programming-for-dummies

362
Using a Stack
Data #3
Figure 4-1:
Stacks store
the oldest
data on the
bottom and
the newest
data on top.
Data #2 Data #3
Data #1 Data #2
Data #1
Adding new data to a stack pushes the
oldest data further down the stack.
Data #3
Data #2
Data #1
Data #2
Data #1
Removing data from a
stack takes away the
newest data.
The only way to remove data from a stack is from its top. Each time you
remove data, the stack shrinks automatically. Because a stack only lets you
remove the last item stored on the stack, it’s often called a Last In, First Out
(LIFO) data structure because the last item stored is always the first item
you can remove from the stack.
Few programming languages offer the stack data structure as a built-in feature.
Instead, you have to create a stack using other data structures, such as
an array or a linked list. When you create another data structure out of a
built-in data structure, the new data structure created is an abstract data
structure. To save you the time and trouble of creating a stack data structure,
many programming language compilers come with libraries (or classes in
object-oriented languages) of subprograms that have created the stack data
structure for you.
Because a stack is just a data structure, you can declare a variable to represent
a stack in Visual Basic.NET by doing the following:
Dim BlowMyStack as New Stack
This command simply identifies a BlowMyStack variable as a stack data
structure. The New command tells the computer to create a new stack.
Adding data to a stack
When you first create a stack, it contains zero items. Each time you add a
new chunk of data to a stack, it expands automatically. Unlike other data
structures, such as collections, you can only add new data to the top of the
stack; you can never add data in a specific location in a stack.