FIT1008 Introduction to Computer Science (FIT2085: for Engineers) Interview Prac 2 – Weeks 8 and 9
Semester 2, 2019
Learning objectives of this practical session
To be able to implement and use basic container classes in Python.
For this prac, you are required to:
• Create a new file/module for each task named task[num].py (that is, task1.py, task2.py, and so on).
• Provide documentation for each basic piece of functionality in your code (see below)
• The documentation needs to include (in the docstring):
– pre and post conditions
– Big O best and worst time complexity – information on any parameters used
• Provide testing for those functions/methods explicitly specified in the questions. The testing needs to include:
– a function to test it
– a comment indicating the purpose of each test
– at least two test cases per test function. These cases need to show that your
functions/methods can handle both valid and invalid inputs.
– assertions for testing the cases, rather than just printing out messages
Your tests should be added to the corresponding test_task[num].py file provided
• VERY IMPORTANT: The code cannot use any of the Python list or string functionality that would make the job trivial (so no slicing, append, prepend, str, len, eq for lists,
and __contains__ for strings, etc). However, you may:
– Track the capacity of your internal array using len (as we have in the ListADT
– Use str, eq, etc. on members of the list (e.g. when implementing __str__ or __eq__).
– Use string methods (e.g. startswith) for command processing (determining that “read small.txt” is a read command with argument “small.txt”).
– Use any functions/methods you wish in your testing code.
To get you started, you will find some supporting files on Moodle under Week 8:
• ListADT.py, Editor.py: Skeleton files for the ListADT and Editor classes, which you may use as starting points.
• test_prac2.py, test_task[num].py, test_common.py: Testing harnesses for each of the tasks.
• Additional text files used by the testing harness.
Your code will be tested with an extended version of this harness, so make sure your code can run inside the harness. To make the testing work, we have made assumptions about the naming of instance variables, so please follow the naming in the skeletons.
The harness you have been provided is not exhaustive, so don’t rely on it to ensure correctness! Remember to add your own tests, as well.
Important: Checkpoint for the end of Week 8
To reach the check point you must complete Tasks 1, 2 and 3. Please remember that reaching the checkpoint is a hurdle and you will get a 0 if you do not reach it (read the pracGuide document for more details).
Task 1 [9 marks]
Consider an array-based implementation of a List ADT, as seen in the lectures. In this task you will need to re-implement this array-based List ADT using a class ListADT that contains the methods included below. The supplied ListADT.py contains a skeleton of the ListADT class you need to implement with some (glaringly uncommented) methods (like is empty(self), is full(self)). We have also provided test_task1.py, which contains code useful to test all ListADT class methods (those already given in the skeleton and those you need to define).
To complete this task, you need to add comments to the provided ListADT methods, and implement the following ones (properly commented) inside the ListADT class (note that the testing for these methods is already provided in test_task1.py):
• • •
init (self, size): Initialises the appropriate instance variables (have a look at ListADT.py). Please define it in such a way you allow users to provide the size upon creation and, if they do not provide it, you give a reasonably big default.
str (self): Returns a string representation of the list (returns the empty string if the list is empty). Structure the string so that there is one item per line. Recall that this method is the one called by str(self)
len (self): Returns the length of the list (that is, the number of elements in the list; 0 if empty). Recall that this method is called from Python by calling len(self)
getitem (self, index): Returns the item at index in the list, if index is non-negative. If it is negative, it will return the last item if index is −1, the second-to last if index is −2, and so on up to minus the length of the list, which returns the first item. The function raises an IndexError if index is out of the range -len(self) to len(self)-1. Recall that this method is called from Python by calling y = self[index]
setitem (self, index, item): Sets the value at index in the list to be item. The index can be negative, behaving as described above. Raises an IndexError if index is out of the range -len(self) to len(self)-1. Recall that this method is called from Python by calling self[index] = value
eq (self, other): Returns True if other is a ListADT which is equivalent to this list (that is, they have exactly the same elements in the same order – but possibly different capacities), and False for any other object. Recall that this method is called from Python by calling self == other. insert(self, index, item): Inserts item into self at position index. For example, insert(self, 0, item) inserts the item in the first position, shuffling back everything else. As before, the index can be negative. IndexError if index is out of the range from -len(self)-1 to len(self). Hint: Note the range here is slightly different to __setitem__. Be especially careful with negative in- dices: self.insert(len(self), item) and self.insert(-1, item) should both have the effect of appending item to the end of the list (which differs slightly from insert for native lists). delete(self, index): Returns the item at index from the list and deletes it, shuffling all items after it towards the start of the list. The index can be negative, behaving as described above. Raises an IndexError if index is out of the range from -len(self) to len(self)-1.
Task 2 [2 marks]
Modify your list implementation so that the size of the underlying array can change. That is, if the list
becomes full, the size of the array is increased to be 1.9 (rounding up) times the current size. Likewise, the
size of the array should decrease by half if the length of the list is less than 1 of the size of the array (i.e., 4
if the elements in the list occupy less than 1/4 of the available space). But make sure the size of the array is never less than 40 (you need to check this upon creation for the user’s value and your default value, and also whenever the size decreases). Also, when resizing the list (both increasing and decreasing), make sure you retain the contents of the list, that is, the elements of the old list are copied into the new one in the same order as they were before.
You are allowed to copy and paste any methods from Task 1 that are not affected by the resizing.
Important: make sure you do update the tests of the methods that are affected by the capacity with at least two new tests per such method.
Background for the rest of the prac
The editor ed was one of the first editors written for UNIX. In this prac we will use the list class you implemented in the previous tasks to implement a version of a line-oriented text editor based on ed. The text editor ed is very similar to the common UNIX text editors vi and vim (it is in fact their predecessor). 1 However, our commands will be different from the ed commands.
To implement a simple line-oriented text editor, the idea is as follows. Suppose a text file called small.txt contains the following three lines of text:
not to utter
where the string “Yossarian decided” is considered to be in line 1, “not to utter” is considered to be in line 2, etc. We want to store every line in the file in a data type that allows users to easily manipulate (delete/add/print) any line by simply providing the line number they want to modify. This means we should use a list data type (as opposed to a stack or a queue).
Documentation for Python string functions can be found at https://docs.python.org/3/library/ stdtypes.html
To properly test your code you should use very large text files. There are plenty of online resources that can be used to download files such as e-books. As example, here is one: http://www.textfiles.com/etext/.
Task 3 [3 marks]
Implement a function read text file(name) that takes the name of a text file as input and reads each line of text into a string, returning a list of strings associated to the file (i.e., the function converts the text in the file into a list of strings). Note that you should use the List ADT implemented in Task 2 for this, that is, the list of strings returned by the function should be an object of the ListADT class implemented in Task 2. Test the function using the small.txt provided above in the Background, and a similarly small file created by you. Please make sure you close the file after reading it (and verify this by checking the closed attribute of the file object). For information on how to read data from a file, please read the tutorial found at https://docs.python.org/3/tutorial/inputoutput.html#reading-and-writing-files.
Important: you need to test the read text file(name) function with a small file containing a few (3+) lines.
Task 4 [6 marks]
(You should reach this point during week 8)
Write a class Editor that allows a user to perform the operations shown below using an ed-style com- mand prompt. The class should have an instance variable called text lines containing a list of strings (corresponding to the list of text lines being edited), where the list is an object of the ListADT class implemented in Task 2. Please implement your interface so that the user enters commands as text (e.g. print 8), as opposed to, say, a numeric menu. Also, make sure each command (except quit, which is trivial) is implemented by a method with the command’s name which takes as argument the string read from input (after taking the command part off the string). For example, command print
1If you want to find out more about ed, you can read the man page (by typing: man ed into a linux or MacOS X terminal), or visit, for example: http://roguelife.org/ ̃fujita/COOKIES/HISTORY/V6/ed.1.html or https://youtu.be/BNYpmLH6IjQ (YouTube tutorial)
num will be implemented by print num(self,rest string) and delete num will be implemented by delete num(self,rest string). For the insert num command we will instead call method insert_num_strings(self,number,list_of_strings), which has a second argument the integer repre- senting num and as third argument the list of strings the user gave you to insert, if any. This will help you (and us) test your command implementations. You must add an appropriate test function for the delete_num and insert_num_strings methods, with a few meaningful test cases.
Hint: An example of user interaction, corresponding method calls, and program responses is shown at the end of the prac sheet.
read filename: which basically calls the function defined in Task 3.
print num: which prints the line of text at position num, and if no num is given prints all the lines. Note that what the user refers to as line 1 is what text lines stores in position 0. Thus, you need to convert the numbers given by the user to the indices used by your array.
delete num: which deletes the line of text at position num, and deletes all the lines if no num is given. Same issue as above for nums.
insert num: which inserts immediately from position num onwards, any line of text subsequently written by the user in the order entered, until the user types a full stop on a line by itself. Note that full stops that are not on a line by themselves should not terminate the input. Do not store the full stop in the list. For example, if the user types insert 3 and then types lines
I am well. How are you? .
then string ”Hello” will be added to text lines at position 2 (remember it starts at 0, so Hello becomes the third line in the list), and string ”I am well. How are you?” will be added at position 3, with all items in the text lines that used to appear from 2 onwards, now appearing from 4 onwards.
quit: which quits the program.
Important: All errors should be caught and a question mark, ?, should be printed when an error occurs, so that the user is given the opportunity to provide correct input data. Negative number lines are possible and should be handled following the convention described for the original list implementation. For example, num = -1 refers to the last line of the text, and inserting from -1 should append. Also note that 0 is not a valid line number, and should be treated as an error.
Tip: When you are testing your code, it is handy to have a source of reasonably large text files that you can use as test data. There is a huge repository of public domain text files at Project Gutenberg’s websites. The Australian Project Gutenberg repository is at http://gutenberg.net.au. You are encouraged to download a couple of ebooks from here and use them to make sure your code can deal with large files. Be sure to download plain-text format, though – your program is not expected to deal with other formats.
Task 5 [2 marks]
Adds the following new command to the Editor interface:
search string: which first calls a method search_string(self, string) that returns a list with all the line numbers (if any) in which string appears, and then prints the elements of the list.
Makesureyoutestthesearch_string(self, string)method,andcatchallerrorsprintingaquestion mark when an error occurs, as before.
Important: Make sure you do not use inbuilt string methods for doing the searching (such as the contains method). You must implement the search yourself. You may chose to use an iterator.
Task 6 [3 marks]
The sequence of actions made in a text editor can be stored during execution to facilitate undoing actions. When a user chooses to undo an operation the most recent successful delete or insert action should have
its changes reversed This indicates that a last in, first out data structure would be suitable for storing the actions.
Implement a Stack ADT of your choosing (using linked nodes or an array) and use it to implement in your editor the undo feature. Add this to your editor using the command undo.
Important: to reduce the time taken for this prac, you do not need to comment or write test cases for your stack ADT (and can copy and paste the one from the lectures). Also, think about what you need to store in the Stack in order to be able to undo commands these two commands, and try not to store more than you need, or you will run out of memory… Note: Inserting zero lines (e.g. ed.insert_num_strings(“1”,)) is still a successful action.
Example: Editor interaction
Below we give a brief example of the expected behaviour of your Editor: the corresponding method call, and the Editor’s response.
text the user types, the
not to utter
Note: Remember that the second argument to insert_num_strings should be a ListADT.
some input. .
ed = Editor()
ed.insert_num_strings(“1”, [“some text.”])
ed.insert_num_strings(“2”, [“this is”,
” some input.”])
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