programming-for-dummies

372
Using Deques
Because you can add data to both ends of a deque, a deque can grow in both
directions, as shown in Figure 4-10.
Data #2 Data #1
Data #3
Figure 4-10:
A deque
can grow
in two
different
directions.
Data #4
Data #4
Data #2 Data #1 Data #3
Data #2 Data #1
Data #3
Unlike a stack that always removes the newest data or a queue that always
removes the oldest data, a deque can never predictably remove either the
oldest or newest data. When you add data to both ends of the deque, the
oldest data tends to get sandwiched and buried in between the newest data
on both ends.
Like stacks and queues, deques only allow you to remove data from a specific
part of the data structure. In general, the more complicated the data
structures, the more code you have to write to manage that data structure.
A list or an array is much simpler to use, but much less flexible than a queue
or a stack. Unfortunately, stacks, queues, and deques add greater complexity
to your program in exchange for their added flexibility.
If you need to store and remove data in an orderly fashion, stacks, queues,
and deques are better suited than arrays, collections, or dictionaries, which
are better suited for long-term storage of data. A queue might be useful for
an online reservation system that handles the oldest request first.
Although the idea of removing the last item first might seem counterintuitive,
stacks are a commonly used data structure. Most programs offer
an Undo command, which lets you undo the last command you gave the
computer. If you give five different commands, the program may store each
command in a stack.
When you want to undo a command, you want to start with the last
command you gave it, which appears on the top of the stack, as shown in
Figure 4-11. Each succeeding Undo command removes an additional command
until you get to the last command, which is the oldest one that was
buried at the bottom of the stack.