Wednesday, December 12, 2012

Collection and differences with dynamic array in visual basic


Collections: Arrays are convenient for storing related data, but accessing individual elements can be a problem. Arrays should be accessed by their contents.  Visual Basic provides an alternative: collections. Similar to an array, a collection stores related items, in this case, objects that have properties and methods. The advantage of a collection over an array is that the collection lets you access its items via a key.
To use a collection, you must first declare a collection variable, as follows:
               Dim Temperatures As New Collection
The keyword New tells Visual Basic to create a new collection and name it Temperatures.

The collection object provides three methods and one property:
  Add method Adds items to the collection
  Remove method Deletes an item from the collection by index or key
  Item method Returns an item by index or by key
  Count property Returns the number of items in the collection

Adding to a Collection: The Add method adds new items to the collection and it has the following syntax:                     collection. Add value, key, before, after

To add a new element to a collection, assign its value to the item argument and its key to the key argument. To place the new item in a specific location in the array, specify one of the arguments before or after (but not both).
               Ex. Temperatures.Add 78, “San Francisco”

Removing an Item from a Collection: The Remove method removes an item from a collection. The index argument can be either the position of the item you want to delete or the item’s key. To remove the city of Atlanta from your collection of temperatures, use the following statement:

               Ex. Temperatures.Remove “Atlanta”
Or, if you know the city’s order in the collection, specify the index in place of the key:
   Ex. Temperatures.Remove 6

Returning Items in a Collection: The Item method returns the value of an item in the collection. As with the Remove method, the index can be either the item’s position in the collection or its key. To recall the temperature in Atlanta, use one of the following statements:

               Ex. T1 = Temperatures.Item(“Atlanta”)
                      T1 = Temperatures.Item(3)

Counting a Collection: The Count property returns the number of items in the collection. To find out how many cities have been entered so far in the Temperatures collection, use the following statement:

               Ex. Temperatures.Count