Array and String

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 */



String


This lesson will discuss C-style strings, which you may have already seen in the array tutorial. In fact, C-style strings are really arrays of chars with a little bit of special sauce to indicate where the string ends. This tutorial will cover some of the tools available for working with strings--things like copying them, concatenating them, and getting their length.

What is a String?
Note that along with C-style strings, which are arrays, there are also string literals, such as "this". In reality, both of these string types are merely just collections of characters sitting next to each other in memory. The only difference is that you cannot modify string literals, whereas you can modify arrays. Functions that take a C-style string will be just as happy to accept string literals unless they modify the string (in which case your program will crash). Some things that might look like strings are not strings; in particular, a character inclosed in single quotes, like this, 'a', is not a string. It's a single character, which can be assigned to a specific location in a string, but which cannot be treated as a string. (Remember how arrays act like pointers when passed into functions? Characters don't, so if you pass a single character into a function, it won't work; the function is expecting a char*, not a char.)



To recap: strings are arrays of chars. String literals are words surrounded by double quotation marks.


"This is a static string"

Remember that special sauce mentioned above? Well, it turns out that C-style strings are always terminated with a null character, literally a '\0' character (with the value of 0), so to declare a string of 49 letters, you need to account for it by adding an extra character, so you would want to say:


char string[50];

This would declare a string with a length of 50 characters. Do not forget that arrays begin at zero, not 1 for the index number. In addition, we've accounted for the extra with a null character, literally a '\0' character. It's important to remember that there will be an extra character on the end on a string, just like there is always a period at the end of a sentence. Since this string terminator is unprintable, it is not counted as a letter, but it still takes up a space. Technically, in a fifty char array you could only hold 49 letters and one null character at the end to terminate the string.

Note that something like


char *my_string;

can also be used as a string. If you have read the tutorial on pointers, you can do something such as:


arry = malloc( sizeof(*arry) * 256 );

which allows you to access arry just as if it were an array. To free the memory you allocated, just use free:

For example:


free ( arry );

Using Strings
Strings are useful for holding all types of long input. If you want the user to input his or her name, you must use a string. Using scanf() to input a string works, but it will terminate the string after it reads the first space, and moreover, because scanf doesn't know how big the array is, it can lead to "buffer overflows" when the user inputs a string that is longer than the size of the string (which acts as an input "buffer").

There are several approaches to handling this problem, but probably the simplest and safest is to use the fgets function, which is declared in stdio.h.

The prototype for the fegets function is:


char *fgets (char *str, int size, FILE* file);

There are a few new things here. First of all, let's clear up the questions about that funky FILE* pointer. The reason this exists is because fgets is supposed to be able to read from any file on disk, not just from the user's keyboard (or other "standard input" device). For the time being, whenever we call fgets, we'll just pass in a variable called stdin, defined in stdio.h, which refers to "standard input". This effectively tells the program to read from the keyboard. The other two arguments to fgets, str and size, are simply the place to store the data read from the input and the size of the char*, str. Finally, fgets returns str whenever it successfully read from the input.

When fgets actually reads input from the user, it will read up to size - 1 characters and then place the null terminator after the last character it read. fgets will read input until it either has no more room to store the data or until the user hits enter. Notice that fgets may fill up the entire space allocated for str, but it will never return a non-null terminated string to you.

Let's look at an example of using fgets, and then we'll talk about some pitfalls to watch out for.

For a example:


#include

int main()
{
/* A nice long string */
char string[256];

printf( "Please enter a long string: " );

/* notice stdin being passed in */
fgets ( string, 256, stdin );

printf( "You entered a very long string, %s", string );

getchar();
}

Remember that you are actually passing the address of the array when you pass string because arrays do not require an address operator (&) to be used to pass their addresses, so the values in the array string are modified.

The one thing to watch out for when using fgets is that it will include the newline character ('\n') when it reads input unless there isn't room in the string to store it. This means that you may need to manually remove the input. One way to do this would be to search the string for a newline and then replace it with the null terminator. What would this look like? See if you can figure out a way to do it before looking below:


char input[256];
int i;

fgets( input, 256, stdin );

for ( i = 0; i < 256; i++ )
{
if ( input[i] == '\n' )
{
input[i] = '\0';
break;
}
}


Here, we just loop through the input until we come to a newline, and when we do, we replace it with the null terminator. Notice that if the input is less than 256 characters long, the user must have hit enter, which would have included the newline character in the string! (By the way, aside from this example, there are other approaches to solving this problem that use functions from string.h.)

Manipulating C strings using string.h
string.h is a header file that contains many functions for manipulating strings. One of these is the string comparison function.


int strcmp ( const char *s1, const char *s2 );

strcmp will accept two strings. It will return an integer. This integer will either be:


Negative if s1 is less than s2.
Zero if s1 and s2 are equal.
Positive if s1 is greater than s2.

Strcmp performs a case sensitive comparison; if the strings are the same except for a difference in cAse, then they're countered as being different. Strcmp also passes the address of the character array to the function to allow it to be accessed.


char *strcat ( char *dest, const char *src );

strcat is short for "string concatenate"; concatenate is a fancy word that means to add to the end, or append. It adds the second string to the first string. It returns a pointer to the concatenated string. Beware this function; it assumes that dest is large enough to hold the entire contents of src as well as its own contents.


char *strcpy ( char *dest, const char *src );

strcpy is short for string copy, which means it copies the entire contents of src into dest. The contents of dest after strcpy will be exactly the same as src such that strcmp ( dest, src ) will return 0.


size_t strlen ( const char *s );

strlen will return the length of a string, minus the termating character ('\0'). The size_t is nothing to worry about. Just treat it as an integer that cannot be negative, which is what it actually is. (The type size_t is just a way to indicate that the value is intended for use as a size of something.)

Here is a small program using many of the previously described functions:


#include /* stdin, printf, and fgets */
#include /* for all the new-fangled string functions */

/* this function is designed to remove the newline from the end of a string
entered using fgets. Note that since we make this into its own function, we
could easily choose a better technique for removing the newline. Aren't
functions great? */
void strip_newline( char *str, int size )
{
int i;

/* remove the null terminator */
for ( i = 0; i < size; ++i )
{
if ( str[i] == '\n' )
{
str[i] = '\0';

/* we're done, so just exit the function by returning */
return;
}
}
/* if we get all the way to here, there must not have been a newline! */
}

int main()
{
char name[50];
char lastname[50];
char fullname[100]; /* Big enough to hold both name and lastname */

printf( "Please enter your name: " );
fgets( name, 50, stdin );

/* see definition above */
strip_newline( name, 50 );

/* strcmp returns zero when the two strings are equal */
if ( strcmp ( name, "Alex" ) == 0 )
{
printf( "That's my name too.\n" );
}
else
{
printf( "That's not my name.\n" );
}
// Find the length of your name
printf( "Your name is %d letters long", strlen ( name ) );
printf( "Enter your last name: " );
fgets( lastname, 50, stdin );
strip_newline( lastname, 50 );
fullname[0] = '\0';
/* strcat will look for the \0 and add the second string starting at
that location */
strcat( fullname, name ); /* Copy name into full name */
strcat( fullname, " " ); /* Separate the names by a space */
strcat( fullname, lastname ); /* Copy lastname onto the end of fullname */
printf( "Your full name is %s\n",fullname );

getchar();

return 0;
}

Safe Programming
The above string functions all rely on the existence of a null terminator at the end of a string. This isn't always a safe bet. Moreover, some of them, noticeably strcat, rely on the fact that the destination string can hold the entire string being appended onto the end. Although it might seem like you'll never make that sort of mistake, historically, problems based on accidentally writing off the end of an array in a function like strcat, have been a major problem.

Fortunately, in their infinite wisdom, the designers of C have included functions designed to help you avoid these issues. Similar to the way that fgets takes the maximum number of characters that fit into the buffer, there are string functions that take an additional argument to indicate the length of the destination buffer. For instance, the strcpy function has an analogous strncpy function


char *strncpy ( char *dest, const char *src, size_t len );

which will only copy len bytes from src to dest (len should be less than the size of dest or the write could still go beyond the bounds of the array). Unfortunately, strncpy can lead to one niggling issue: it doesn't guarantee that dest will have a null terminator attached to it (this might happen if the string src is longer than dest). You can avoid this problem by using strlen to get the length of src and make sure it will fit in dest. Of course, if you were going to do that, then you probably don't need strncpy in the first place, right? Wrong. Now it forces you to pay attention to this issue, which is a big part of the battle.

Loop structure

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). 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()
{
// The loop goes while x < 10, and x increases by one every loop
for ( int x = 0; x < 10; x++ ) {
// Keep in mind that the loop condition checks
// the conditional statement before it loops again.
// consequently, when x equals 10 the loop breaks.
// x is updated before the condition is checked.
printf("%d",x);
}
getch();
}

This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls cout<< x <


WHILE
- WHILE loops are very simple. The basic structure is

while ( condition ) { Code to execute while the condition is true } The true represents a boolean expression which could be x == 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x ==5 || v == 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is the same as a for loop without the initialization and update sections. However, an empty condition is not legal for a while loop as it is with a for loop.

Example:


#include

int main()
{
int x = 0; // Don't forget to declare variables

while ( x < 10 ) { // While x is less than 10
printf("%d",x);
x++; // Update x so the condition can be met eventually
}

}

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 basically a reversed while loop. A while loop says "Loop while the condition is true, and execute this block of code", a do..while loop says "Execute this block of code, and loop while the condition is true".

Example:


#include

using namespace std;

int main()
{
int x;

x = 0;
do {
// "Hello, world!" is printed at least one time
// even though the condition is false
printf("\n Hello world");
} while ( x != 0 );

}

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.

Decision and Control Statements

The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user-entered password, your program can decide whether a user is allowed access to the program.

Without a conditional statement such as the if statement, programs would run almost the exact same way every time, 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)

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. Switch statements serves as a simple way to write long if statements when the requirements are met. Often it can be used to process input from a user.

Below is a sample program, in which not all of the proper functions are actually declared, but which shows how one would use switch in a program.


#include

using namespace std;

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

int main()
{
int input;

cout<<"1. Play game\n";
cout<<"2. Load game\n";
cout<<"3. Play multiplayer\n";
cout<<"4. Exit\n";
cout<<"Selection: ";
cin>> input;
switch ( input ) {
case 1: // Note the colon, not a semicolon
playgame();
break;
case 2: // Note the colon, not a semicolon
loadgame();
break;
case 3: // Note the colon, not a semicolon
playmultiplayer();
break;
case 4: // Note the colon, not a semicolon
cout<<"Thank you for playing!\n";
break;
default: // Note the colon, not a semicolon
cout<<"Error, bad input, quitting\n";
break;
}
cin.get();
}

This program will compile, but cannot be run until the undefined functions are given bodies, but it serves as a model (albeit simple) for processing input. If you do not understand this then try mentally putting in if statements for the case statements. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. You could easily make a few small functions if you wish to test the code.