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CS 305 Lab 7: Stacks and Queues

The purpose of this lab is to give you experience with stacks and queues. In this lab,
the stack is implemented with a linked list. The queue is implemented with an array.
This lab has a total of 100 possible points (30 points from pre-lab and 70 points
from lab). You have two lab sessions to complete this lab. You have two lab
sessions for this lab.
Sit with your assigned partner.
Objectives
Upon completion of the laboratory exercise, you will be able to do the following:
 Read implementations for stacks and queues.
 Define a peek function for a stack.
 Define a function to determine if a string has properly nested ( ) and [ ], using
a stack data structure
 Define a full function for a queue.
 Use stack and queue data structures to add and remove items.
Part 1: Logging in (choose one person)
1. Follow the procedure from prior labs to log in and Mobaxterm. Go into your cs305
directory. Make a new lab7 directory:
cd /drives/p/cs305
mkdir lab7
2. Get the lab 7 files. Download stack.zip and queue.zip from moodle to your
lab7 folder. On Mobaxterm:
ls
Hopefully, you can see the file. Unzip both files:
unzip stack.zip
unzip queue.zip
Part 2: Stacks
1. Go into the stack folder. Open stack.h to see the struct definitions and the
function prototypes. The stack is implemented as a linked list, where the front of the
list is the top element of the stack. The operations on a stack include: initStack,
empty, push, and pop. You will implement peek as part of this lab.
2. Open main.c and look at the function test1. What are the values of the stack s
just after 15 is pushed to the stack?
top of stack —
bottom of stack —
3. Compile and run the program to see if your answer to #2 is correct.
gcc –o s stack.c main.c
./s
4. Complete the function definition for peek. It should test if the stack is empty. If
so, print “Stack is empty. No data item to peek.\n” and return BAD. BAD is a
macro defined in stack.h. If there is data in the stack, return the top element of the
stack (but don’t remove it from the stack for the peek operation).
Comment out test1() in main and uncomment test2() in main. Compile and run
your code to be sure that peek is working correctly.
5. Add a function prototype in stack.h and a function definition in stack.c to free
the memory associated with the stack. It should look like:
void freeStack(Stack S);
Because the stack might have nodes in it, you should free the nodes by popping
whatever is in the stack until it is empty. Once the stack is empty, you can free S
(the pointer to the StackType).
Complete the function called test3 in main that tests that freeing the stack works.
You should put items in the stack and then free the stack.
Checkpoint 1 [20 points]: Show your lab instructor/assistant the results of
your program running with calls to test2 and test3 uncommented. Show
your answer to the question above.
Part 3: Using Stacks
1. Now, you will complete the function testMatch in main.c that uses a stack to
solve the nested delimiter problem for matching ( ) and [ ]. Here is the pseudocode:
While there is a unprocessed character c in input:
if c is the left delimiter (, then push it to the stack
if c is the left delimiter [, then push it to the stack
if c is the right delimiter ), then pop the stack; if what is popped is (, keep
going; if what is
popped is not (, then print “Not in proper format”
if c is the right delimiter ], then pop the stack; if what is popped is [, keep
going; if what is
popped is not [, then print “Not in proper format”
Else, move to next character
If stack is empty, print “In proper format”. Else, print “Not in proper format”
Uncomment testMatch in main and comment out the other tests.
2. Open stack.h. Change the StackData type to char and define BAD as ‘!’.
Compile and run the code. Try different inputs for expressions to see if your function
is working properly.
Checkpoint 2 [20 points]: Show your lab instructor/assistant the results of
your program running for detecting properly formatted nested ( ) and [ ]
for a variety of inputs. Show the lab instructor/assistant your code.
Part 4: Queues
1. The queue data structure for this lab is implemented as an array. Go in the queue
folder. Open queue.h to see the struct definitions and the function prototypes.
2. Open main.c. Look at the code and predict what the queue has in in before you
test this function. What does the queue contain (in order such that the left-most
item will be the next item dequeued) after 14 is enqueued?
front of queue______________________________________________________back of queue
3. Compile and run the code:
gcc –o q queue.c main.c
./q
4. Is your prediction in #2 correct? YES NO
5. Open queue.c to see how the functions are implemented. The head is the index
in the array that is one less than the position of the first item in the queue. The tail
is the index of the last item in the queue. When head is equal to tail, the queue is
empty. Note that by keeping track of head and tail, we do not need to physically
move items in the data array (copying/shifting them to the left as items are
removed from the queue).
a. What is the value of q-head just after 4 in enqueued in main? ____________
(You can print this value if to confirm your prediction.)
b. What is the value of q-tail just after 4 is enqueued in main? __________
(You can print this value if to confirm your prediction.)
c. What is the value of q-head just after 20 is enqueued in main? _______
d. What is the value of q-tail just after 20 is enqueued in main? _____
9. Open queue.h. At the top of the file, you will see a macro definition for MAX_Q. Set
it to 5. Recompile the code and run it.
What are the queue’s contents after 14 is enqueued (this is what is printed after
“Queue contents:”)? _______________________
Hmmm, what happened to the rest of the items in the queue?
____________________________
What are the values of q-data, q-head, and q-tail just after 14 in enqueued in
main? (you can check this by printing out these values.)
q-data:
0 1 2 3 4
q-tail: __________ q-head: _________
10. Well, what you saw is what happens when a queue is allowed to become
overfull. This is a buggy implementation. One option is to re-allocate memory to
store twice the size of data and copy everything over to this new array and store
the new array as data. Another option is to not allow the queue to become over-full.
Open queue.c. At the bottom, there is a function called full. As implemented, full
always returns 0 (meaning that is never full). Replace the return statement with a
correct return statement. The queue is full when tail is one less than the value of
head (it has wrapped around the array). Be sure that this works when tail is the
largest position of the array and head is at 0. You can use modulus (%) to help you
(see how it is used in dequeue and enqueue).
Compile and run your program.
Now what happens? _________________________________
What is in the queue after 14 is enqueued in main?
front of queue________________________________________ back of queue
11. Another thing that we should always do when writing C programs is to free any
memory that was allocated on the heap before returning from main. We know the
underlying representation of this queue is a static-sized array, but anyone else
using this queue ADT would not know that. Add a function to queue.c and a
prototype to queue.h called:
void freeQueue(Queue Q);
The function definition should simply call free(Q).
In main, just before the return statement, call:
freeQueue(q);
Checkpoint 3 [15 points]: Show your lab instructor/assistant the result of
your program running with MAX_Q set to 5 and your code. Show your lab
instructor/assistant the result of your program running with MAX_Q set to
15. Show your answers to the questions above.
Keep MAX_Q at 15. Another operation that could be implemented for queues is
length. While this is not a “standard” queue operation, it could certainly be useful
for a programmer. For example, knowing the length of a queue could help a
program determine what should be done. At Disneyland, the length of the queue
might trigger the staff to put more cars on a ride, for example.
1. Add a function to queue.c and a prototype to queue.h called:
int length(Queue Q);
2. Using the values of head and tail, return the current length of the queue. Think
about the two cases:
If tail = head, this is calculated by:
________________________________________________
(queue does not wrap around end of array)
If tail < head, this is calculated by: _______________________________________________
(queue wraps around end of array)
3. Add code to main.c to test that length is working properly. Print the length of the
queue before and after enqueuing and dequeuing items.
Checkpoint 4 [15 points]: Show your lab instructor/assistant the result of
your program running and your code. Show your answers to the questions
above.
If you finish early and want more practice, try writing code to:
 use two stacks to mimic the behavior of a queue
 use two queues to mimic the behavior of a stack
 use an extra stack to keep a stack in sorted order
You are finished. Close Mobaxterm and any other applications. Log off the
computer.
If any checkpoints are not finished, they are due March 15. You may submit
screenshots, code files, and answers to questions electronically on moodle.

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