An introduction to pointers

Pointers can be confusing, and at times, you may wonder why you would ever want to use them. The truth is, they can make some things much easier. For example, using pointers is one way to have a function modify a variable passed to it. It is also possible to use pointers to dynamically allocate memory allowing certain programming techniques, such as linked lists and resizable arrays. Pointers are what they sound like...pointers. They point to locations in memory. Picture a big jar that holds the location of another jar. In the other jar holds a piece of paper with the number 12 written on it. The jar with the 12 is an integer, and the jar with the memory address of the 12 is a pointer.

Pointer syntax can also be confusing, because pointers can both give the memory location and give the actual value stored in that same location. When a pointer is declared, the syntax is this: variable_type *name; Notice the *. This is the key to declaring a pointer, if you use it before the variable name, it will declare the variable to be a pointer.

As I have said, there are two ways to use the pointer to access information about the memory address it points to. It is possible to have it give the actual address to another variable, or to pass it into a function. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing.

In order to have a pointer actually point to another variable it is necessary to have the memory address of that variable also. To get the memory address of the variable, put the & sign in front of the variable name. This makes it give its address. This is called the address operator, because it returns the memory address.

For example:

#include <iostream>

using namespace std;

int main()
{
  int x;            // A normal integer
  int *p;           // A pointer to an integer

  p = &x;           // Read it, "assign the address of x to p"
  cin>> x;          // Put a value in x, we could also use *p here
  cin.ignore();
  cout<< *p <<"\n"; // Note the use of the * to get the value
  cin.get();
}
The cout outputs the value in x. Why is that? Well, look at the code. The integer is called x. A pointer to an integer is then defined as p. Then it stores the memory location of x in pointer by using the address operator (&). If you wish, you can think of it as if the jar that had the integer had a ampersand in it then it would output its name (in pointers, the memory address) Then the user inputs the value for x. Then the cout uses the * to put the value stored in the memory location of pointer. If the jar with the name of the other jar in it had a * in front of it would give the value stored in the jar with the same name as the one in the jar with the name. It is not too hard, the * gives the value in the location. The unasterisked gives the memory location.

Notice that in the above example, pointer is initialized to point to a specific memory address before it is used. If this was not the case, it could be pointing to anything. This can lead to extremely unpleasant consequences to the computer. You should always initialize pointers before you use them.

It is also possible to initialize pointers using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists. This difficult topic is too complex for this text. An understanding of the keywords new and delete will, however, be tremendously helpful in the future.

The keyword new is used to initialize pointers with memory from free store (a section of memory available to all programs). The syntax looks like the example:

int *ptr = new int;
It initializes ptr to point to a memory address of size int (because variables have different sizes, number of bytes, this is necessary). The memory that is pointed to becomes unavailable to other programs. This means that the careful coder should free this memory at the end of its usage.

The delete operator frees up the memory allocated through new. To do so, the syntax is as in the example.

delete ptr;
After deleting a pointer, it is a good idea to reset it to point to 0. When 0 is assigned to a pointer, the pointer becomes a null pointer, in other words, it points to nothing. By doing this, when you do something foolish with the pointer (it happens a lot, even with experienced programmers), you find out immediately instead of later, when you have done considerable damage.

Functions on c++

Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four.

For example:

#include <cstdlib>   // Include rand()

using namespace std; // Make rand() visible

int a = rand(); // rand is a standard function that all compilers have
Do not think that 'a' will change at random, it will be set to the value returned when the function is called, but it will not change again.

The general format for a prototype is simple:

return-type function_name ( arg_type arg1, ..., arg_type argN );
There can be more than one argument passed to a function or none at all (where the parentheses are empty), and it does not have to return a value. Functions that do not return values have a return type of void. Lets look at a function prototype:

int mult ( int x, int y );
This prototype specifies that the function mult will accept two arguments, both integers, and that it will return an integer. Do not forget the trailing semi-colon. Without it, the compiler will probably think that you are trying to write the actual definition of the function.

When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a block with the code that the function is to execute, just as you would write it for the main function. Any of the arguments passed to the function can be used as if they were declared in the block. Finally, end it all with a cherry and a closing brace. Okay, maybe not a cherry.

Lets look at an example program:

#include <iostream>

using namespace std;

int mult ( int x, int y );

int main()
{
  int x;
  int y;
 
  cout<<"Please input two numbers to be multiplied: ";
  cin>> x >> y;
  cin.ignore();
  cout<<"The product of your two numbers is "<< mult ( x, y ) <<"\n";
  cin.get();
}

int mult ( int x, int y )
{
  return x * y;
}
This program begins with the only necessary include file and a directive to make the std namespace visible. Everything in the standard headers is inside of the std namespace and not visible to our programs unless we make them so. Next is the prototype of the function. Notice that it has the final semi-colon! The main function returns an integer, which you should always have to conform to the standard. You should not have trouble understanding the input and output functions. It is fine to use cin to input to variables as the program does. But when typing in the numbers, be sure to separate them by a space so that cin can tell them apart and put them in the right variables.

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

cout<<"The product of your two numbers is "<< x * y <<"\n";
The mult function is actually defined below main. Due to its prototype being above main, the compiler still recognizes it as being defined, and so the compiler will not give an error about mult being undefined. As long as the prototype is present, a function can be used even if there is no definition. However, the code cannot be run without a definition even though it will compile. The prototype and definition can be combined into one also. If mult were defined before it is used, we could do away with the prototype because the definition can act as a prototype as well.

Return is the keyword used to force the function to return a value. Note that it is possible to have a function that returns no value. If a function returns void, the retun statement is valid, but only if it does not have an expression. In otherwords, for a function that returns void, the statement "return;" is legal, but redundant.

The most important functional (Pun semi-intended) question is why do we need a function? Functions have many uses. For example, a programmer may have a block of code that he has repeated forty times throughout the program. A function to execute that code would save a great deal of space, and it would also make the program more readable. Also, having only one copy of the code makes it easier to make changes. Would you rather make forty little changes scattered all throughout a potentially large program, or one change to the function body? So would I.

Another reason for functions is to break down a complex program into logical parts. For example, take a menu program that runs complex code when a menu choice is selected. The program would probably best be served by making functions for each of the actual menu choices, and then breaking down the complex tasks into smaller, more manageable tasks, which could be in their own functions. In this way, a program can be designed that makes sense when read. And has a structure that is easier to understand quickly. The worst programs usually only have the required function, main, and fill it with pages of jumbled code.

Loops on c++

Loops are used to repeat a block of code. You should understand the concept of C++'s true and false, because it will be necessary when working with loops (the conditions are the same as with if statements). There are three types of loops: for, while, and do..while. Each of them has their specific uses. They are all outlined below.

FOR - for loops are the most useful type. The layout is

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 <iostream>

using namespace std; // So the program can see cout and endl

int main()
{
  // The loop goes while x < 10, and x increases by one every loop
  for ( int x = 0; x < 10; x++ ) {
    // Keep in mind that the loop condition checks
    //  the conditional statement before it loops again.
    //  consequently, when x equals 10 the loop breaks.
    // x is updated before the condition is checked.   
    cout<< x <<endl;
  }
  cin.get();
}
This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls cout<< x <<endl; and it adds 1 to x until the condition is met. Keep in mind also that the variable is incremented after the code in the loop is run for the first time.

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

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

Example:

#include <iostream>

using namespace std; // So we can see cout and endl

int main()
{
  int x = 0;  // Don't forget to declare variables
 
  while ( x < 10 ) { // While x is less than 10
    cout<< x <<endl;
    x++;             // Update x so the condition can be met eventually
  }
  cin.get();
}
This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is that when it reaches the brace at the end it jumps back up to the beginning of the loop, which checks the condition again and decides whether to repeat the block another time, or stop and move to the next statement after the block.

DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure is

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 <iostream>

using namespace std;

int main()
{
  int x;

  x = 0;
  do {
    // "Hello, world!" is printed at least one time
    //  even though the condition is false
    cout<<"Hello, world!\n";
  } while ( x != 0 );
  cin.get();
}
Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.

If statements for c++

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

Without a conditional statement such as the if statement, programs would run almost the exact same way every time. If statements allow the flow of the program to be changed, and so they allow algorithms and more interesting code.

Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison with the relational operators, the operator will return 1 if the comparison is true, or 0 if the comparison is false. For example, the check 0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use a cout statement to output the result of those various comparisons (for example cout<< ( 2 == 1 );)

When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other.

There are a number of operators that allow these checks.

Here are the relational operators, as they are known, along with examples:

>     greater than              5 > 4 is TRUE
<     less than                 4 < 5 is TRUE
>=    greater than or equal     4 >= 4 is TRUE
<=    less than or equal        3 <= 4 is TRUE
==    equal to                  5 == 5 is TRUE
!=    not equal to              5 != 4 is TRUE
It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding. Now that you understand TRUE and FALSE in computer terminology as well as the comparison operators, let us look at the actual structure of if statements.

The structure of an if statement is as follows:

if ( TRUE )
  Execute the next statement
To have more than one statement execute after an if statement that evaluates to true, use braces, like we did with the body of a function. Anything inside braces is called a compound statement, or a block.

For example:

if ( TRUE ) {
  Execute all statements inside the braces
}
There is also the else statement. The code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.

It can look like this:

if ( TRUE ) {
  // Execute these statements if TRUE
}
else {
  // Execute these statements if FALSE
}
One use for else is if there are two conditional statements that may both evaluate to true, yet you wish only one of the two to have the code block following it to be executed. You can use an else if after the if statement; that way, if the first statement is true, the else if will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements.

Let's look at a simple program for you to try out on your own.

#include <iostream>   

using namespace std;
       
int main()                            // Most important part of the program!
{
  int age;                            // Need a variable...
 
  cout<<"Please input your age: ";    // Asks for age
  cin>> age;                          // The input is put in age
  cin.ignore();                       // Throw away enter
  if ( age < 100 ) {                  // If the age is less than 100
     cout<<"You are pretty young!\n"; // Just to show you it works...
  }
  else if ( age == 100 ) {            // I use else just to show an example
     cout<<"You are old\n";           // Just to show you it works...
  }
  else {
    cout<<"You are really old\n";     // Executed if no other statement is
  }
  cin.get();
}
Boolean operators allow you to create more complex conditional statements. For example, if you wish to check if a variable is both greater than five and less than ten, you could use the boolean AND to ensure both var > 5 and var < 10 are true. In the following discussion of boolean operators, I will capitalize the boolean operators in order to distinguish them from normal english. The actual C++ operators of equivalent function will be described further into the tutorial - the C++ symbols are not: OR, AND, NOT, although they are of equivalent function.

When using if statements, you will often wish to check multiple different conditions. You must understand the Boolean operators OR, NOT, and AND. The boolean operators function in a similar way to the comparison operators: each returns 0 if evaluates to FALSE or 1 if it evaluates to TRUE.

NOT: The NOT operator accepts one input. If that input is TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1) evalutes to 0, and NOT (0) evalutes to 1. NOT (any number but zero) evaluates to 0. In C and C++ NOT is written as !. NOT is evaluated prior to both AND and OR.

AND: This is another important command. AND returns TRUE if both inputs are TRUE (if 'this' AND 'that' are true). (1) AND (0) would evaluate to zero because one of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND (1) evaluates to 1. (any number but 0) AND (0) evaluates to 0. The AND operator is written && in C++. Do not be confused by thinking it checks equality between numbers: it does not. Keep in mind that the AND operator is evaluated before the OR operator.

OR: Very useful is the OR statement! If either (or both) of the two values it checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. The OR is written as || in C++. Those are the pipe characters. On your keyboard, they may look like a stretched colon. On my computer the pipe shares its key with \. Keep in mind that OR will be evaluated after AND.

It is possible to combine several boolean operators in a single statement; often you will find doing so to be of great value when creating complex expressions for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is because 1 && 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).

Try some of these - they're not too hard. 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

The basics of C++

C++ is a programming language of many different dialects, just as each spoken language has many different dialects. In C++, dialects are not because the speakers live in the North or South; it is because there are several different compilers. There are several common compilers: Borland C++, Microsoft C++, GNU C++, etc.. There are also many front-end environments for the different compilers, the most common is Dev-C++ around GNU's G++ compiler. Some are free, others are not. Please see the compiler listing on this site for information on how to get one and set it up.

Each of these compilers is slightly different. Each one should support the ANSI/ISO standard C++ functions, but each compiler will also have nonstandard functions (these functions are similar to slang spoken in different parts of a country. Sometimes the use of nonstandard functions will cause problems when you attempt to compile source code (the actual C++ written by a programmer and saved as a text file) with a different compiler. These tutorials use ANSI/ISO standard C++ and should not suffer from this problem with sufficiently modern compilers.

If you don't have a compiler, I strongly suggest that you get a compiler. A simple compiler is sufficient for out use, but you should get one in order to get the most from these tutorials.

C++ is a different breed of programming language. A C++ program begins with a function, a collection of commands that do something. The function that begins a C++ program is called main. From main, we can also call other functions whether they be written by us or by others. To access a standard function that comes with the compiler, you include a header with the #include directive. What this does is take everything in the header and paste it into your program. Let's look at a working program:

#include <iostream>

using namespace std;

int main()
{
  cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
  cin.get();
}
Let's look at the elements of the program. The #include is a preprocessor directive that tells the compiler to put code from the header called iostream into our program. By including header files, you an gain access to many different functions. For example, the cout function requires iostream.

The next imporant line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The 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.

The next line of the program may seem strange. If you have programmed in another language, you might expect that print would be the function used to display text. In C++, however, the cout function is used to display text. It uses the << symbols, known as insertion operators. The quotes tell the compiler that you want to output the literal string as-is. The \n is actually one character that stands for a newline. It moves the cursor on your screen to the next line. The semicolon is added onto the end of all function calls in C++. You will see later that the semicolon is used to end most commands in C++.

The next command is cin.get(). This is another function call that expects the user to hit the return key. Many compiler environments will open a new console window, run the program, and then close the window. This command keeps that window from closing because the program is not done yet. It gives you time to see the program run.

Upon reaching the end of main, the closing brace, our program will return the value of 0 (and integer, hence why we told main to return an int) to the operating system. This return value is important as it can be used to tell the OS whether our program succeeded or not. A return value of 0 means success and is returned automatically (but only for main, other functions require you to manually return a value), but if we wanted to return something else, such as 1, we would have to do it with a return statement:

#include <iostream>

using namespace std;

int main()
{
  cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
  cin.get();

  return 1;
}
The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .cpp (or whatever extension your compiler requires) file. If you are using a command-line compiler, such as Borland C++ 5.5, you should read the compiler instructions for information on how to compile. Otherwise compiling and running should be as simple as clicking a button with your mouse.

Comments are critical for all but the most trivial programs. When you tell the compiler a section of text is a comment, it will ignore it when running the code, allowing you to use any text you want to describe the real code. To create a comment use either //, which tells the compiler that the rest of the line is a comment, or /* and then */ to block off everything between as a comment. Certain compiler environments will change the color of a commented area, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program. When you are learning to program, it is useful to be able to comment out sections of code in order to see how the output is affected.

So far you should be able to write a simple program to display information typed in by you, the programmer. It is also possible for your program to accept input. The function you use is known as cin, and is followed by the insertion operator >>.

Of course, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float.

A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable. To declare a variable you use the syntax type <name>. It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. The declaration of a variable or set of variables should be followed by a semicolon (Note that this is the same procedure used when you call a function). If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake.

Here are some variable declaration examples:

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.

#include <iostream>

using namespace std;

int main()
{
  int thisisanumber;

  cout<<"Please enter a number: ";
  cin>> thisisanumber;
  cin.ignore();
  cout<<"You entered: "<< thisisanumber <<"\n";
  cin.get();
}
Let's break apart this program and examine it line by line. The keyword int declares thisisanumber to be an integer. The function cin>> reads a value into thisisanumber; the user must press enter before the number is read by the program. cin.ignore() is another function that reads and discards a character. Remember that when you type intput into a program, it takes the enter key too. We don't need this, so we throw it away. Keep in mind that the variable was declared an integer; if the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored). Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Notice that when printing out a variable quotation marks are not used. Were there quotation marks, the output would be "You Entered: thisisanumber." The lack of quotation marks informs the compiler that there is a variable, and therefore that the program should check the value of the variable in order to replace the variable name with the variable when executing the output function. Do not be confused by the inclusion of two separate insertion operators on one line. Including multiple insertion operators on one line is acceptable as long as each insertion operator outputs a different piece of information; you must separate string literals (strings enclosed in quotation marks) and variables by giving each its own insertion operators (<<). Trying to put two variables together with only one << will give you an error message, do not try it. Do not forget to end functions and declarations with a semicolon. If you forget the semicolon, the compiler will give you an error message when you attempt to compile the program. Variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C++ == is used for that task. The equal sign is still extremely useful. It sets the left input to the equal sign, which must be one, and only one, variable equal to the value on the right side of the equal sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.

Here are a few examples:

a = 4 * 6; // (Note use of comments and of semicolon) a is 24
a = a + 5; // a equals the original value of a with five added to it
a == 5     // Does NOT assign five to a. Rather, it checks to see if a equals 5.
The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is useful in other areas of C++; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators.

For example:

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

Make Directory

Now, let's make a couple of directories so we can put our file away in a place that makes sense to us. The command for making a directory is "md" (for "make directory") and is followed by the name of the directory you wish to make.

·         C:\> md dosclass
(to make the directory DOSCLASS)

·         C:\> dir
(to view the directory with the new DOSCLASS subdirectory)

·         C:\> cd dosclass
(to change to the DOSCLASS directory)

·         C:\DOSCLASS> md samples
(to make the directory "samples" inside the DOSCLASS directory)

·         C:\DOSCLASS> dir
(to view the directory with the new SAMPLES subdirectory)

Copy

Now that we have created the directories, we can put the practice file we created into the new directory, SAMPLES (C:\DOSCLASS\SAMPLES).

To keep things simple, let's "cd" back to the root directory where the practice file is.

C:\>cd \

And now let's copy that file to the SAMPLES directory which is inside the DOSCLASS directory. In order to copy something, you must first issue the command (copy), then identify the file to be copied (source), and then the directory to which you wish to copy the file (destination).

C:\>copy practice.txt dosclass\samples

A somewhat unfriendly yet useful diagram of the command format would look something like this (where things in brackets are optional).

copy [volume+pathname+]filename [volume+pathname+]directory

Delete and Undelete

There is a slight problem now. The PRACTICE.TXT file was copied into the SAMPLES directory, but there is no reason to have two copies of PRACTICE.TXT. To delete a file, use the DEL command (from "delete") followed by the name of the file.

C:\>del practice.txt

Again, you can delete the file from any directory by including the full pathname.

Z:\>del c:\practice.txt

If you accidentally delete something, there is a possibility of retrieving it using the "undelete" command. This, however, will only work for files just deleted.

C:\undelete

A list of any files that can be undeleted will appear. You will need to replace the first character of the file because DOS removes the first letter of a deleted file (that's how it keeps track of what can be written over).

Rename

Like the above commands, you needn't be in the same directory as the file you wish to rename provided you include the pathname of the file you wish to change. But the second argument of this command, the new filename, will not accept a pathname designation. In other words, the second argument should just be a filename (with no pathname):

C:\>ren \dosclass\samples\practice.txt prac.txt

Extras

Parent directory (..)
If you wish to move up one level in the hierarchy of the file structure (change to the parent directory), there is a shortcut--two consecutive periods: ".."

C:\DOSCLASS\SAMPLES>cd ..

This works in a pathname as well:

C:\DOSCLASS\SAMPLES>ren ..\practice.txt prac.txt

If the file PRACTICE.TXT were in the directory above SAMPLES (the DOSCLASS directory), the above command would change it to PRAC.TXT.

F3
The F3 function key can be a time saver if you're prone to typos. Pressing the F3 button will retype the last command for you.

Breaking
Sometimes, you may start a procedure such as a long directory listing and wish to stop before it is completed. The Break command often works when running batch files and other executable files. To stop a procedure, press [CTRL] + [BREAK]. [CTRL] is located in the lower right-hand corner of the keyboard. [BREAK] is located in the upper right hand corner of the keyboard and is also labeled [PAUSE].

Wildcards (*) and (?)
Another benefit of the command-prompt interface is the ability to use wildcards. If you want, for example, to copy only files with the .txt extension, you employ a wildcard:

C:\>copy *.txt a:\backup\txtfiles

Viewing

Text files, on the other hand can be viewed quickly with the type command:

C:\>cd dosclass\samples
C:\DOSCLASS\SAMPLES>type practice.txt | more

Editing

Or you can view the file in the provided text editor (just as we did when we first created practice.txt):

C:\DOSCLASS\SAMPLES>edit practice.txt

Printing

If you want to send a text file to the printer, there is the print command. But there are two steps:

C:\DOSCLASSES\SAMPLES>print practice.txt
Name of list device [PRN]: lpt2

If you wish to print to a networked printer, usually lpt2 is the correct response. For local printers, the correct response is usually lpt1.

Change Directory

To move to a different directory, use the command "cd" (for "change directory") followed by the directory you wish to move to. The backslash by itself always represents the root directory.

C:\>cd \

·         C:\> edit practice.txt

·         Type your name or some other message.

·         Then press the Alt key, followed by the f key to display the File menu.(You can also use the mouse.) Press [ALT] + [F]

·         Then press the s key to save the file. Press [S]

·         To exit, press the Alt key, followed by the f key followed by the x key. Press [ALT] + [F] + [X]

C:\dir

 Volume in drive C is TC_PC

 Volume Serial Number is 1843-17E5

 Directory of C:\

SYSTMP       <DIR>   07-07-94   3:03P

PRACTICE TXT          07-07-94   3:39P

DOS sees the new file as C:\PRACTICE.TXT.

Directory

If you need more help in orienting yourself, it sometimes helps to take a look at the files and directories available where you are by using the DIR command (short for directory).

C:\>dir

This will give you a listing of all the files and directories contained in the current directory in addition to some information about the directory itself. You will see the word volume in this information. Volume is simply another word for a disk that the computer has access to. Your hard disk is a volume, your floppy disk is a volume, a server disk (hard disk served over a network) is a volume. Now you know fancy words for all the parts of the format DOS uses to represent a file.

Volume: C:
Pathname: \DEMO\DOS&WIN\SAMPLES\
Filename: SAMPLE

Here are some helpful extensions of the DIR command:

·         C:\>dir | more
(will display the directory one screen at a time with a < more> promptÑ--use control-C to escape)

·         C:\>dir /w
(wide: will display the directory in columns across the screen)

·         C:\>dir /a
(all: will display the directory including hidden files and directories)

Now that we have a grasp on where we are in DOS and how to find out, let's take a look at how we manage the files.

If you need more help in orienting yourself, it sometimes helps to take a look at the files and directories available where you are by using the DIR command (short for directory).

C:\>dir

This will give you a listing of all the files and directories contained in the current directory in addition to some information about the directory itself. You will see the word volume in this information. Volume is simply another word for a disk that the computer has access to. Your hard disk is a volume, your floppy disk is a volume, a server disk (hard disk served over a network) is a volume. Now you know fancy words for all the parts of the format DOS uses to represent a file.

Volume: C:
Pathname: \DEMO\DOS&WIN\SAMPLES\
Filename: SAMPLE

Here are some helpful extensions of the DIR command:

·         C:\>dir | more
(will display the directory one screen at a time with a < more> promptÑ--use control-C to escape)

·         C:\>dir /w
(wide: will display the directory in columns across the screen)

·         C:\>dir /a
(all: will display the directory including hidden files and directories)

Now that we have a grasp on where we are in DOS and how to find out, let's take a look at how we manage the files.