Array-Based Lists

 Introduction
 A list is a collection of items of the same kind. The list can be based on numbers (natural or floating-point), text, dates, times, or such values as long as the items are of the same type. Lists are at the core of computer programming. They are used in databases, spreadsheets, word processors and applications that are not primarily list-oriented.We are going to create a list of double-precision numbers.

 List Setup
 There are various ways to create a list. We will take a look at the simplest approach, which is to use a normal array. With this technique, you can start by creating a class. Here is an example:
```using System;

class ListOfNumbers
{
public ListOfNumbers()
{
}
}

class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();

return 0;
}
}```
 To start the list, you can declare an array as a member variable:
```using System;

class ListOfNumbers
{
private double[] Item = new double[20];

public ListOfNumbers()
{
}
}

class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();
return 0;
}
}```
 A primary accessory you need when using a list is a count of the number of items in the list when they are added or deleted. This accessory is primarily a private member variable as a simple natural number. When the class is declared, this member variable should be set to 0 to indicate that the list is primarily empty:
```class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}
}```
 Since the size member variable was declared private, if you plan to get the count of items in the list from outside the class, you can provide a property to take care of this. This can simply be done as follows:
```using System;

class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}

public int Count
{
get
{
return size;
}
}
}

class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();

Console.WriteLine("Number of Items: {0}", lstNumbers.Count);
return 0;
}
}```
 This would produce:
 `Current Number of Items: 0`

 Creating a list consists of adding items to it. Items are usually added one at a time. The easiest way to do this is to add an item at the end of the existing list. This is the easiest of your operations. To add an item to the list, you first check whether the list is already full. A list is full if its count of items is equal to or higher than the maximum number you had set. If the list is not empty, you can add an item at the end and increase the count by one. Here is how this can be done:
``` class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}

public int Count
{
get
{
return size;
}
}

// Adds a new item to the list if the list is not full
// Increases the number of items in the list
// Returns true if the item was added, otherwise returns false
{
// Make sure the list is not yet full
if( size < 20 )
{
// Since the list is not full, add the "item" at the end
Item[size] = item;
// Increase the count and return the new position
size++;

// Indicate that the item was successfully added
return true;
}

// If the item was not added, return false;
return false;
}
}```
 Once you have a means of adding items to the list, you can effectively create a list of items. Here is an example:
```class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();

Console.WriteLine("Number of Items: {0}", lstNumbers.Count);

Console.WriteLine();
return 0;
}
}```
 Item Retrieval
 After adding items to a list, you can retrieve them to do what you intended the list for. To retrieve an item, you can locate an item by its position, the same way you would do for an array. Having this index, you can check if the position specified is negative or higher than the current count of items. If it is, there is nothing much to do since the index would be wrong. If the index is in the right range, you can retrieve its corresponding item. The method to do this can be implemented as follows:
```class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}

public int Count
{
get
{
return size;
}
}

// Adds a new item to the list if the list is not full
// Increases the number of items in the list
// Returns true if the item was added, otherwise returns false
{
// Make sure the list is not yet full
if( size < 20 )
{
// Since the list is not full, add the "item" at the end
Item[size] = item;
// Increase the count and return the new position
size++;

// Indicate that the item was successfully added
return true;
}

// If the item was not added, return false;
return false;
}

// Retrieves an item from the list based on the specified index
public double Retrieve(int pos)
{
// Make sure the index is in the range
if( pos >= 0 && pos <= size )
return Item[pos];

// If the index was wrong, return 0
return 0;
}
}```
 You can then call such a method to locate an item and use its value. Here is an example:
```class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();

Console.WriteLine("Number of Items: {0}", lstNumbers.Count);

for(int i = 0; i < lstNumbers.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, lstNumbers.Retrieve(i));

Console.WriteLine();
return 0;
}
}```
 Complex Operations on a List

 Item Insertion
 Inserting a new item in the list allows you to add one at a position of your choice. To insert a new item in the list, you must provide the new item and the desired position. Before performing this operation, you must first check two things. First, the list must not be empty. Second, the specified position must be in the allowed range. The method can be implemented as follows:
```class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}

public int Count
{
get
{
return size;
}
}

// Adds a new item to the list if the list is not full
// Increases the number of items in the list
// Returns true if the item was added, otherwise returns false
{
// Make sure the list is not yet full
if( size < 20 )
{
// Since the list is not full, add the "item" at the end
Item[size] = item;
// Increase the count and return the new position
size++;

// Indicate that the item was successfully added
return true;
}

// If the item was not added, return false;
return false;
}

// Retrieves an item from the list based on the specified index
public double Retrieve(int pos)
{
// Make sure the index is in the range
if( pos >= 0 && pos <= size )
return Item[pos];

// If the index was wrong, return 0
return 0;
}

// Before performing this operation, check that
// 1. The list is not full
// 2. The specified position is in an allowable range
// Inserts a new item at a specified position in the list
// After the new item is inserted, the count is increased
public bool Insert(double item, int pos)
{
// Check that the item can be added to the list
if( size < 20 && pos >= 0 && pos <= size )
{
// Since there is room,
// starting from the end of the list to the new position,
// push each item to the next or up
// to create room for the new item
for(int i = size; i > pos-1; i--)
Item[i+1] = Item[i];

// Now that we have room, put the new item in the position created
Item[pos] = item;

// Since we have added a new item, increase the count
size++;

// Indicate that the operation was successful
return true;
}

// Since the item could not be added, return false
return false;
}
}```

 Item Deletion
 Another operation you can perform on a list consists of deleting an item. This is also referred to as removing the item. To delete an item from the list, you can provide its position. Before performing the operation, you can first check that the specified position is valid. The method to perform this operation can be implemented as follows:
```using System;

class ListOfNumbers
{
private double[] Item = new double[20];
private int size;

public ListOfNumbers()
{
size = 0;
}

public int Count
{
get
{
return size;
}
}

// Adds a new item to the list if the list is not full
// Increases the number of items in the list
// Returns true if the item was added, otherwise returns false
{
// Make sure the list is not yet full
if( size < 20 )
{
// Since the list is not full, add the "item" at the end
Item[size] = item;
// Increase the count and return the new position
size++;

// Indicate that the item was successfully added
return true;
}

// If the item was not added, return false;
return false;
}

// Retrieves an item from the list based on the specified index
public double Retrieve(int pos)
{
// Make sure the index is in the range
if( pos >= 0 && pos <= size )
return Item[pos];

// If the index was wrong, return 0
return 0;
}

// Before performing this operation, check that
// 1. The list is not full
// 2. The specified position is in an allowable range
// Inserts a new item at a specified position in the list
// After the new item is inserted, the count is increased
public bool Insert(double item, int pos)
{
// Check that the item can be added to the list
if( size < 20 && pos >= 0 && pos <= size )
{
// Since there is room,
// starting from the end of the list to the new position,
// push each item to the next or up
// to create room for the new item
for(int i = size; i > pos-1; i--)
Item[i+1] = Item[i];

// Now that we have room, put the new item in the position created
Item[pos] = item;

// Since we have added a new item, increase the count
size++;

// Indicate that the operation was successful
return true;
}

// Since the item could not be added, return false
return false;
}

// Removes an item from the list
// First check that the specified position is valid
// Deletes the item at that position and decreases the count
public bool Delete(int pos)
{
// Make sure the position specified is in the range
if( pos >= 0 && pos <= size )
{
// Since there is room, starting at the specified position,
// Replace each item by the next
for(int i = pos; i < size; i++)
Item[i] = Item[i+1];

// Since an item has been  removed, decrease the count
size--;

// Indicate that the operation was successful
return true;
}

// Since the position was out of range, return false
return false;
}
}

class Exercise
{
static int Main()
{
ListOfNumbers lstNumbers = new ListOfNumbers();

Console.WriteLine("Number of Items: {0}", lstNumbers.Count);

for(int i = 0; i < lstNumbers.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, lstNumbers.Retrieve(i));

lstNumbers.Delete(2);

Console.WriteLine("\nAfter deleting the the third item...");
for(int i = 0; i < lstNumbers.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, lstNumbers.Retrieve(i));

Console.WriteLine();
return 0;
}
}```
 This would produce:
```Number of Items: 5
Item 1: 224.52
Item 2: 60.48
Item 3: 1250.64
Item 4: 8.86
Item 5: 1005.36

After deleting the the third item...
Item 1: 224.52
Item 2: 60.48
Item 3: 8.86
Item 4: 1005.36```