Chapter 8- ARRAYS

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? 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:


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 relatvely 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 */

Chapter 7- STRUCTURES

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. 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:


#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();
}

Chapter 6- Introduction to POINTERS...

Pointers are an extremely powerful programming tool. They can make some things much easier, help improve your program's efficiency, and even allow you to handle unlimited amounts of data. For example, using pointers is one way to have a function modify a variable passed to it. It is also possible to use pointers to dynamically allocate memory, which means that you can write programs that can handle nearly unlimited amounts of data on the fly--you don't need to know, when you write the program, how much memory you need. Wow, that's kind of cool. Actually, it's very cool, as we'll see in some of the next tutorials. For now, let's just get a basic handle on what pointers are and how you use them.

What are pointers? Why should you care?
Pointers are aptly name: they "point" to locations in memory. Think of a row of safety deposity boxes of various sizes at a local bank. Each safety deposity 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 jewelery. 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 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 that 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.
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 becuase 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 = malloc( sizeof(*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!

Taking Stock of Pointers
Pointers may feel like a very confusing topic at first but I think anyone can come to appreciate and understand them. If you didn't feel like you absorbed everything about them, just take a few deep breaths and re-read the lesson. You shouldn't feel like you've fully grasped every nuance of when and why you need to use pointers, though you should have some idea of some of their basic uses.

Chapter 5- SWITCH CASE

Switch case statements are a substitute for long if statements that compare a variable to several "integral" values ("integral" values are simply values that can be expressed as an integer, such as the value of a char). The basic format for using switch case is outlined below. The value of the variable given into switch is compared to the value following each of the cases, and when one value matches the value of the variable, the computer continues executing the program from that point.


switch ( ) {
case this-value:
Code to execute if == this-value
break;
case that-value:
Code to execute if == that-value
break;
...
default:
Code to execute if does not equal the value following any of the cases
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:


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. It can be useful to put some kind of output to alert you to the code entering the default case if you don't expect it to. Switch statements serve 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.


#include

void playgame();
void loadgame();
void playmultiplayer();

int main()
{
int input;

printf( "1. Play game\n" );
printf( "2. Load game\n" );
printf( "3. Play multiplayer\n" );
printf( "4. Exit\n" );
printf( "Selection: " );
scanf( "%d", &input );
switch ( input ) {
case 1: /* Note the colon, not a semicolon */
playgame();
break;
case 2:
loadgame();
break;
case 3:
playmultiplayer();
break;
case 4:
printf( "Thanks for playing!\n" );
break;
default:
printf( "Bad input, quitting!\n" );
break;
}
getchar();

}


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.

Chapter 4- Functions

Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype gives basic structural information: it 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:


#include /* Include rand() */

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:


return-type function_name ( arg_type arg1, ..., arg_type argN );

arg_type just means the type for each argument -- for instance, an int, a float, or a char. It's exactly the same thing as what you would put if you were declaring a variable.

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. Let's look at a function prototype:


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 (surrounded by curly braces) 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.

Let's look at an example program:


#include

int mult ( int x, int y );

int main()
{
int x;
int y;

printf( "Please input two numbers to be multiplied: " );
scanf( "%d", &x );
scanf( "%d", &y );
printf( "The product of your two numbers is %d\n", mult( x, y ) );
getchar();
}

int mult (int x, int y)
{
return x * y;
}

This program begins with the only necessary include file. 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 if you've followed the previous tutorials.

Notice how printf actually takes the value of what appears to be the mult function. What is really happening is printf is accepting the value returned by mult, not mult itself. The result would be the same as if we had use this print instead


printf( "The product of your two numbers is %d\n", x * y );

The mult function is actually defined below main. Because its prototype is above main, the compiler still recognizes it as being declared, and so the compiler will not give an error about mult being undeclared. 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.

Prototypes are declarations of the function, but they are only necessary to alert the compiler about the existence of a function if we don't want to go ahead and fully define the function. If mult were defined before it is used, we could do away with the prototype--the definition basically acts 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 usually redundant. (It can be used to exit the function before the end of the function.)

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.

chapter 3- LOOP IN C

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( "%d\n", x );
} 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);
}

This way, if one player can't take her turn, the game doesn't stop for everybody; we just skip her and keep going with the next player's turn.

Chapter 2- If statement

Without a conditional statement such as the if statement, programs would run almost the exact same way every time, always following the same sequence of function calls. If statements allow the flow of the program to be changed, which leads to 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 printf statement to output the result of those various comparisons (for example printf ( "%d", 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:


> 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 well as the comparison operators, let us look at the actual structure of if statements.
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's not! 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;
}

More interesting conditions using boolean operators
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 along 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) evaluates to 0, and NOT (0) evaluates to 1. NOT (any number but zero) evaluates to 0. In 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. If you have questions about them, feel free to stop by our forums.


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)

If you find you enjoyed this section, then you might want to look more at Boolean Algebra.