I
Setting up Code Blocks ; A Free C and C++ Compiler, on Windows
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.
Setting up Code Blocks ; A Free C and C++ Compiler, on Windows
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.
Step 1: Download Code Blocks
- Go to this website: http://www.codeblocks.org/downloads
- Follow the link to "Download the binary release" (direct link)
- Go to the Windows 2000 / XP / Vista / 7 section
- Look for the file that includes mingw in the name. (The name as of this writing was codeblocks-10.05mingw-setup.exe; the 10.05 may be different).
- Save the file to your desktop. It is roughly 74 megabytes.
- Double click the installer.
- Hit next several times. Other setup tutorials will assume you have installed in C:\Program Files\CodeBlocks (the default install location), but you may install elsewhere if you like
- Do a Full Installation
- Launch Code::Blocks
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.
The printf function is the standard C way of displaying output on the screen. The quotes tell the compiler that you want to output the literal string as-is (almost). The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail); for the time being, just remember that there are a few sequences that, when they appear in a string literal, are actually not displayed literally by printf and that '\n' is one of them. The actual effect of '\n' is to move the cursor on your screen to the next line. Notice the semicolon: it tells the compiler that you're at the end of a command, such as a function call. You will see that the semicolon is used to end many lines in C.
The next command is getchar(). This is another function call: it reads in a single character and waits for the user to hit enter before reading the character. This line is included because many compiler environments will open a new console window, run the program, and then close the window before you can see the output. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run.
Finally, at the end of the program, we return a value from main to the operating system by using the return statement. This return value is important as it can be used to tell the operating system whether our program succeeded or not. A return value of 0 means success.
The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .c file, and then compile it. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse (perhaps the "build" or "run" button).
You might start playing around with the printf function and get used to writing simple C programs.
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 <variable type> <name of variable>;. (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 <stdio.h>
int main()
{
/* wrong! The variable declaration must appear first */
printf( "Declare x next" );
int x;
return 0;
}
Fixed
#include <stdio.h>
int main()
{
int x;
printf( "Declare x first" );
return 0;
}
Reading inputUsing 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 <stdio.h>
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.
When the program runs, each call to scanf checks its own input string to see what kinds of input to expect, and then stores the value input into the variable.
The second printf statement also contains the same '%d'--both scanf and printf use the same format for indicating values embedded in strings. In this case, printf takes the first argument after the string, the variable this_is_a_number, and treats it as though it were of the type specified by the "format specifier". In this case, printf treats this_is_a_number as an integer based on the format specifier.
So what does it mean to treat a number as an integer? If the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored) when stored in the variable. Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty.
Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the / divides, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C == is used for that task. The equal sign is still extremely useful. It sets the value of the variable on the left side of the equals sign equal to the value on the right side of the equals sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.
Here are a few examples:
a = 4 * 6; /* (Note use of comments and of semicolon) a is 24 */
a = a + 5; /* a equals the original value of a with five added to it */
a == 5 /* Does NOT assign five to a. Rather, it checks to see if a equals 5.*/
The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is extremely useful in many areas of C; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators. For example:
a < 5 /* Checks to see if a is less than five */
a > 5 /* Checks to see if a is greater than five */
a == 5 /* Checks to see if a equals five, for good measure */
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 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 <stdio.h>
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 (i.e., 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. 
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 <stdio.h>
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 <stdio.h>
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 isdo {
} 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 <stdio.h>
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);
}
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. 
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 <stdlib.h> /* 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 <stdio.h>
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 return statement is valid, but only if it does not have an expression. In other words, 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.
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 ( <variable> ) {
case this-value:
Code to execute if <variable> == this-value
break;
case that-value:
Code to execute if <variable> == that-value
break;
...
default:
Code to execute if <variable> 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 <stdio.h>
void playgame()
{
printf( "Play game called" );
}
void loadgame()
{
printf( "Load game called" );
}
void playmultiplayer()
{
printf( "Play multiplayer game called" );
}
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. II
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:
<variable_type> *<name>;
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 <stdio.h>
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 <stdlib.h>
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!
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.
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 <stdio.h>
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();
}
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