[SOLVED] Cmput 175 – lab 4: stacks

Goal: Become familiar with a new data structure: Stack.
Exercise 1:
You are tasked with creating a web browser simulator. Your web browser simulator should use
two stacks: one to enable the back button functionality, and one to enable the forward button
functionality. Your simulator will not actually display any webpages, but just the URL address
of the current page that the user is on. (See sample run at the end of this exercise.) When the
user wishes to enter a new webpage address, s/he signals this by entering ‘=’. When the user
wishes to go back, ‘<’ is entered. When the user wishes to go forward, ‘>’ is entered. The user
can quit by entering ‘q’ when prompted.1. Complete the worksheet at the end of this lab to show the contents of the back Stack and
forward Stack after every valid entry in the sample output. DO NOT submit your
worksheet on eClass, but be prepared to show this to your TA during any help or demo
sessions.2. Download and save stack.py from eClass. This file contains implementation #2 of the Stack
covered in the lectures. DO NOT modify or submit stack.py.
3. Download and save a copy of lab4_browser.py from eClass. (Be sure that you save it in the
same directory as stack.py.) This file contains a COMPLETED main() function which
controls the flow of operation of a web browser simulation. In the following steps, you will
complete the functions that this main() function calls.4. Complete getAction(). This function prompts the user to enter either a ‘=’ (to enter a new
website address), ‘<’ (back button), ‘>’ (forward button), or ‘q’ to quit the browser
simulation. If the user enters something other than these 4 characters, an error message is
displayed before re-prompting for a valid entry. This function has no inputs. This function
returns the valid character entered by the user (str).
*Reminder: Be sure to write a docstring for each of your functions.5. Complete goToNewSite(). This function is called when the user enters ‘=’ during getAction().
This function prompts the user to enter a new website address, and returns that address as a
string. It also updates the two stacks, as appropriate. (Hint: experiment with how the back
and forward buttons work on a real web browser like Firefox or Chrome. After a new address
is entered, can you still go forward?) Note that you do not need to explicitly return the two
stacks because the Stack (as we implemented it) is a mutable object – so bck and fwd are
actually just aliases for the stacks called back and forward in your main function. The inputs
for this function are the current website (str), a reference to the Stack holding the webpage
addresses to go back to, and a reference to the Stack holding the webpage addresses to go
forward to.6. Complete goBack(). This function is called when the user enters ‘<’ during getAction(). An
error message is displayed if there are no webpages stored in the back history, and the current
site is returned (str). Otherwise, the previous webpage is retrieved (and returned as a string),
and the two stacks are updated as appropriate. The inputs for this function are the current
website (str), a reference to the Stack holding the webpage addresses to go back to, and a
reference to the Stack holding the webpage addresses to go forward to.7. Complete goForward(). This function is called when the user enters ‘>’ during getAction().
An error message is displayed if there are no webpages stored in the forward history, and the
current site is returned (str). Otherwise, the next website is retrieved (and returned as a
string), and the two stacks are updated as appropriate. The inputs for this function are the
current website (str), a reference to the Stack holding the webpage addresses to go back to,
and a reference to the Stack holding the webpage addresses to go forward to.Sample run:
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: 123
Invalid entry.
Enter = to enter a URL, < to go back, > to go forward, q to quit: >
Cannot go forward.
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: <
Cannot go back.
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: =
URL: www.google.ca
Currently viewing www.google.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: <
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: >
Currently viewing www.google.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: =
URL: docs.python.org
Currently viewing docs.python.org
Enter = to enter a URL, < to go back, > to go forward, q to quit: <
Currently viewing www.google.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: <
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: =
URL: www.beartracks.ualberta.ca
Currently viewing www.beartracks.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: >
Cannot go forward.
Currently viewing www.beartracks.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: <
Currently viewing www.cs.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: >
Currently viewing www.beartracks.ualberta.ca
Enter = to enter a URL, < to go back, > to go forward, q to quit: q
Browser closing…goodbye.Worksheet to Match Sample Run:
The first 3 steps are already done to get you started…
Step 1: Web browser opened; home page displayed
current
back forward
Cannot go forward, cannot go back. No change to current.
Step 2: Go to new site; www.google.ca displayed
current
back forward
Step 3: Go back to previous page; www.cs.ualberta.ca displayed
current
back forward
Complete the rest (draw extra boxes for Stack elements if needed)…
Step 4: Go forward to next page; www.google.ca displayed
current
back forward
www.cs.ualberta.ca
www.google.ca
www.cs.ualberta.ca
www.google.ca
www.cs.ualberta.ca (empty)
(empty) (empty)
(empty) www.google.ca
Step 5: Go to new site; docs.python.org displayed
current
back forward
Step 6: Go back to previous page; www.google.ca displayed
current
back forward
Step 7: Go back to previous page; www.cs.ualberta.ca displayed
current
back forwardStep 8: Go to new site; www.beartracks.ualberta.ca displayed
current
back forward
Step 9: Try to go forward, but can’t; www.beartracks.ualberta.ca displayed
current
back forward
docs.python.org
www.google.ca
www.cs.ualberta.ca
www.beartracks.ualberta.ca
www.beartracks.ualberta.ca
Step 10: Go back to previous page; www.cs.ualberta.ca displayed
current
back forwardStep 11: Go forward to next page; www.beartracks.ualberta.ca displayed
current
back forward
www.cs.ualberta.ca
www.beartracks.ualberta.ca