CSC 161  Grinnell College  Spring, 2009 
Imperative Problem Solving and Data Structures  
This reading introduces the concept of an Abstract Data Type (ADT) and describes a stack as a specific example.
Most of this reading is an edited version of Henry M. Walker, Introduction to Computing and Computer Science with Pascal, Little, Brown, and Company, 1986, Sections 17.117.2, 17.4, with programming examples translated from Pascal to C. This material is used with permission from the copyright holder.
When we want to work with data on a general level, we often need to describe two basic characteristics: the data we will be storing, and the operations we will want to perform on these data. In computer science, these two characteristics combine to give he concept of an abstract data type or ADT which allows us to work with data on a conceptual level without worrying about various programming details.
The stack discussed in this lab provides one example of an abstract data type. The queue, discussed in a forthcoming lab, provides a second example.
A stack mimics the information that we might keep in a pile on our desk. For example, on our desk, we may keep separate piles for
These piles have several properties. First, each pile contains the same type of information (e.g., bills, magazines, or notes). In addition, for each pile, we can do several tasks.
These operations allow us to do all of our normal processing of data at our desk. For example, when we receive bills in the mail, we add them to our pile of bills until payday comes. Then, we take our bills, one at a time, off the top of our pile and pay them until our money runs out.
When discussing these operations, it is customary to call the addition of an item to the top of the pile a Push operation and the deletion of an item from the top a Pop operation.
More formally, a stack is defined as an abstract data type that can store data and that has the following operations:
This specification says nothing about how we will program the various stack operations; rather, it tells us how stacks can be used. We can also infer some limitations on how we can use the data. For example, stack operations allow us to work with only the top item on the stack. We cannot look at other pieces of data lower down in the stack without first using Pop operations to clear away items above the desired one.
A Push operation always puts the new item on top of the stack, and this is the first item returned by a Pop operation. Thus, the last piece of data added to the stack will be the first item removed.
One common implementation of a stack involves the use of an array.
More precisely, we store each piece of data as an element of an array. We place the first data item at one end of the array. Then, for a Push operation, we add a data item to the next array element. For a Pop operation, we return the item at the top of our data, and we record that the top has moved down. This processing requires several parts, including an array (StackArray) of data to store our data items, a variable (topPosition) to keep track of our top element, and a constant (MaxStack) to keep track of the size of our array. Conceptually, this setup fits together as shown in the figure.
With this figure, we trace what happens in our stack operations. We start with topPosition equal to 1, since we have no data in the array. Then, we perform a Push, we increment topPosition by one, and we store our new item in this new topPosition. Similarly, for a Pop operation, we return the item at the topPosition, and we move the topPosition down by one. Finally, for Full or Empty functions, we compare the topPosition with MaxStack1 or 1, respectively.
Additional details arise because we must check if the stack is full or empty before actually performing a Push or Pop operation, respectively.
Finally, we note that sometimes it is convenient to package the elements together in a struct construction, perhaps with a typedef. For example, the following declarations might be used when the stack is to store strings.
#define MaxStack 50; /* MaxStack stands for the size of all stack arrays */ typedef struct { int topPosition; char * stackArray [MaxStack]; } stringStack; /* type for a stack of strings */
This document is available on the World Wide Web as
http://www.walker.cs.grinnell.edu/courses/161.sp09/readings/readingstacksarrays.shtml
created 28 April 1997 last revised 13 April 2009 

For more information, please contact Henry M. Walker at walker@cs.grinnell.edu. 