Introduction to File Processing
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Introduction to File Processing |
In Lesson 2, we saw that a piece of information used in an application is primarily represented as a group of bits. So far, if we requested information from the user, when the application exited, we lost all information that the user had entered. This is because such information was only temporarily stored in the random access memory (RAM). In some cases, you will want to "keep" information that the user has entered so you can make the same values available the next time the user opens the application. In some other cases, whether you request information from the user or inherently provide it to the user, you may want different people working from different computers to use or share the same data. In these and other scenarios, you must store the information somewhere and retrieve it when necessary. This is the basis of file processing.
A file is a series of bytes of data that are arranged in a particular way to produce a usable document. For easy storage, location, and management, the bytes are stored on a medium such as a hard disc, a floppy disc, a compact disc, or any valid and support type of storage. When these bytes belong to a single but common entity and hold values that are stored on a medium, the group is referred to as a file. For even greater management, files can be stored in a parent object called a directory or a folder. Since a file is a unit of storage and it stores information, it has a size, which is the number of bits it uses to store its values. To manage it, a file has a location also called a path that specifies where and/or how the file can be retrieved. Also, for better management, a file has attributes that indicate what can be done on the file or that provide specific information that the programmer or the operating system can use when dealing with the file.
File processing consists of creating, storing, and/or retrieving the contents of a file from a recognizable medium. For example, it is used to save word-processed files to a hard drive, to store a presentation on floppy disk, or to open a file from a CD-ROM. A stream is the technique or means of performing file processing. In order to manage files stored in a computer, each file must be able to provide basic pieces of information about itself. This basic information is specified when the file is created but can change during the life time of a file. To create a file, a user must first decide where it would be located: this is a requirement. A file can be located on the root drive. Alternatively, a file can be positioned inside of an existing folder. Based on security settings, a user may not be able to create a file just anywhere in the (file system of the) computer. Once the user has decided where the file would reside, there are various means of creating files that the users are trained to use. When creating a file, the user must give it a name following the rules of the operating system combined with those of the file system. The most fundamental piece of information a file must have is a name. Once the user has created a file, whether the file is empty or not, the operating system assigns basic pieces of information to it. Once a file is created, it can be opened, updated, modified, renamed, etc.
To support file processing, the .NET Framework provides the System.IO namespace that contains many different classes to handle almost any type of file operation you may need to perform. Therefore, to perform file processing, include the System.IO namespace in your calls. The parent class of file processing is Stream. With Stream, you can store data to a stream or you can retrieve data from a stream. Stream is an abstract class, which means that you cannot use it to declare a variable in your application. As an abstract class, Stream is used as the parent of the classes that actually implement the necessary operations. You will usually use a combination of classes to perform a typical operation. For example, some classes are used to create a stream object while some others are used to write data to the created stream. |
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using System;
namespace VendingMachine
{
// This class is used to create and manage an ice cream
// and to process an order
sealed class IceCream
{
// This is the base price of an ice cream
// Optional values may be added to it
public const decimal BasePrice = 1.55M;
// These arrays are used to build the components of various ice creams
// In C#, we can allocate an array's memory in the body of the class
private string[] Flavor;
private string[] Container;
private string[] Ingredient;
// Additional factor used to process an ice cream order
private int Scoops;
private decimal TotalPrice;
// Variables that will hold the user's choice
// These are declared "globally" so they can be shared among methods
int ChoiceFlavor;
int ChoiceContainer;
int ChoiceIngredient;
// This default constructor is the best place for us to initialize the array
public IceCream()
{
Flavor = new string[10];
Flavor[0] = "Vanilla";
Flavor[1] = "Cream of Cocoa";
Flavor[2] = "Chocolate Chip";
Flavor[3] = "Organic Strawberry";
Flavor[4] = "Butter Pecan";
Flavor[5] = "Cherry Coke";
Flavor[6] = "Chocolate Brownies";
Flavor[7] = "Caramel Au Lait";
Flavor[8] = "Chunky Butter";
Flavor[9] = "Chocolate Cookie";
Ingredient = new string[4];
Ingredient[0] = "No Ingredient";
Ingredient[1] = "Peanuts";
Ingredient[2] = "M & M";
Ingredient[3] = "Cookies";
Container = new string[3];
Container[0] = "Cone";
Container[1] = "Cup";
Container[2] = "Bowl";
}
// This method requests a flavor from the user and returns the choice
public void ChooseFlavor()
{
// Make sure the user selects a valid number that represents a flavor...
do
{
// In case the user types a symbol that is not a number
try
{
Console.WriteLine("What type of flavor do you want?");
for(int i = 0; i < Flavor.Length; i++)
Console.WriteLine("{0} - {1}", i+1, Flavor[i]);
Console.Write("Your Choice? " );
ChoiceFlavor = int.Parse(Console.ReadLine());
}
catch(FormatException) // display an appropriate message
{
Console.WriteLine("You must enter a valid number and no other character!");
}
// If the user typed an invalid number out of the allowed range
// let him or her know and provide another chance
if( ChoiceFlavor < 1 || ChoiceFlavor > Flavor.Length )
Console.WriteLine("Invalid Choice - Try Again!\n");
} while( ChoiceFlavor < 1 || ChoiceFlavor > Flavor.Length );
}
// This method allows the user to select a container
public void ChooseContainer()
{
// Make sure the user selects a valid number that represents a container
do
{
// If the user types a symbol that is not a number
try
{
Console.WriteLine("What type of container do you want?");
for(int i = 0; i < Container.Length; i++)
Console.WriteLine("{0} - {1}", i+1, Container[i]);
Console.Write("Your Choice? ");
ChoiceContainer = int.Parse(Console.ReadLine());
}
catch(FormatException) // display an appropriate message
{
Console.WriteLine("You must enter a valid number and no other character!");
}
// If the user typed an invalid number out of the allowed range
// let him or her know and provide another chance
if( ChoiceContainer < 1 || ChoiceContainer > Container.Length )
Console.WriteLine("Invalid Choice - Try Again!");
} while( ChoiceContainer < 1 || ChoiceContainer > Container.Length );
}
public void ChooseIngredient()
{
do
{
try
{
Console.WriteLine("Do you want an ingredient or not");
for(int i = 0; i < Ingredient.Length; i++)
Console.WriteLine("{0} - {1}", i+1, Ingredient[i]);
Console.Write("Your Choice? ");
ChoiceIngredient = int.Parse(Console.ReadLine());
}
catch(FormatException)
{
Console.WriteLine("You must enter a valid number and no other character!");
}
if( ChoiceIngredient < 1 || ChoiceIngredient > Ingredient.Length )
Console.WriteLine("Invalid Choice - Try Again!");
} while( ChoiceIngredient < 1 || ChoiceIngredient > Ingredient.Length );
}
public void SpecifyNumberOfScoops()
{
do
{
try
{
Console.Write("How many scoops(1, 2, or 3)? ");
Scoops = int.Parse(Console.ReadLine());
}
catch(FormatException)
{
Console.WriteLine("You must enter a valid number and no other character!");
}
if( Scoops < 1 || Scoops > 3 )
Console.WriteLine("Invalid Choice - Try Again!");
} while( Scoops < 1 || Scoops > 3 );
}
// This method is used to process a customer order
// It uses the values of the above methods
public void ProcessAnOrder()
{
// int ChoiceFlavor;
// int ChoiceContainer;
// int ChoiceIngredient;
decimal PriceIngredient, PriceScoop;
// Let the user know that this is a vending machine
Console.WriteLine("Ice Cream Vending Machine");
// Let the user select the components of the ice cream
ChooseFlavor();
ChooseContainer();
ChooseIngredient();
SpecifyNumberOfScoops();
// If the user selects an ingredient instead of "No Ingredient",
// add $0.50 to the order
if( ChoiceIngredient == 2 || ChoiceIngredient == 3 || ChoiceIngredient == 4 )
PriceIngredient = 0.50M;
else
PriceIngredient = 0.00M;
// Instead of multiplying a number scoops to a value,
// We will use an incremental value depending on the number of scoops
if( Scoops == 1 )
PriceScoop = 0.65M;
else if( Scoops == 2 )
PriceScoop = 1.05M;
else
PriceScoop = 1.55M;
// Calculate the total price of the ice cream
TotalPrice = BasePrice + PriceScoop + PriceIngredient;
// Create the ice cream...
// And display a receipt to the user
DisplayReceipt();
}
// This method is used to display a receipt to the user
public void DisplayReceipt()
{
Console.WriteLine("\nIce Cream Order");
Console.WriteLine("Flavor: {0}", Flavor[ChoiceFlavor-1]);
Console.WriteLine("Container: {0}", Container[ChoiceContainer-1]);
Console.WriteLine("Ingredient: {0}", Ingredient[ChoiceIngredient-1]);
Console.WriteLine("Scoops: {0}", Scoops);
Console.WriteLine("Total Price: {0:C}\n", TotalPrice);
}
}
}
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using System;
class Exercise
{
static int Main()
{
VendingMachine.IceCream IS = new VendingMachine.IceCream();
IS.ProcessAnOrder();
return 0;
}
}
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Ice Cream Vending Machine What type of flavor do you want? 1 - Vanilla 2 - Cream of Cocoa 3 - Chocolate Chip 4 - Organic Strawberry 5 - Butter Pecan 6 - Cherry Coke 7 - Chocolate Brownies 8 - Caramel Au Lait 9 - Chunky Butter 10 - Chocolate Cookie Your Choice? 8 What type of container do you want? 1 - Cone 2 - Cup 3 - Bowl Your Choice? 0 Invalid Choice - Try Again! What type of container do you want? 1 - Cone 2 - Cup 3 - Bowl Your Choice? 1 Do you want an ingredient or not 1 - No Ingredient 2 - Peanuts 3 - M & M 4 - Cookies Your Choice? 8 Invalid Choice - Try Again! Do you want an ingredient or not 1 - No Ingredient 2 - Peanuts 3 - M & M 4 - Cookies Your Choice? 3 How many scoops(1, 2, or 3)? 9 Invalid Choice - Try Again! How many scoops(1, 2, or 3)? 1 Ice Cream Order Flavor: Caramel Au Lait Container: Cone Ingredient: M & M Scoops: 1 Total Price: $2.70 |
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The Name of a File |
Before performing file processing, one of your early decisions will consist of specifying the type of operation you want to perform. For example, you may want to create a brand new file. You may want to open an existing file. Or you may want to perform a routine operation on a file. In all or most cases, whether you are creating a new file or manipulating an existing, you must specify the name of the file. You can do this by declaring a string variable but most classes uses to create a stream as we will learn later can take a string that represents the file.
If you are creating a new file, there are certainly some rules you must observe. The name of a file follows the directives of the operating system. On MS DOS and Windows 3.X (that is, prior to Microsoft Windows 9X), the file had to use the 8.3 format. The actual name had to have a maximum of 8 characters with restrictions on the characters that could be used. The user also had to specify three characters after a period. The three characters, known as the file extension, were used by the operating system to classify the file. That was all necessary for those 8-bit and 16-bit operating systems. Various rules have changed. For example, the names of folders and files on Microsoft Windows >= 95 can have up to 255 characters. The extension of the file is mostly left to the judgment of the programmer but the files are still using extensions. Applications can also be configured to save different types of files; that is, files with different extensions.
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At the time of this writing, the rules for file names for Microsoft Windows were on the MSDN web site at Windows Development\Windows Base Services\Files and I/O\SDK Documentation\Storage\Storage Overview\File Management\Creating, Deleting, and Maintaining Files\Naming a File (because it is a web site and not a book, its pages can change anytime). |
Based on this, if you declare a string variable to hold the name of the file, you can simply initialize the variable with the necessary name and its extension. Here is an example:
Practical
Learning: Introducing Streamingusing System;
class Exercise
{
static int Main()
{
string NameOfFile = "Employees.spr";
return 0;
}
}
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using System;
using System.IO;
namespace VendingMachine
{
// This class is used to create and manage an ice cream
// and to process an order
sealed class IceCream
{
. . . No Change
// This method is used to display a receipt to the user
public void DisplayReceipt()
{
Console.WriteLine("\nIce Cream Order");
Console.WriteLine("Flavor: {0}", Flavor[ChoiceFlavor-1]);
Console.WriteLine("Container: {0}", Container[ChoiceContainer-1]);
Console.WriteLine("Ingredient: {0}", Ingredient[ChoiceIngredient-1]);
Console.WriteLine("Scoops: {0}", Scoops);
Console.WriteLine("Total Price: {0:C}\n", TotalPrice);
}
public void SaveOrder()
{
string NameOfFile;
Console.Write("Please enter your initials or the name we will use to remember your order: ");
NameOfFile = Console.ReadLine();
}
}
}
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using System;
class Exercise
{
static int Main()
{
char Answer;
VendingMachine.IceCream IS = new VendingMachine.IceCream();
IS.ProcessAnOrder();
Console.Write("Do you want us to remember this order the next time you come to get your ice cream (y/n)? ");
Answer = char.Parse(Console.ReadLine());
if( Answer == 'y' || Answer == 'Y' )
IS.SaveOrder();
return 0;
}
}
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The Path to a File |
If you declare a string as above, the file will be created in the folder as the application. Otherwise, you can create your new file anywhere in the hard drive. To do that, you must provide a complete path where the file will reside. A path is a string that specifies the drive (such as A:, C:, or D:). The sections of a complete path string are separated by a backslash. For example, a path can the made of a folder followed by the name of the file. An example would be
C:\Palermo.tde
A path can also consist of a drive followed by the name of the folder in which the file will be created. Here is an example:
C:\Program Files\Palermo.tde
A path can also indicate that the file will be created in a folder that itself is inside of another folder. In this case, remember that the names of folder must be separated by backslashes.
In Lesson 1, we saw that the backslash character is used to create or manage escape sequences and it can be included in a string value to make up an escape sequence. Because of this, every time you include a backslash in a string, the compiler thinks that you are trying to provide an escape sequence. In this case, if the combination of the backslash and the character that follows the backslash is not recognized as an escape sequence, you would get an error. To solve this problem, you have two alternatives. To indicate that the backslash must be considered as a character in its own right, you can double it. Here are examples:
using System;
class Exercise
{
static int Main()
{
string NameOfFile = "C:\\Documents and Settings\\Business Records\\Employees.spr";
return 0;
}
}
Alternative, you can keep one backslash in each placeholder but precede the value of the string with the @ symbol. Here is an example:
using System;
class Exercise
{
static int Main()
{
string NameOfFile = @"C:\Documents and Settings\Business Records\Employees.spr";
return 0;
}
}
In the same way, you can declare a string variable to represent the name of an existing file that you plan to use in your program. You can also represent its path.
When providing a path to the file, if the drive you specify doesn't exist or cannot be read, the compiler would consider that the file doesn't exist. If you provide folders that don't exist in the drive, the compiler would consider that the file doesn't exist. This also means that the compiler will not create the folder(s) (the .NET Framework provides all means to create a folder but you must ask the compiler to create it; simply specifying a folder that doesn't exist will not automatically create it, even if you are creating a new file). Therefore, it is your responsibility to make sure that either the file or the path to the file is valid. As we will see in the next section, the compiler can check the existence of a file or path.
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File Existence |
While Stream is used as the parent of all file processing classes, the .NET Framework provides the File class equipped with methods to create, save, open, copy, move, delete, or provide detailed information about, files. Based on its functionality, the File class is typically used to assist the other classes with their processing operations. To effectively provide this support, all File's methods are static; which means that you will usually not need to declare a File variable to access them.
One of the valuable operations that the File class can perform is to check the existence of the file you want to use. For example, if you are creating a new file, you may want to make sure it doesn't exist already because if you try to create a file that exists already, the compiler may first delete the old file before creating the new one. This could lead to unpredictable result, especially because such a file is not sent to the Recycle Bin. On the other hand, if you are trying to open a file, you should first make sure the file exists, otherwise the compiler will not be able to open a file it cannot find.
To check the existence of a file, the File class provides the Exists method. Its syntax is:
public static bool Exists(string path);
If you provide only the name of the file, the compiler would check it in the folder of the application. If you provide the path to the file, the compiler would check its drive, its folder(s) and the file itself. In both cases, if the file exists, the method returns true. If the compiler cannot find the file, the method returns false. It's important to know that if you provided a complete path to the file, any slight mistake would produce a false result.
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using System;
using System.IO;
namespace VendingMachine
{
// This class is used to create and manage an ice cream
// and to process an order
sealed class IceCream
{
. . . No Change
// This method is used to display a receipt to the user
public void DisplayReceipt()
{
Console.WriteLine("\nIce Cream Order");
Console.WriteLine("Flavor: {0}", Flavor[ChoiceFlavor-1]);
Console.WriteLine("Container: {0}", Container[ChoiceContainer-1]);
Console.WriteLine("Ingredient: {0}", Ingredient[ChoiceIngredient-1]);
Console.WriteLine("Scoops: {0}", Scoops);
Console.WriteLine("Total Price: {0:C}\n", TotalPrice);
}
public void SaveOrder()
{
string NameOfFile;
Console.Write("Please enter your initials or the name we will use to remember your order: ");
NameOfFile = Console.ReadLine();
if( File.Exists(NameOfFile) )
{
char Answer;
Console.WriteLine("The file you entered exists already.");
Console.Write("Do you want to replace it(y/n)?" );
Answer = char.Parse(Console.ReadLine());
if( Answer == 'y' || Answer == 'Y' )
Console.WriteLine("The former order with the same name will be replaced");
else if( Answer == 'n' || Answer == 'N' )
{
Console.WriteLine("Please enter a name we will use to remember this order: ");
NameOfFile = Console.ReadLine();
}
else
Console.WriteLine("Invalid Answer - We will close");
return;
}
else
Console.WriteLine("The name you entered is not registered in our previous orders");
}
public void OpenOrder()
{
string NameOfFile;
Console.Write("Please enter the name you previously gave to remember your order: ");
NameOfFile = Console.ReadLine();
if( File.Exists(NameOfFile) )
Console.WriteLine("The file would have been opened");
else
Console.WriteLine("The name you entered is not registered in our previous orders");
}
}
}
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using System;
class Exercise
{
static int Main()
{
VendingMachine.IceCream IS = new VendingMachine.IceCream();
char Answer = 'n';
Console.Write("Have you ordered here before(y/n)? ");
Answer = char.Parse(Console.ReadLine());
if( Answer == 'y' || Answer == 'Y' )
IS.OpenOrder();
else
{
IS.ProcessAnOrder();
Console.Write("Do you want us to remember this order the next time you come to get your ice cream (y/n)? ");
Answer = char.Parse(Console.ReadLine());
if( Answer == 'y' || Answer == 'Y' )
IS.SaveOrder();
}
return 0;
}
}
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Access to a File |
In order to perform an operation on a file, you must specify to the operating system how to proceed. One of the options you have is to indicate the type of access that will be granted on the file. This access is specified using the FileAccess enumerator. The members of the FileAccess enumerator are:
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File Sharing |
In standalone workstations, one person is usually able to access and open a file then perform the necessary operations on it. In networked computers, you may create a file that different people can access at the same time or you may make one file access another file to retrieve information. For example, suppose you create an application for a fast food restaurant that has two or more connected workstations and all workstations save their customers orders to a common file. In this case, you must make sure that any of the computers can access the file to save an order. An employee from one of these workstations must also be able to open the file to retrieve a customer order for any necessary reason. You can also create a situation where one file holds an inventory of the items of a store and another file holds the customers orders. Obviously one file would depend on another. Based on this, when an operation must be performed on a file, you may have to specify how a file can be shared. This is done through the FileShare enumerator.
The values of the FileShare enumerator are:
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The Mode of a File |
Besides the access to the file, another option you will most likely specify to the operating system is referred to as the mode of a file. It is specified through the FileMode enumerator. The members of the FileMode Enumerator are:
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