Routine Operations on an Array-Based List

 Creating a collection 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 collection. To add an item to the collection, you first check whether the list is already full. For an array-based list, the collection is full if its count of items is equal to or higher than the maximum number you had set. If the collection 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 Collection
{
public int MaxCount = 20;

// This collection will be a list of decimal numbers
private double[] Item;
// This is the size of the collection
private int size;

#region This section is used to set up the collection
// Our default constructor, used to initialize the collection
public Collection()
{
this.Item = new double[MaxCount];
this.size = 0;
}

// This represents the number of items in the collection
public int Count
{
get { return this.size; }
}
#endregion

#region Operations on the collection
// 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 (this.size < 20)
{
// Since the list is not full, add the "item" at the end
this.Item[this.size] = item;
// Increase the count and return the new position
this.size++;

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

// If the item was not added, return false;
return false;
}
#endregion
}```
 Once you have a means of adding items to the list, you can effectively create a list of items. Here is an example: ```class Program { static int Main(string[] args) { Collection list = new Collection(); Console.WriteLine("Number of Items: {0}", list.Count); list.Add(224.52); list.Add(60.48); list.Add(1250.64); list.Add(8.86); list.Add(1005.36); Console.WriteLine("Number of Items: {0}", list.Count); return 0; } }``` This would produce: ```Number of Items: 0 Number of Items: 5 Press any key to continue . . .```

 Getting an Item From a List
 After adding items to a collection, you can retrieve them to do what you intended the list for. To retrieve an item, you can locate it by its position, the same way you would do for an array. Having this index, you can check whether 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:
```using System;

class Collection
{
public int MaxCount = 20;

// This collection will be a list of decimal numbers
private double[] Item;
// This is the size of the collection
private int size;

#region This section is used to set up the collection
// Our default constructor, used to initialize the collection
public Collection()
{
this.Item = new double[MaxCount];
this.size = 0;
}

// This represents the number of items in the collection
public int Count
{
get { return this.size; }
}
#endregion

#region Operations on the collection
// 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 < MaxCount)
{
// Since the list is not full, add the "item" at the end
this.Item[this.size] = item;
// Increase the count and return the new position
this.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 this.Item[pos];

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

class Program
{
static int Main(string[] args)
{
Collection list = new Collection();

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

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}\n", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}", list.Count);

return 0;
}
}```

This would produce:

```Number of Items: 0
Item 1: 224.52
Item 2: 60.48
Item 3: 1250.64
Item 4: 8.86
Item 5: 1005.36
Number of Items: 5

Press any key to continue . . .```
 Inserting an Item in the List

Inserting a new item in the collection allows you to add one at a position of your choice. To insert an item in the list, you must provide the new item and the desired position. Before performing this operation, you must 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:

```using System;

class Collection
{
public int MaxCount = 20;

// This collection will be a list of decimal numbers
private double[] Item;
// This is the size of the collection
private int size;

#region This section is used to set up the collection
// Our default constructor, used to initialize the collection
public Collection()
{
this.Item = new double[MaxCount];
this.size = 0;
}

// This represents the number of items in the collection
public int Count
{
get { return this.size; }
}
#endregion

#region Operations on the collection
// 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 < MaxCount)
{
// Since the list is not full, add the "item" at the end
this.Item[this.size] = item;
// Increase the count and return the new position
this.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 this.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 itm, 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--)
this.Item[i + 1] = this.Item[i];

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

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

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

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

class Program
{
static int Main(string[] args)
{
Collection list = new Collection();

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

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}\n", list.Count);

list.Insert(-707.16, 2);

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}\n", list.Count);

return 0;
}
}```

This would produce:

```Number of Items: 0
Item 1: 224.52
Item 2: 60.48
Item 3: 1250.64
Item 4: 8.86
Item 5: 1005.36
Number of Items: 5

Item 1: 224.52
Item 2: 60.48
Item 3: -707.16
Item 4: 1250.64
Item 5: 8.86
Item 6: 1005.36
Number of Items: 6

Press any key to continue . . .```
 Removing an Item From the List

Another operation you can perform on a collection consists of deleting an item. This is also referred to as removing the item. To delete an item from the collection, 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 Collection
{
public int MaxCount = 20;

// This collection will be a list of decimal numbers
private double[] Item;
// This is the size of the collection
private int size;

#region This section is used to set up the collection
// Our default constructor, used to initialize the collection
public Collection()
{
this.Item = new double[MaxCount];
this.size = 0;
}

// This represents the number of items in the collection
public int Count
{
get { return this.size; }
}
#endregion

#region Operations on the collection
// 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 < MaxCount)
{
// Since the list is not full, add the "item" at the end
this.Item[this.size] = item;
// Increase the count and return the new position
this.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 this.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 itm, 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--)
this.Item[i + 1] = this.Item[i];

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

// Since we have added a new item, increase the count
this.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 < this.size; i++)
this.Item[i] = this.Item[i + 1];

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

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

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

class Program
{
static int Main(string[] args)
{
Collection list = new Collection();

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

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}\n", list.Count);

list.Insert(-707.16, 2);
list.Insert(-369952.274, 4);

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}\n", list.Count);

list.Delete(5);
list.Delete(3);

for (int i = 0; i < list.Count; i++)
Console.WriteLine("Item {0}: {1}", i + 1, list.Retrieve(i));
Console.WriteLine("Number of Items: {0}\n", list.Count);

return 0;
}
}```

This would produce:

```Number of Items: 0
Item 1: 224.52
Item 2: 60.48
Item 3: 1250.64
Item 4: 8.86
Item 5: 1005.36
Number of Items: 5

Item 1: 224.52
Item 2: 60.48
Item 3: -707.16
Item 4: 1250.64
Item 5: -369952.274
Item 6: 8.86
Item 7: 1005.36
Number of Items: 7

Item 1: 224.52
Item 2: 60.48
Item 3: -707.16
Item 4: -369952.274
Item 5: 1005.36
Number of Items: 5

Press any key to continue . . .```