
This would make an integer array with 100 slots (the places in which values of an array are stored). 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. The indices for a 100 element array range from 0 to 99. Be careful not to "walk off the end" of the array by trying to access element 100!
What can you do with this simple knowledge? Let's 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:
char astring[100];
will allow you to declare a char array of 100 elements, or slots. Then you can receive input into it from the user, and when the user types in a string, it will go in the array, the first character of the string will be at position 0, the second character at position 1, and so forth. It is relatively easy to work with strings in this way because it allows support for any size string you can imagine all stored in a single variable with each element in the string stored in an adjacent location--think about how hard it would be to store nearly arbitrary sized strings using simple variables that only store one value. Since we can write loops that increment integers, it's very easy to scan through a string:
char astring[10];
int i = 0;
/* Using scanf isn't really the best way to do this; we'll talk about that
in the next tutorial, on strings */
scanf( "%s", astring );
for ( i = 0; i < 10; ++i )
{
if ( astring[i] == 'a' )
{
printf( "You entered an a!\n" );
}
}
Let's look at something new here: the scanf function call is a tad different from what we've seen before. First of all, the format string is '%s' instead of '%d'; this just tells scanf to read in a string instead of an integer. Second, we don't use the ampersand! It turns out that when we pass arrays into functions, the compiler automatically converts the array into a pointer to the first element of the array. In short, the array without any brackets will act like a pointer. So we just pass the array directly into scanf without using the ampersand and it works perfectly.
Also, notice that to access the element of the array, we just use the brackets and put in the index whose value interests us; in this case, we go from 0 to 9, checking each element to see if it's equal to the character a. Note that some of these values may actually be uninitialized since the user might not input a string that fills the whole array--we'll look into how strings are handled in more detail in the next tutorial; for now, the key is simply to understand the power of accessing the array using a numerical index. Imagine how you would write that if you didn't have access to arrays! Oh boy.
Multidimensional arrays are arrays that have more than one index: instead of being just a single line of slots, multidimensional arrays can be thought of as having values that spread across two or more dimensions. Here's an easy way to visualize a two-dimensional array:
[][][][][] [][][][][] [][][][][] [][][][][] [][][][][]
The syntax used to actually declare a two dimensional array is almost the same as that used for declaring a one-dimensional array, except that you include a set of brackets for each dimension, and include the size of the dimension. For example, here is an array that is large enough to hold a standard checkers board, with 8 rows and 8 columns:
int two_dimensional_array[8][8];
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 make three dimensional, four dimensional, or even higher dimensional arrays, though past three dimensions, it becomes quite hard to visualize.
Setting the value of an array element is as easy as accessing the element and performing an assignment. For instance,
[] =
for instance,
/* set the first element of my_first to be the letter c */ my_string[0] = 'c';
or, for two dimensional arrays
[][] = ;
Let me note again that 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, (the elements 0 through 9) but the next location could be anything. Writing to random memory could cause unpredictable effects--for example you might end up writing to the video buffer and change the video display, or you might write to memory being used by an open document and altering its contents. Usually, the operating system will not allow this kind of reckless behavior and will crash the program if it tries to write to unallocated memory.
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. We've already seen one example of using loops to access arrays; here is another, more interesting, example!
#include
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 */
}
printf( "Array Indices:\n" );
for ( x = 0; x < 8;x++ ) {
for ( y = 0; y < 8; y++ )
{
printf( "[%d][%d]=%d", x, y, array[x][y] );
}
printf( "\n" );
}
getchar();
}
Just to touch upon a final point made briefly above: arrays don't require a reference operator (the ampersand) when you want to have a pointer to them. For example:
char *ptr; char str[40]; ptr = str; /* Gives the memory address without a reference operator(&) */
As opposed to
int *ptr; int num; ptr = # /* Requires & to give the memory address to the ptr */
Lesson 14: Accepting 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.
The full declaration of main looks like this:
int main ( int argc, char *argv[] )
The integer, argc is the argument count. 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
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 */
printf( "usage: %s filename", argv[0] );
}
else
{
// We assume argv[1] is a filename to open
FILE *file = fopen( argv[1], "r" );
/* fopen returns 0, the NULL pointer, on failure */
if ( file == 0 )
{
printf( "Could not open file\n" );
}
else
{
int x;
/* read one character at a time from file, stopping at EOF, which
indicates the end of the file. Note that the idiom of "assign
to a variable, check the value" used below works because
the assignment statement evaluates to the value assigned. */
while ( ( x = fgetc( file ) ) != EOF )
{
printf( "%c", x );
}
fclose( file );
}
}
}
Binary Trees in C
The binary tree is a fundamental data structure used in computer science. The binary tree is a useful data structure for rapidly storing sorted data and rapidly retrieving stored data. A binary tree is composed of parent nodes, or leaves, each of which stores data and also links to up to two other child nodes (leaves) which can be visualized spatially as below the first node with one placed to the left and with one placed to the right. It is the relationship between the leaves linked to and the linking leaf, also known as the parent node, which makes the binary tree such an efficient data structure. It is the leaf on the left which has a lesser key value (i.e., the value used to search for a leaf in the tree), and it is the leaf on the right which has an equal or greater key value. As a result, the leaves on the farthest left of the tree have the lowest values, whereas the leaves on the right of the tree have the greatest values. More importantly, as each leaf connects to two other leaves, it is the beginning of a new, smaller, binary tree. Due to this nature, it is possible to easily access and insert data in a binary tree using search and insert functions recursively called on successive leaves.
The typical graphical representation of a binary tree is essentially that of an upside down tree. It begins with a root node, which contains the original key value. The root node has two child nodes; each child node might have its own child nodes. Ideally, the tree would be structured so that it is a perfectly balanced tree, with each node having the same number of child nodes to its left and to its right. A perfectly balanced tree allows for the fastest average insertion of data or retrieval of data. The worst case scenario is a tree in which each node only has one child node, so it becomes as if it were a linked list in terms of speed. The typical representation of a binary tree looks like the following:
10 / \ 6 14 / \ / \ 5 8 11 18
The node storing the 10, represented here merely as 10, is the root node, linking to the left and right child nodes, with the left node storing a lower value than the parent node, and the node on the right storing a greater value than the parent node. Notice that if one removed the root node and the right child nodes, that the node storing the value 6 would be the equivalent a new, smaller, binary tree.
The structure of a binary tree makes the insertion and search functions simple to implement using recursion. In fact, the two insertion and search functions are also both very similar. To insert data into a binary tree involves a function searching for an unused node in the proper position in the tree in which to insert the key value. The insert function is generally a recursive function that continues moving down the levels of a binary tree until there is an unused leaf in a position which follows the rules of placing nodes. The rules are that a lower value should be to the left of the node, and a greater or equal value should be to the right. Following the rules, an insert function should check each node to see if it is empty, if so, it would insert the data to be stored along with the key value (in most implementations, an empty node will simply be a NULL pointer from a parent node, so the function would also have to create the node). If the node is filled already, the insert function should check to see if the key value to be inserted is less than the key value of the current node, and if so, the insert function should be recursively called on the left child node, or if the key value to be inserted is greater than or equal to the key value of the current node the insert function should be recursively called on the right child node. The search function works along a similar fashion. It should check to see if the key value of the current node is the value to be searched. If not, it should check to see if the value to be searched for is less than the value of the node, in which case it should be recursively called on the left child node, or if it is greater than the value of the node, it should be recursively called on the right child node. Of course, it is also necessary to check to ensure that the left or right child node actually exists before calling the function on the node.
Because binary trees have log (base 2) n layers, the average search time for a binary tree is log (base 2) n. To fill an entire binary tree, sorted, takes roughly log (base 2) n * n. Let's take a look at the necessary code for a simple implementation of a binary tree. First, it is necessary to have a struct, or class, defined as a node.
struct node
{
int key_value;
struct node *left;
struct node *right;
};
The struct has the ability to store the key_value and contains the two child nodes which define the node as part of a tree. In fact, the node itself is very similar to the node in a linked list. A basic knowledge of the code for a linked list will be very helpful in understanding the techniques of binary trees. Essentially, pointers are necessary to allow the arbitrary creation of new nodes in the tree.
There are several important operations on binary trees, including inserting elements, searching for elements, removing elements, and deleting the tree. We'll look at three of those four operations in this tutorial, leaving removing elements for later.
We'll also need to keep track of the root node of the binary tree, which will give us access to the rest of the data:
struct node *root = 0;
It is necessary to initialize root to 0 for the other functions to be able to recognize that the tree does not yet exist. The destroy_tree shown below which will actually free all of the nodes of in the tree stored under the node leaf: tree.
void destroy_tree(struct node *leaf)
{
if( leaf != 0 )
{
destroy_tree(leaf->left);
destroy_tree(leaf->right);
free( leaf );
}
}
The function destroy_tree goes to the bottom of each part of the tree, that is, searching while there is a non-null node, deletes that leaf, and then it works its way back up. The function deletes the leftmost node, then the right child node from the leftmost node's parent node, then it deletes the parent node, then works its way back to deleting the other child node of the parent of the node it just deleted, and it continues this deletion working its way up to the node of the tree upon which delete_tree was originally called. In the example tree above, the order of deletion of nodes would be 5 8 6 11 18 14 10. Note that it is necessary to delete all the child nodes to avoid wasting memory.
The following insert function will create a new tree if necessary; it relies on pointers to pointers in order to handle the case of a non-existent tree (the root pointing to NULL). In particular, by taking a pointer to a pointer, it is possible to allocate memory if the root pointer is NULL.
insert(int key, struct node **leaf)
{
if( *leaf == 0 )
{
*leaf = (struct node*) malloc( sizeof( struct node ) );
(*leaf)->key_value = key;
/* initialize the children to null */
(*leaf)->left = 0;
(*leaf)->right = 0;
}
else if(key < (*leaf)->key_value)
{
insert( key, &(*leaf)->left );
}
else if(key > (*leaf)->key_value)
{
insert( key, &(*leaf)->right );
}
}
The insert function searches, moving down the tree of children nodes, following the prescribed rules, left for a lower value to be inserted and right for a greater value, until it reaches a NULL node--an empty node--which it allocates memory for and initializes with the key value while setting the new node's child node pointers to NULL. After creating the new node, the insert function will no longer call itself. Note, also, that if the element is already in the tree, it will not be added twice.
struct node *search(int key, struct node *leaf)
{
if( leaf != 0 )
{
if(key==leaf->key_value)
{
return leaf;
}
else if(keykey_value)
{
return search(key, leaf->left);
}
else
{
return search(key, leaf->right);
}
}
else return 0;
}
C File I/O and Binary File I/O
In this tutorial, you'll learn how to do file IO, text and binary, in C, using fopen, fwrite, and fread, fprintf, fscanf, fgetc and fputc.
FILE *
For C File I/O you need to use a FILE pointer, which will let the program keep track of the file being accessed. (You can think of it as the memory address of the file or the location of the file).
For example:
FILE *fp;
fopen
To open a file you need to use the fopen function, which returns a FILE pointer. Once you've opened a file, you can use the FILE pointer to let the compiler perform input and output functions on the file.
FILE *fopen(const char *filename, const char *mode);
In the filename, if you use a string literal as the argument, you need to remember to use double backslashes rather than a single backslash as you otherwise risk an escape character such as \t. Using double backslashes \\ escapes the \ key, so the string works as it is expected. Your users, of course, do not need to do this! It's just the way quoted strings are handled in C and C++.
fopen modes
The allowed modes for fopen are as follows:
r - open for reading w - open for writing (file need not exist) a - open for appending (file need not exist) r+ - open for reading and writing, start at beginning w+ - open for reading and writing (overwrite file) a+ - open for reading and writing (append if file exists)
Note that it's possible for fopen to fail even if your program is perfectly correct: you might try to open a file specified by the user, and that file might not exist (or it might be write-protected). In those cases, fopen will return 0, the NULL pointer.
Here's a simple example of using fopen:
FILE *fp;
fp=fopen("c:\\test.txt", "r");
This code will open test.txt for reading in text mode. To open a file in a binary mode you must add a b to the end of the mode string; for example, "rb" (for the reading and writing modes, you can add the b either after the plus sign - "r+b" - or before - "rb+")
fclose
When you're done working with a file, you should close it using the function
int fclose(FILE *a_file);
fclose returns zero if the file is closed successfully.
An example of fclose is
fclose(fp);
Reading and writing with fprintf, fscanf fputc, and fgetc
To work with text input and output, you use fprintf and fscanf, both of which are similar to their friends printf and scanf except that you must pass the FILE pointer as first argument. For example:
FILE *fp;
fp=fopen("c:\\test.txt", "w");
fprintf(fp, "Testing...\n");
It is also possible to read (or write) a single character at a time--this can be useful if you wish to perform character-by-character input (for instance, if you need to keep track of every piece of punctuation in a file it would make more sense to read in a single character than to read in a string at a time.) The fgetc function, which takes a file pointer, and returns an int, will let you read a single character from a file:
int fgetc (FILE *fp);
Notice that fgetc returns an int. What this actually means is that when it reads a normal character in the file, it will return a value suitable for storing in an unsigned char (basically, a number in the range 0 to 255). On the other hand, when you're at the very end of the file, you can't get a character value--in this case, fgetc will return "EOF", which is a constant that indicates that you've reached the end of the file. To see a full example using fgetc in practice, take a look at the example here.
The fputc function allows you to write a character at a time--you might find this useful if you wanted to copy a file character by character. It looks like this:
int fputc( int c, FILE *fp );
Note that the first argument should be in the range of an unsigned char so that it is a valid character. The second argument is the file to write to. On success, fputc will return the value c, and on failure, it will return EOF.
Binary file I/O - fread and fwrite
For binary File I/O you use fread and fwrite.
The declarations for each are similar:
size_t fread(void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);
size_t fwrite(const void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);
Both of these functions deal with blocks of memories - usually arrays. Because they accept pointers, you can also use these functions with other data structures; you can even write structs to a file or a read struct into memory.
Let's look at one function to see how the notation works.
fread takes four arguments. Don't be confused by the declaration of a void *ptr; void means that it is a pointer that can be used for any type variable. The first argument is the name of the array or the address of the structure you want to write to the file. The second argument is the size of each element of the array; it is in bytes. For example, if you have an array of characters, you would want to read it in one byte chunks, so size_of_elements is one. You can use the sizeof operator to get the size of the various datatypes; for example, if you have a variable int x; you can get the size of x with sizeof(x);. This usage works even for structs or arrays. E.g., if you have a variable of a struct type with the name a_struct, you can use sizeof(a_struct) to find out how much memory it is taking up.
e.g.,
sizeof(int);
The third argument is simply how many elements you want to read or write; for example, if you pass a 100 element array, you want to read no more than 100 elements, so you pass in 100.
The final argument is simply the file pointer we've been using. When fread is used, after being passed an array, fread will read from the file until it has filled the array, and it will return the number of elements actually read. If the file, for example, is only 30 bytes, but you try to read 100 bytes, it will return that it read 30 bytes. To check to ensure the end of file was reached, use the feof function, which accepts a FILE pointer and returns true if the end of the file has been reached.
fwrite is similar in usage, except instead of reading into the memory you write from memory into a file.
For example,
FILE *fp;
fp=fopen("c:\\test.bin", "wb");
char x[10]="ABCDEFGHIJ";
fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), fp)
