ASP pre-defines several collections.  In addition, we may define our own collections by instantiating instances of the "Scripting.Dictionary" object.

These are the pre-defined ASP Collections:

• Application.Contents
• Application.StaticObjects
• Session.Contents
• Session.StaticObjects
• Request.ClientCertificate *
• Request.Cookies *
• Request.Form *
• Request.QueryString *
• Request.ServerVariables *
• Response.Cookies


* All pre-defined collections of the Request object are read only.

If List were an array, its individual items might be List(0), List(1), List(2), etc.

But if List were a collection, we would not use 0,1,2, etc., to distinguish the items.  Instead we would use string values (called keys).  For example, List("Monday"), List("Tuesday"), List("Wednesday"), etc.

To reference a individual item of a collection, you must know the value of a key.

List("Tuesday")

To cycle (iterate) through the keys of a collection without knowing them, use a For Each loop:

For Each Key in List
    Response.Write Key & " has a value of " & List(Key) & "<br>"
Next

TheCollection is the name of the particular collection, either pre-defined or user-defined.
Key is a variable name we choose.

For I = 0 to List.Count-1
    Response.Write List.Keys(I) & " has a value of " & List.Items(I) & "<br>"
Next

NOTE:  Keys and Items are special functions that retrieve the entire list of keys or items from the collection.  Therefore, the loop above is very inefficient because it calls the Keys and Items methods again and again, for each item in the collection.  If the list is long, this could take a long time and waste a lot of computing power.  A better version would call the Keys and Items methods only once each, storing their contents in variables and using the variables in the loop--like this:
KeyArray = List.Keys
ItemArray = List.Items
For I = 0 to List.Count-1
    Response.Write KeyArray(I) & " has a value of " & ItemArray(I) & "<br>"
Next

We define our own collections by instantiated an instance of the "Scripting.Dictionary" object--like this:

Set List = CreateObject("Scripting.Dictionary")

Recall that Set is required anytime we instantiate an object (as opposed to simply assigning a value to a variable).
The line of code above creates a new List object whose data is empty.

How to Add Items to a Collection

List.Add "Monday", "Turkey"
List.Add "Tuesday", "Pork"
List.Add "Wednesday", "Beef"
List.Add "Thursday", "Ham"
List.Add "Friday", "Fish"
List.Add "Saturday", "Chicken"
List.Add "Sunday", "Mutton"

• The sample code above uses the Add method to add 7 items to the collection named List.  The first line adds an item whose key is "Monday" and whose value is "Turkey".  Thus List("Monday") equals "Turkey".
• If we try to add an item whose key already exists, a run-time error occurs.  Therefore, it is often wise to check for the existence of a key beforehand, like this:

If Not List.Exists("Monday") Then List.Add "Monday", "Turkey"

List.Remove("Tuesday")

• The line above deletes the item in List whose key is "Tuesday".
• If we try to delete an item whose key does not exist, a run-time error occurs.  Therefore, it is often wise to check for the existence of a key beforehand, like this:

If List.Exists("Tuesday") Then List.Remove("Tuesday")

• The line above deletes all items of the List object.  List's data is now empty.

List.Key("Saturday") = "Sabado"

• The line above changes the key named "Saturday" to "Sabado".  The value "Chicken" is not changed.

List.Item("Sunday") = "Vegetables"

• The line above changes the value of the item whose key is "Sunday" to "Vegetables".