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5 Sep
2019

CSP1150/CSP5110: Programming Principles | Good Grade Guarantee!

CSP1150/CSP5110 Workshop 6 Page 1CSP1150/CSP5110: Programming PrinciplesWorkshop 6: Files and Exception HandlingTask 1 – Concept Revision in the Python ShellLet’s revise the concepts from this module by typing some simple statements into the Python Shell.Feel free to deviate from the workshop and experiment!Before typing each of these statements, try to work out which exception they will raise.If you are wrong, take a few moments to read the error message and understand why.1. potato2. 5/03. int(‘0.999’)4. round(3.14159, ‘2’)5. open(‘exam_answers.txt’, ‘r’)Task 2 – Unclosed FilesThis task simply demonstrates the importance of closing a file once you have finished using it.Create a new file in the Python Editor and copy-paste the following code into it:Save and run the program, then go to the folder you saved the program into and open “example.txt”in Notepad. Write something into the file and try to save it. You should see an error:Since the program didn’t close the file, it is still “locked” and can’t be written to by other programs.Cancel the saving and close “example.txt”. Add the following line to the program and run it again:Open “example.txt” again. Is the file still empty? If so, it’s because the data that the program wrotewas written into a “buffer” in memory (which is faster), rather than writing it to the file on the diskitself. The data in the buffer is only written to the file on the disk when you close the file. Add a lineto close the file, and now the program should work as intended:
f = open(‘example.txt’, ‘w’)
Python
f = open(‘example.txt’, ‘w’)f.write(‘Write this to the file.’)Pythonf = open(‘example.txt’, ‘w’)f.write(‘Write this to the file.’)f.close()PythonCSP1150/CSP5110 Workshop 6 Page 2Task 3 – “To Do List” Case StudyWe’re going to put together a slightly bigger than usual program in the next few tasks – don’t worry,we’ll break it down into separate functions and tackle them one at a time. Create a new file in thePython Editor and copy-paste the following code into it (or download the file from Blackboard):(This code is the same as the code in the “Creating a Module” section of Lecture 4)Save this file (filename not important) in the same folder as convert.py, then run the code to test it.Once you’ve made sure that it is working, enhance convert.py to make it more useful in these ways:1. Add a second parameter named “rounding” to each of the functions. The parameter shouldhave a default value of 2, and should be used to round the result of the function.Test your work by calling the functions in your second file: convert.cm2inches(7) should return a value of 2.75 convert.m2yards(10, 1) should return a value of 10.92. Add a third parameter named “addUnit” to each of the functions. The parameter shouldhave a default of False. If True is specified for the parameter when the function is called,the function should return a string of the result with the unit of measurement at the end,e.g. ‘2.75 inches’ instead of 2.75. Test your work by calling the functions: convert.m2feet(5, 0, True) should return 16.0 feet convert.km2miles(2, addUnit=True) should return 1.24 miles# — getList(filename), returns a list of strings —# (open/create file and return lines of text as a list of strings)def getList(filename):data = [] # this line is a placeholderreturn data# — showList(data), returns nothing —# (receive list of strings and display them, or “nothing in list” message)def showList(data):return# — addToList(filename, data), returns a list of strings —# (prompt for an item to add to the list of strings and append to the file)def addToList(filename, data):return data# — deleteFromList(filename, data), returns a list of strings —# (prompt for item number to delete from the list of strings and write list to the file)def deleteFromList(filename, data):return data# — main part of program —FILENAME = ‘list.txt’ # define the filename used to store the listlineList = getList(FILENAME) # call the getList function to read the file into a listwhile True: # this endless loop displays the list and prompts the user for a commandshowList(lineList) # call showList to show the current content of the list
# show the instructions for the possible commands –print(‘nType “a” to add an item.’)
[a]dd, [d]elete, e[x]it
if len(lineList) > 0: # only show the delete instruction if the list has itemsprint(‘Type “d” to delete an item.’)print(‘Type “x” to exit.’)command = input(‘> ‘) # prompt for a command# if “a”, calladdToList to prompt for item and add to listif command == ‘a’:lineList = addToList(FILENAME, lineList)# if “d”, call deleteFromList to prompt for item number and delete from listelif command == ‘d’ and len(lineList) > 0:lineList = deleteFromList(FILENAME, lineList)elif command == ‘x’: # if “x”, break out of the loop to end the programprint(‘Goodbye!’)breakelse: # if anything else, show an error messageprint(‘Invalid command.n’)PythonCSP1150/CSP5110 Workshop 6 Page 3Save the file and run it. At the moment, it runs but does nothing useful. Look through the code andread the following notes about what the program will do once it is complete:

The program will become a “To Do List” application which uses a text file to store its data.It will allow the user to see the items on the list, add items to it and delete items from it.

When the program starts, it will call a function to read the text file and store the lines of thefile into a “list” variable, which we covered in Module 3. If the text file does not exist, it willbe created (and since it has no text in it yet, the list will be empty).

This is the only time the text file needs to be read – after that, the program will be
interacting with the list variable, writing data to the text file when needed.

The program then enters an endless loop which calls a function that displays the list itemsand shows instructions regarding how to add and delete list items, or exit the program.
 The program then waits for the user’s input. If they choose to add or delete, functions areused to do this – modifying the list variable and writing the data to the text file. The loopallows the user to continue adding and deleting items until they choose to exit the program.Below is a usage example of the program (red lines indicate iterations of the program’s main loop).The user begins with an empty list. They enter “a” toadd an item and then enter “Sleep all day”.When the list is shown on the next loop, it now has anitem to show. The user enters “a” again then enters“Work on my assignments”.When the list is shown on the next loop, it now hastwo items to show. The user enters “d” then enters“1” to delete item number 1 from the list.When the list is shown on the next loop, it now hasonly one item show. The user enters “x” to exit theprogram, breaking out of the endless loop.
Your To Do List is empty.Type “a” to add an item.Type “x” to exit.> aAdd: Sleep all dayItem added to list.
To Do List:1) Sleep all dayType “a” to add an item.Type “d” to delete an item.Type “x” to exit.> aAdd: Work on my assignmentsItem added to list.
To Do List:1) Sleep all day2) Work on my assignmentsType “a” to add an item.Type “d” to delete an item.Type “x” to exit.> dItem number to delete: 1Item deleted from list.
To Do List:1) Work on my assignmentsType “a” to add an item.Type “d” to delete an item.Type “x” to exit.> xGoodbye!
CSP1150/CSP5110 Workshop 6 Page 4The “main” part of the program is already complete – all that is left to do is write the code for thefour functions: getList, showList, addToList and deleteFromList. The following tasks describe exactlywhat those functions need to do, and it is up to you to write the pseudocode and actual code.Remember to spend some time looking over the code provided and reading the details on theprevious page. Having a good understanding of a situation and what is required is extremely helpful.Task 4 – Writing the getList function (“To Do List” Case Study)The first function we are going to write is called getList. Here is the placeholder code we have so far:The getList function requires one parameter: The filename of the text file used to store the list data.The getList function must return a list of strings (the lines of text from the file).The purpose of the getList function is to open the text file use to store the list data, read all of thelines from the file into a list, and return that list. If the text file does not exist, the function mustcreate it. This will result in it returning an empty list, since there will be no lines of data to read.Currently, the function assigns an empty list to the data variable – this is simply a placeholder tomake sure that the code you start out with does not crash. You will need to edit this line.Write pseudocode to plan your code, and then write the code of the getList function. Hints:

First try to open the file in read (“r”) mode, and if a FileNotFoundError exception is raised,open the file in Write/Read (“w+”) mode (which will create an empty file and allow reading).
 Use the “readlines()” method on the file variable to read all lines of the file into a list. Remember to close the file once you have read it.Be sure to test your code thoroughly:

Add “print(lineList)” after the “lineList = getList(FILENAME)” line in themain part of the program so that you can see exactly what your function returned.

Remember to remove this once you’ve written the code for the showList function!

Since we haven’t written the function to add items to the list, the file will be empty and yourfunction will return an empty list. Feel free to type a few lines into the file using Notepad.
# — getList(filename), returns a list of strings —# (open/create file and return lines of text as a list of strings)def getList(filename):data = [] # this line is a placeholderreturn dataPythonCSP1150/CSP5110 Workshop 6 Page 5Task 5 – Writing the showList function (“To Do List” Case Study)Now that we can read the data, let’s write the showList. Here is the placeholder code so far:The showList function requires one parameter: A list of strings containing the To Do List items.The showList function does not return anything.The purpose of the showList function is to display the items on the To Do List in a user-friendly way.If the list is empty, it should show a message saying that there are no items in the list. If there is atleast one item in the list, it should show the items with a number before each one. See the usageexample on Page 3 for examples of how the output of the showList function should look.Currently, the function contains a return statement – this is simply a placeholder to make sure thatthe code you start out with does not crash. You can remove this line once the code is written.Write pseudocode to plan your code, and then write the code of the showList function. Hints:

Determine whether the list is empty and show the appropriate output by using an “if”statement and built-in “len()” function.

If the list is not empty, use a “for” loop to loop through the items in the list.

There is an example of this in Lecture 3, Slide 37.Create a variable before the loop (and increment it inside the loop) for the itemnumber. Print this number in front of each item as seen in the examples on page 3.It is more user-friendly to start the item number at 1, but remember – the index ofthe items in the list starts at 0, so your item number will be 1 higher than its index!


Use the “rstrip()” method to remove line breaks from the end of the items in the list asyou loop through it and print the items.

There is an example of this in Lecture 6, Slide 27.
Be sure to test your code thoroughly: Make sure that if the file is empty, the “Your To Do List is empty.” message is shown. Add some lines of text to the file and make sure that your program displays them correctly. Spend some time making the output to look nice (see the usage example on Page 3).# — showList(data), returns nothing —# (receive list of strings and display them, or “nothing in list”…def showList(data):returnPythonCSP1150/CSP5110 Workshop 6 Page 6Task 6 – Writing the addToList function (“To Do List” Case Study)Now let’s write the addToList function so that we can add items. Here is the placeholder code so far:The addToList function requires two parameters: The filename and the list of strings containing theTo Do List items. The addToList function must return the list of strings (with the new item added).The purpose of the addToList function is to prompt the user for an item and append it to the list ofitems, as well as appending the item to the text file.Currently, the function contains a return statement – this will remain at the end of the addToListfunction’s code, since the function must return the modified list (with the new item appended).Write pseudocode to plan your code, and then write the code of the addToList function. Hints:

Once you have used the built-in “input()” function to prompt for the item to add,concatenate a “n” to the end of the item to make sure it will have a line break in the file.
 Use the “append()” method to add the item to the list (see Lecture 3, Slide 11). Open the text file in Append (“a”) mode and write the item to it, then close the file. Print a message confirming that the item has been added, and then return the list.Be sure to test your code thoroughly: Add items to the list and ensure that they are shown correctly by the showList function. Open the text file in Notepad to confirm the items are correctly saved in the file.

Add an item consisting of spaces or an empty string (by just pressing enter at the prompt).

It is up to you whether you wish to add code to prevent this from being saved.
# — addToList(filename, data), returns a list of strings —# (prompt for an item to add to the list of strings and append to…def addToList(filename, data):return dataPythonCSP1150/CSP5110 Workshop 6 Page 7Task 7 – Writing the deleteFromList function (“To Do List” Case Study)The final function we need to write is deleteFromList. Here is the placeholder code so far:The deleteFromList function requires two parameters: The filename and the list of stringscontaining the To Do List items. The deleteFromList function must return the list of strings (with thespecified item deleted).The purpose of the deleteFromList function is to prompt the user for an item number and delete itfrom the list of items, as well as deleting the item from the text file. Deleting the item from the listinvolves deleting the appropriate index from the list, and deleting the item from the text file involveserasing the content of the file and rewriting it using the list.The function must first prompt the user for an item number to delete (the showList function showsthese numbers at the start of each item). This must be converted to an integer, and then used todelete the appropriate index from the list.Currently, the function contains a return statement – this will remain at the end of the function(to return the modified list) as well as being in the exception handlers (to return the unchanged list).Write pseudocode to plan your code, and then write the code of the deleteFromList function. Hints:

Converting the input to an integer can cause a ValueError, and trying to reference an indexnumber in the list can cause an IndexError if there is no item with that index number.

Place these statements into a “try” block, and handle both of the exceptions. The
exception handlers simply need to print an error message and return the list(bringing the user back to the main part of the program without changing anything).

Remember that the item numbers start at 1 but the index of the list starts at 0.

Subtract 1 from the number the user enters to get the appropriate index number.Use a “del” statement to remove an item from a list. See Lecture 3, Slide 11.

Open the text file in Write (“w”) mode (which will erase the current data) and use the“writelines()” method on the file variable to write the list to it, then close the file.
 Print a message confirming that the item has been deleted, and then return the list.That’s it – the application is complete. You can now view, add and delete items from the list, and atext file is used to store the list so that it is remembered. Be sure to test your code thoroughly.That’s all for this workshop. If you haven’t completed the workshop or readings,find time to do so before next week’s class.# — deleteFromList(filename, data), returns a list of strings —
# (prompt for item number to delete from the list of strings anddef deleteFromList(filename, data):

return dataPython

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