Assignment
Goal
In this project you will implement a simple text editor. The editor will be able to read from a file, modify it, then allow you to save your edits.
Program Features
The editor is interactive and command driven. When you start the program, first a welcome message is printed, then the ">" prompt is printed. Commands for the text editor are input at the ">" prompt.
Here is a sample input file:
Abbott: You throw the ball to first base. Costello: Then who gets it?
Abbott: Naturally. Costello: Naturally.
And execution:
Welcome to the CS1064 Text Editor.
>open example.txt
Abbott: You throw the ball to first base. |0: 41|
^
Costello: Then who gets it? | 42: 69|
Abbott: Naturally. | 70: 88|
Costello: Naturally. | 89:109|
>move 45
Abbott: You throw the ball to first base. |0: 41|
Costello: Then who gets it? |42: 69|
^
Abbott: Naturally. | 70: 88|
Costello: Naturally. | 89:109|
>move 49
Abbott: You throw the ball to first base. |0: 41|
Costello: Then who gets it? |42: 69|
^
Abbott: Naturally. | 70: 88|
Costello: Naturally. | 89:109|
>erase
Abbott: You throw the ball to first base. |0: 41|
Costell: Then who gets it? | 42: 68|
^
Abbott: Naturally. | 69: 87|
Costello: Naturally. | 88:108|
>insert O
Abbott: You throw the ball to first base. |0: 41|
CostellO: Then who gets it? | 42: 69|
^
Abbott: Naturally. | 70: 88|
Costello: Naturally. | 89:109|
>quit
Would you like to save before you exit? (Y/N): Y Enter filename: example-edited.txt
The example execution above reads example.txt from the disk then prints a formatted version of the file including the "cursor" (the ^). The ^ (the cursor) is the place where edits occur in your file. After printing the file and cursor, the first two commands move the cursor: the first command moves it to position 45, the next command moves it to position 49. We then start editing the file, we erase the character at position 49, and then we insert a new character "O". Finally we try to quit, and we are prompted to save the file.
As noted above, the way the file is printed by our program is different than the way the file is stored on disk. The formatted output contains additional information on each line, plus the cursor (^). The | 0: 41| at the end of the line are the positions (indexes) in the file of the first and last characters for that line. So the following example shows the first line of the file, the cursor is initially at position 0 in the file, the first character on this line is at position 0 (the A) and the last character on this line (the '\n') is at position 41.
Abbott: You throw the ball to first base. |0: 41|
^
Note that the file contents are printed along with the "cursor" (the ^) after the file is opened and also after each command.
For simplicity, commands and the parameters for those commands will be separated by whitespace. This makes it easier to read the commands using the techniques you've already seen so far.
Representing the File
Computer programs rarely modify file data "in place". Instead they load the data from a file into the program (into a list, string, etc.), let you to modify the data, and only when you explicitly save the file is something written to disk. So the file data will "live" entirely inside of variables within your program. The file should be represented with a string (the actual file data) and an integer (the cursor).
Your program's representation of the file (the string and integer) is what will be modified by each command entered into the program. For example, the open command loads the file data from the disk and stores it into the string. Every time you edit the file with insert or erase you are changing the string. When you move the cursor you will change the integer value. In addition to the string and integer, you should also keep track of: the file name (string), whether or not the file has been opened (boolean) and whether or not the file has been modified (boolean).
To represent the file, you should store and read it all as one string:
f = open(file_name, "r")
# read all of the file data as string data = f.read()
After the read function, data contains all of the text data, formatting information ("\n") and all. So for the example above data would contain:
# Modify me!
"Abbott: You throw the ball to first base.\n Costello: Then who gets it?\nAbbott: Naturally.\nCostello: Naturally."
The Commands
Your program must implement the following six commands. The first part of each command is the name of the command itself. If the command takes any parameters, they are listed in after the command name. Parameters in square brackets are optional.
open filename
Opens filename and reads the file data into your program. The filename should be the full name with the extension (for example, example.txt). The name cannot have any spaces.
Initially a file is unmodified, so entering quit right after opening the file should not result in a "save" prompt. When opening the file, the cursor should start at 0.
saveas filename
Saves the current file data (the string) into the specified filename. After this command the file is no longer modified. The filename should be the full name with the extension (for example, test.txt). The name cannot have any spaces.
move position
Takes one parameter that must be an integer, i.e. 'move 86'. This would move the cursor to index 86 in the file. If the index is out of range (bigger than the file, negative, or not an integer) an error message is printed, and the original cursor (before the move command) value should be retained.
insert string
Inserts a string in the file at the cursor position. Takes one parameter i.e. 'insert hello world' would insert the string 'hello world' at the cursor position. String slicing may be of use here.
erase [number]
If no parameter is specified, erase deletes a single character at the cursor location, if a parameter is specified, it should be the number of characters to delete. So 'erase 1' is equivalent to just 'erase', while 'erase 3' would delete up to 3 characters starting at the cursor, erase will delete less than 3 if the erasures would go beyond the end of the string. If something other than a number is provided an error message should be printed and the string should be unchanged.
quit
"Terminates" the program. If the file is modified and hasn't been saved, the user is prompted to the save the changes to another file. No further commands should be read after this and the program should exit normally (i.e. reach the end of your program, don't use exit() etc.).
Input and Output Files
The input file will simply contain characters and the file may be arbitrarily long. The file may contain multiple lines but there won't be any further complex formatting. The output file would work the same way. When you saveas, you want to write whatever the current file data (which is represented by a string) is into a new file, preserving newlines.
Tricky Details
Here are some additional details that you need to aware of to get full credit. If a command is entered that is not one of the six listed above print an error message:
Costello: Naturally. |419:439|
>banjo
That's not an editor command.
Abbott: You throw the ball to first base. |0: 41|
^
Opening a file.
If a file hasn't been opened, no commands but "open" and "quit" are allowed, even nonsense commands result in an error message about the file not being open.
Welcome to the CS2304 Text Editor.
>banjo
No file is currently open.
To open a file enter: open filename
>open test.txt
Abbott: You throw the ball to first base. |0: 41|
^
If the file can't be opened print an error message, then print a prompt
Welcome to the CS2304 Text Editor.
>open doesnotexist.txt
Could not open file: doesnotexist.txt. Please try a different file.
> quit
Move
If an invalid cursor is entered (a negative number, one bigger than the file size, or non integer)
move 1000000, an error message should be printed, the cursor should remain unchanged, and then a prompt should be printed.
Costello: Naturally. | 89:109|
>move 100000
Invalid cursor.
Abbott: You throw the ball to first base. |0: 41|
^
Quit
If the file has been modified but not saved and you try to quit then the editor will prompt you to save first. If the file is unmodified or has already been saved the editor should quit quietly.
>erase
bbott: You throw the ball to first base. |0: 40|
^
Costello: Then who gets it? | 41: 68|
Abbott: Naturally. | 69: 87|
Costello: Naturally. | 88:108|
>quit
Would you like to save before you exit? (Y/N): Y Enter filename: sample.txt
Plan of Attack
I've provided you with a starting Python file on the course website, this file contains a function (print_file) that will print the file and cursor as shown above. After that, your program should contain a loop that reads commands until it sees the quit command. Commands can either have a parameter or not, so you'll have to handle that possibility. Use a sequence of if/elif statements to check which of the commands was entered and perform the correct action. If the command is quit, then break out of the loop so that you stop processing input.
Don't try to write the whole program at once. Consider implementing the quit command first, just to make sure that your program exits the loop correctly when you type that. Then move on to open and saveas, and see if you can load a file and display it on the screen properly and then write it back to a different file. After that, try implementing the remaining commands one at a time.
What to Submit
For this assignment you should submit your p3.py file. Your file must be named p3.py.
This assignment will be graded automatically. Test your programs thoroughly before submitting them. Make sure that your programs produce correct results for every logically valid test case you can think of. Do not waste submissions on untested code, or on code that does not compile with the supplied code from the course website.
Web--CAT will assign a score based on runtime testing of your submission; your best score will be counted; the TAs will later verify that your best submission meets the stated restrictions, and assess penalties if not.