Question

Use python 3 to solve:

Every web browser has both a back and a forward button which allows the user to navigate to previously seen web pages.

Your task is to implement this functionality using two stacks:

·        The input to your program will be a sequence of commands. The ">" and "<" tokens will be used to indicate forward and back, respectively. All other input will be web addresses (you can assume they will always be valid).

·        You will not actually have to do any web navigation and you will simply be keeping track of the current page by outputting the given address.

·        There are a few situations you should take care to handle.

o   First, if a user enters "<" or ">" without there being a previous or next page to go to, you should output a corresponding error message but allow the user to continue browsing.

o   Secondly, if a user enters "<" and then a new web address, the user's previous browsing history in the ">" direction should be erased.

o   For example, if a user is on "www.google.com", enters "<", and then enters "www.yahoo.com", the history of visiting "www.google.com" should be lost. This means that if the user then enters ">", this should generate an error message and not "www.google.com". This behaviour can also be seen below in the sample data. You can also test an actual web browser (specifically Firefox or Chrome) if you feel there is other ambiguity. Also, keep in mind that navigation should start with a home page which we will set as "www.cs.ualberta.ca".

You may choose how your program receives the input.

Sample Input

www.google.com
<
<
>
www.yahoo.com
www.ualberta.ca
www.beartracks.ualberta.ca
>
<
<
>
<
www.microsoft.com
>
<
<

Sample Output

Starting at Home Page: www.cs.ualberta.ca
Current page: www.google.com
Current page: www.cs.ualberta.ca
< is an invalid action
Current page: www.google.com
Current page: www.yahoo.com
Current Page: www.ualberta.ca
Current Page: www.beartracks.ualberta.ca
> is an invalid action
Current Page: www.ualberta.ca
Current page: www.yahoo.com
Current Page: www.ualberta.ca
Current page: www.yahoo.com
Current page: www.microsoft.com
> is an invalid action
Current page: www.yahoo.com
Current Page: www.google.com

Answer 1

back_stack=['www.cs.ualberta.ca']
forward_stack=[]

def main():
   print ('Starting at Home Page: www.cs.ualberta.ca')

   while True:
       input_str = input('')
       if(input_str=='<'):
           if(len(back_stack)<=1):
               print ('< is an invalid action')
           else:
               url = back_stack.pop()
               forward_stack.append(url)
               print ('Current Page: '+back_stack[-1])

       elif input_str=='>':
           if(len(forward_stack)==0):
               print ('> is an invalid action')
           else:
               url = forward_stack.pop()
               back_stack.append(url)
               print ('Current Page: '+back_stack[-1])  
       else:
           back_stack.append(input_str)
           forward_stack=[]
           print ('Current Page: '+input_str)

if __name__ == '__main__':
   main()