Computer Science II — CSci 1200
Lab 10
Stacks and Queues
Introduction to Stacks and Queues
Stacks and queues are very simple sequence containers in which items are
only added and removed from the end. In a stack, all work is done on just
one end, called the top. Hence, when an item is removed, it will be the item
most recently added. As a result, a stack is called a LIFO structure, for
“Last In First Out”. In a queue, items are added to the end, usually called
the rear or back, and removed from the other end, usually called the front.
Hence, when an item is removed it will be the item that has been in the
queue longer than any other item currently in the queue. As a result, a queue
is called a FIFO structure, for “First In First Out”. A fundamental property
distinguishing stacks and queues from other containers is that items in the
middle of the sequence may not be accessed or removed. One effect
of this is than neither stacks nor queues have iterators.
Stacks and queues are interesting because their simple functionality pro-
vides an appropriate model for many common operations. Here are some
examples:
•The management of memory associated with function calls uses a
stack. Each function call causes the creation of a “frame” for the
new function, including space for parameters and local variables. This
frame is “pushed” onto the memory stack at the start of operations
for the function. When a function ends, this frame is “popped” off
the memory stack, and the previous function, whose memory is now
on the top of the stack, resumes its execution.
•The allocation of computing resources, including the CPU, printers
and communication channels, is often based on the FIFO nature of
a queue. Certainly, these allocation methods are more sophisticated
than purely FIFO, but FIFO is the starting idea.
A simple example will help illustrate further the use of a stack. The
goal is to determine whether or not an expression has a balanced set of
parentheses. Here the term parentheses is meant to include the characters
’(’, ’[’, ’{’, ’)’, ’]’, ’}’. We use a stack of chars. Each time an “open”
parenthesis char — ’(’, ’[’, ’{’ — is read in the input, the char is pushed
