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ECS 32B: Programming Assignment #2
Summer Session #2 2020
Contents
1 Changelog 1
2 General Submission Details 1
3 Grading Breakdown 2
4 Gradescope Autograder Details 2
4.1 Released Test Cases vs. Hidden Test Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4.2 Released Test Cases’ Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4.3 Changing Gradescope Active Submission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
5 Ayayron➋: An Editor That No One in Their Right Mind Would Ever Use 3
5.1 Part #1 - Loading Keys: load_keys() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5.2 Part #2 - Reading the Edited File’s Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.3 Part #3 - Printing the Current File Contents: print_current() . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.4 Part #4 - Saving the Current File’s Contents: save() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.5 Part #5 - Moving the Cursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.5.1 Part #5.1 - Leftward/Rightward Movement: move_left() and move_right() . . . . . . . . . . . . . . . . 8
5.5.2 Part #5.2 - Upward/Downward Movement and Scrolling the Window: move_up() and move_down() . . . 11
5.6 Part #6 - Insertion: insert() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.7 Part #7 - Edit History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.7.1 Part #7.1 - Undoing an Insertion: undo() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.7.2 Part #7.2 - Redoing an Insertion: redo() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1 Changelog
You should always refer to the latest version of this document.
❼ v.1: Initial version.
❼ v.2: Made it clearer in part #3 the number of whitespace in certain portions (see red text).
❼ v.3: Added autograder details.
❼ v.4: Removed minor error in the example output at top of page 6 where I erroneously had line 5 printed twice.
2 General Submission Details
Partnering on this assignment is prohibited. If you have not already, you should read the section on
academic misconduct in the syllabus.
This assignment is due the night of Saturday, August 29. Gradescope will say 12:30 AM on Sunday, August 30, due to
the “grace period” (as described in the syllabus). Rely on the grace period for extra time at your own risk.
Some students tend to email me very close to the deadline. This is a bad idea. There is no guarantee that I will check
my email right before the deadline.
✯This content is protected and may not be shared, uploaded, or distributed.
1
3 Grading Breakdown
The assignment is out of 160 points. Here is a tentative breakdown of the worth of each part.
❼ Part #1: 15
❼ Part #2: 20
❼ Part #3: 25
❼ Part #4: 15
❼ Part #5: 30
– Part #5.1: 5
– Part #5.2: 25
❼ Part #6: 20
❼ Part #7: 35
– Part #7.1: 20
– Part #7.2: 15
4 Gradescope Autograder Details
More autograder details:
❼ Depending on how your code behaves on certain test cases (that it fails), the autograder may take a long time to run. In
a way, this is good in that it will incentivize you to avoid a cycle of making a tiny change, submitting to the autograder,
making another tiny change, etc. (You should be comfortable with testing your code by hand on your end.) That aside,
you should submit occassionally, and you should be very careful about working on your code during the grace period
(like many of you tend to do). As I said towards the start of this document, doing so is risky. Moreover, if you do a
submission late enough such that you don’t even see your score until after the deadline, you risk getting a zero (e.g. if
you messed something up that caused a syntax error) that you can’t change your active submission to avoid, since you
presumably won’t have known that that last submission had a syntax error.
❼ The autograder ignores trailing whitespace when comparing your output to the expected output.
❼ Sometimes (often?), when Gradescope shows you the differences between the expected output and your output, it will
be messy or not that helpful. Sometimes, that’s just the way it is. Other times, the messages could be useful if you
took a bit of time to read them closely and interpret them correctly. One thing to note is that when Gradescope is
displaying the output differences, it does it as if we want to change the expected output to your output (instead of the
other way around), so if things seem flipped, that may be why.
❼ When a test case in a later part fails, do keep in mind that it could always be due to an error in your code for an earlier
part. Don’t ask me how likely this is; that depends on your code.
4.1 Released Test Cases vs. Hidden Test Cases
You will not see the results of certain test cases until after the deadline; these are the hidden test cases, and Gradescope
will not tell you whether you have gotten them correct or not until after the deadline. One purpose of this is to promote the
idea that you should test if your own code works and not depend solely on the autograder to do it. Unfortunately, Gradescope
will display a dash as your score until after the deadline, but you can still tell if you have passed all of the visible test cases
if you see no test cases that are marked red for your submission.
4.2 Released Test Cases’ Inputs
See the document p2_visible_cases_inputs.pdf and the file test_editor.py that are on Canvas.
4.3 Changing Gradescope Active Submission
Once the deadline occurs, whatever submission is your active submission will be the one that dictates your final score.
By default, your active submission will be your latest submission. However, you can change which submission is your active
submission, which I talk about a small video that you can find on Canvas here. This video is from my ECS 32A course last
summer session, so the autograder referenced therein is for a different assignment. Note that due to the hidden test cases,
your score will show as a dash for all of the submissions, but you can still identify how many of the released test cases you
got correct for each submission.
2
5 Ayayron➋: An Editor That No One in Their Right Mind Would Ever Use
In this assignment, you will implement a text editor. The only actual editing that this editor will permit is insertion of a
phrase on a given line (based on where the cursor is) and the trimming of trailing whitespace. Other helpful operations will
include movement of the cursor, scrolling, and saving to a file.
Your entire submission will be in a file called editor.py. You can find a skeleton version of editor.py and a complete
implementation of the Stack implementation from lecture in stack.py on Canvas. You can change the parameter names of the
methods in editor.py, if you wish. You will only submit editor.py to the autograder; the autograder will use its own copy of
stack.py, meaning that you are not allowed to modify this file. So far, the code should just be able to tell you when you’ve
entered an invalid command and should allow you to quit.
1 >>> e = Editor (’ ignore ’, ’ ignore ’)
2 >>> e. run ()
3 Enter command : d
4
5 Enter command : a
6
7 Enter command : z
8 Invalid command .
9
10 Enter command : o
11 Invalid command .
12
13 Enter command : end
14 >>>
Although the parts below come in a certain order, you don’t have to do them in the exact order shown. Needless to say,
some parts have to be done (or at the very least, should be done) before others; for example, it wouldn’t make much sense to
implement the window scrolling before you implement the reading of the input file’s contents. I personally found part #5.2
to be the hardest part. In some autograder cases, I may try to reduce the dependency of certain parts on each other. For
example, even if you cannot quite get part #5.2 fully working, it will be possible to get some points for parts #6 and #7.
Although you understandably may not understand every single detail of these specifications on your first read through,
you may find it worthwhile to read the entire specifications before starting anyways.
I would recommmend skimming the list of string and list methods here and here. I am not prohibiting any of these on
this assignment. I personally found the string methods rstrip() and rjust() to be useful.
Do not forget to close any file that you open, once you are done using it. Or just use the with operator.
You will find that you’ll need to add members to the Editor class that are not explicitly stated in the direcitons; you can
figure these out as you go.
5.1 Part #1 - Loading Keys: load_keys()
As you can see in editor.py, there are default values of each of the key members, from self.down_key to self.redo_key. Each
such member contains the key that must be entered in order to do that operation; for example, to undo the last edit/insertion,
the user would need to press the key whose value is in self.undo_key.
The last parameter (or, you could say, third explicit parameter), settings_filename, is the name of a settings file. This file
is used in order to override the default keys. It is not always the case that one wants to override the default keys, which is
why the settings_filename has a default value of None.
Your task for this part is to implement the load_keys() method. If properly formatted, the settings file should have
eight lines, with one character per line. In order, the characters should be the following:
1. Down key.
2. Up key.
3. Left key.
4. Right key.
5. Insert key.
6. Save key.
7. Undo key.
8. Redo key.
You may assume that the file always has eight lines. The load_keys() method should raise a ValueError if any of the lines
is empty or contains more than one character (ignoring the newline character).
This next detail is something that I hate to waste a paragraph on, but it is worth pointing out because it relates to
potential bugs you may encounter. You may recall that each line read from the file will end with a newline character.
3
Unfortunately, some editors (e.g. Sublime Text) do not automatically insert a newline character at the end of the last line
of a text file. This is arguably a bug/flaw in such editors1
, and you should expect that any text file used by the autograder
will have a newline character at the end of its last line. However, if you create your own text files and find that your code
crashes on the last line, then it could be because of the lack of a newline character at the end of the last line, depending on
which editor you used.
I personally found it easier to first check if any line in the file has the wrong length and then read the keys. This required
two passes through the file, causing me to use the seek() method (with 0 as the argument) to tell the file object to start from
the beginning of the file. Obviously, doing two passes is arguably inefficient, and you don’t need to do that if you don’t want
to; you don’t need to use seek().
Below is are examples (along with relevant text files) of what the result of load_keys() should be.
1 >>> e = Editor (" ignore for now ", " ignore this for now too ") # default ( no third argument )
2 >>> e. down_key
3 ’s ’
4 >>> e. redo_key
5 ’r ’
6 >>> e = Editor (" ignore for now ", " ignore this for now too ", " settings1 . txt ")
7 >>> e. down_key
8 ’d ’
9 >>> e. redo_key
10 ’y ’
11 >>> e. up_key
12 ’u ’
13 >>> e = Editor (" ignore for now ", " ignore this for now too ", " bad_settings1 . txt ")
14 Traceback ( most recent call last ):
15 ...
16 ValueError : At least one line in settings is too long .
17 >>> e = Editor (" ignore for now ", " ignore this for now too ", " bad_settings2 . txt ")
18 Traceback ( most recent call last ):
19 ...
20 ValueError : At least one line in settings is empty .
5.2 Part #2 - Reading the Edited File’s Contents
The first explicit argument to the Editor initializer, called infilename, is the name of a file whose contents the editor will
start with when editing. You need to read and store this file’s contents. There are a few options that you have for storing the
1
I say this because the POSIX standards define a text file as a file organized into lines, where each line ends in a newline character. This is as
opposed to a binary file (which admittedly may seem like nonsense, because this definition implies that a text file that does not end in a newline
character should be called a binary file even if it clearly only contains human-readable text, but that’s beside the point).
4
file’s contents. The most intuitive (and probably best) option is a list of strings. Another option is a list of lists of characters.
The primary benefit of a list of lists of characters is that you will not have to deal with the fact that strings are immutable
in part #6, but other than that, a list of strings is perhaps preferable. If you go with the list of lists of characters, you may
find the fact that list(...) can convert a string into a list of its characters to be useful, as shown below:
1 >>> list (" abcd ")
2 [’a’, ’b’, ’c’, ’d’]
Your initializer (or a helper function called by the initializer) should raise a ValueError in either of the following cases:
❼ At least one line in the file is longer than the window width (see part #3), which is 20 characters (not counting the
newline character).
❼ The input file is empty.
❼ The input file has more than 30 lines.
No example output is shown, since part #3 indirectly does this. As stated in parts #3 and #4, the editor trims trailing
whitespace (see part #3 for a description of what this means), so you may want to make use of the string method rstrip()
in this part.
5.3 Part #3 - Printing the Current File Contents: print_current()
Whenever the edited file’s contents are displayed, it will be within a window that is 20 characters wide and 10 characters
tall. (These values 20 and 10 are stored in the window_width and window_height fields of the Editor class, as you can see at the
start of the initializer.) Through this window, we see only a part of the edited file’s contents at any given time. The file’s
contents will never be wider than the window, because the Editor initializer would have raised a ValueError and, as you’ll see
in part #6, the user will not be allowed to extend a line so that it’s wider than the window2
. Initially, the window should
start at line 1; in part #5.2, you will allow the window to be scrolled down. For example, suppose that we have the following
text file.
1 abcdef
2 ab
3
4 xyz
5 Hi there
6 Hi how are you
7 I am good
8 blah
9 blah blah blah blah
10 stream of
11 conciousness
12 don ’t fail me now
13 blah
14
15 that is all
Without the window being moved, print_current() would result in only the first 10 lines being printed, as shown below.
1 *
2 12345678901234567890
3 * 1 abcdef
4 2 ab
5 3
6 4 xyz
7 5 Hi there
8 6 Hi how are you
9 7 I am good
10 8 blah
11 9 blah blah blah blah
12 10 stream of
13 12345678901234567890
(Update: Just so it’s clear, there are two whitespaces between the asterisk and the number 1, in the case of the asterisk
on the left column. If the asterisk were on line number 10, then there would be one whitespace between the asterisk and the
number 10.)
Below is the above, but shown on the Python IDLE interpreter instead. This is what it should look like so far if you
have completed this part. Note that the run() method already calls print_current() when appropriate. In fact, I would highly
recommend against modifying run() unless you have a good reason to do so. You do not need to store the text files that you
use in a folder called text_files; I did that for my own file organization.
2As I said above, no one would want to use this editor.
5
1 >>> e = Editor (’ text_files / input3 . txt ’, ’ ignore for now ’)
2 >>> e. run ()
3 *
4 12345678901234567890
5 * 1 blah
6 2 blah blah
7 3 blah blah blah
8 4
9 5 what is up
10 6
11 7
12 8
13 9
14 10
15 12345678901234567890
16 Enter command :
As you can see, rows containing 12345678901234567890 are printed above and below the window’s contents, for convenience.
(It is sometimes useful to know which column you are on in an editor, even if that editor is terrible.) You can also see two
asterisks in the above output as well. Those asterisks mark the current location of the cursor. In part #5, you will allow the
cursor to be moved, and in part #6, you will allow the user to make insertions starting at the current location of the editor.
Before getting to those parts, you can always manually change the position of the cursor in the code in order to check if
your print_current() implementation works regardless of the cursor’s location, so as to minimize the chance that you have to
modify print_current() once you are done with this part. Lastly, the line numbers of the file are shown. The max line number
is 30 (since files that are longer than that are rejected by the Editor initializer). Since line numbers may be one digit or two
digits, you need to be mindful of how many whitespaces to print before it; this is why I suggest the rjust() string method.
Regarding trailing whitespaces: “Trailing whitespace” refers to the occurrence of whitespace at the end of a line.
When printing strings out, it is difficult to visually identify trailing whitespace. For example, for all you know, there could
be 30 blank whitespaces after the asterisk printed in the first line of the above example output. Do not worry about
trailing whitespace in this assignment. The autograder will ignore trailing whitespace when comparing the output of
your code with mine. In some cases, part of the file itself may include trailing whitespace. For example, line #1 in the above
example output could have four whitespaces after the abcdef, and line #3 could contain six whitespaces in it. You do not
need to worry about trailing whitespace here either. The editor will trim trailing whitespace, meaning that all trailing
whitespace will be removed, e.g. line #1 would have the four trailing whitespaces removed, and line #3 would be reduced to
a completely empty line. This primarily matters for part #4.
Suppose that in the above example, the window instead was currently starting at line #4 and the cursor was at the sixth
column and the eighth row. In that case, the output of print_current() would look like below. Again, it will not be possible
to move the cursor or scroll the window until you’ve done part #5. However, you may want to experiment with moving the
window artificially (by changing the relevant members that you hopefully will add to your Editor class) before moving on
from this part.
1 *
2 12345678901234567890
3 4 xyz
4 5 Hi there
5 6 Hi how are you
6 7 I am good
7 8 blah
8 9 blah blah blah blah
9 10 stream of
10 * 11 conciousness
11 12 don ’t fail me now
12 13 blah
13 12345678901234567890
Below is an example (the file and the initial view) of what happens when the edited file lacks enough lines to fill the entire
height of the window.
File:
1 blah blah
2 blah blah blah
3
4 what is up
Initial output of print_current():
1 *
2 12345678901234567890
3 * 1 blah
6
4 2 blah blah
5 3 blah blah blah
6 4
7 5 what is up
8 6
9 7
10 8
11 9
12 10
13 12345678901234567890
5.4 Part #4 - Saving the Current File’s Contents: save()
Implement the save() method so that it saves the contents of the currently edited file. The file that you should save to
is the one whose name was given as the second explicit argument in the initializer of Editor. Its original contents should be
erased. Note that the same file can be the one that was read and written to (in which case the first and second arguments
to the Editor initializer would both be that file’s name).
This method is already called by the run() method when the user enters the save key.
If you chose to store the edited file’s contents in a list of lists of characters, then you may find the string method join()
to be useful, as shown below.
1 >>> "". join ([ ’a’, ’b’, ’c’, ’d’])
2 ’abcd ’
Below is an example of how saving should behave.
Input file (first argument to initializer):
1 Line 1
2 Line 2
3 Line 3
4 Line 4
5 Line 5
6 Line 6
7 Line 7
8 Line 8
9 Line 9
10 Line 10
11 Line 11
12 Line 12
Interpreter interaction:
1 >>> e = Editor (’ text_files / input4 . txt ’, ’ text_files / output4 . txt ’)
2 >>> e. run ()
3 *
4 12345678901234567890
5 * 1 Line 1
6 2 Line 2
7 3 Line 3
8 4 Line 4
9 5 Line 5
10 6 Line 6
11 7 Line 7
12 8 Line 8
13 9 Line 9
14 10 Line 10
15 12345678901234567890
16 Enter command : e
17
18 *
19 12345678901234567890
20 * 1 Line 1
21 2 Line 2
22 3 Line 3
23 4 Line 4
24 5 Line 5
25 6 Line 6
26 7 Line 7
27 8 Line 8
28 9 Line 9
29 10 Line 10
30 12345678901234567890
7
31 Enter command : q
32 >>>
Resulting output file (second argument to initializer), which is exactly the same as the input file since no modifications
were made:
1 Line 1
2 Line 2
3 Line 3
4 Line 4
5 Line 5
6 Line 6
7 Line 7
8 Line 8
9 Line 9
10 Line 10
11 Line 11
12 Line 12
As stated before, the editor removes trailing whitespace. Thus, if the initial file contains trailing whitespace, the output
file will not contain it. I would deal with this by using the string method rstrip() while writing out each line to the output
file.
Input file: (there are three whitespaces after “Line 3”)
1 Line 1
2 Line 2
3 Line 3
4 Line 4
5 Line 5
6 Line 6
7 Line 7
8 Line 8
9 Line 9
10 Line 10
11 Line 11
12 Line 12
Interpreter interaction:
1 [ Basically the same as in the previous example .]
Resulting output file:
1 Line 1
2 Line 2
3 Line 3
4 Line 4
5 Line 5
6 Line 6
7 Line 7
8 Line 8
9 Line 9
10 Line 10
11 Line 11
12 Line 12
There is a file method called writelines() that may eliminate the need for a loop in this part. In my own implementation,
I didn’t use writelines(), since I chose not to store the newline character at the end of each line.
5.5 Part #5 - Moving the Cursor
5.5.1 Part #5.1 - Leftward/Rightward Movement: move_left() and move_right()
The move_left() and move_right() methods should change the “x-coordinate” of the cursor, i.e. change which column the
cursor is on. If moving the cursor would cause it to go out of bounds (i.e. to before the first column or after the 20th column),
then the movement should be prevented.
Below are examples of how your code should behave once you have successfully implemented move_left() and move_right().
Note that the run() method already calls these methods when the appropriate directional commands are entered.
1 >>> e = Editor (’ text_files / input1 . txt ’, ’ ignore ’)
2 >>> e. run ()
3 *
4 12345678901234567890
8
5 * 1 abcdef
6 2 ghi
7 3 jklm
8 4 jkj
9 5 jklasj
10 6 jklajs ; lkfj
11 7 qiowuioj
12 8 lkjw ; lkj
13 9 qwklejklj ;c
14 10 jkaljsoiu
15 12345678901234567890
16 Enter command : d
17
18 *
19 12345678901234567890
20 * 1 abcdef
21 2 ghi
22 3 jklm
23 4 jkj
24 5 jklasj
25 6 jklajs ; lkfj
26 7 qiowuioj
27 8 lkjw ; lkj
28 9 qwklejklj ;c
29 10 jkaljsoiu
30 12345678901234567890
31 Enter command : d
32
33 *
34 12345678901234567890
35 * 1 abcdef
36 2 ghi
37 3 jklm
38 4 jkj
39 5 jklasj
40 6 jklajs ; lkfj
41 7 qiowuioj
42 8 lkjw ; lkj
43 9 qwklejklj ;c
44 10 jkaljsoiu
45 12345678901234567890
46 Enter command : d
47
48 *
49 12345678901234567890
50 * 1 abcdef
51 2 ghi
52 3 jklm
53 4 jkj
54 5 jklasj
55 6 jklajs ; lkfj
56 7 qiowuioj
57 8 lkjw ; lkj
58 9 qwklejklj ;c
59 10 jkaljsoiu
60 12345678901234567890
61 Enter command : d
62
63
64
65 [... omitting some parts in order to save electronic trees ...]
66
67
68
69 *
70 12345678901234567890
71 * 1 abcdef
72 2 ghi
73 3 jklm
74 4 jkj
75 5 jklasj
76 6 jklajs ; lkfj
77 7 qiowuioj
78 8 lkjw ; lkj
9
79 9 qwklejklj ;c
80 10 jkaljsoiu
81 12345678901234567890
82 Enter command : d
83
84
*
85 12345678901234567890
86 * 1 abcdef
87 2 ghi
88 3 jklm
89 4 jkj
90 5 jklasj
91 6 jklajs ; lkfj
92 7 qiowuioj
93 8 lkjw ; lkj
94 9 qwklejklj ;c
95 10 jkaljsoiu
96 12345678901234567890
97 Enter command : d
98
99
*
100 12345678901234567890
101 * 1 abcdef
102 2 ghi
103 3 jklm
104 4 jkj
105 5 jklasj
106 6 jklajs ; lkfj
107 7 qiowuioj
108 8 lkjw ; lkj
109 9 qwklejklj ;c
110 10 jkaljsoiu
111 12345678901234567890
112 Enter command : d
113
114
*
115 12345678901234567890
116 * 1 abcdef
117 2 ghi
118 3 jklm
119 4 jkj
120 5 jklasj
121 6 jklajs ; lkfj
122 7 qiowuioj
123 8 lkjw ; lkj
124 9 qwklejklj ;c
125 10 jkaljsoiu
126 12345678901234567890
127 Enter command : d
128
129
*
130 12345678901234567890
131 * 1 abcdef
132 2 ghi
133 3 jklm
134 4 jkj
135 5 jklasj
136 6 jklajs ; lkfj
137 7 qiowuioj
138 8 lkjw ; lkj
139 9 qwklejklj ;c
140 10 jkaljsoiu
141 12345678901234567890
142 Enter command : d
143
144
*
145 12345678901234567890
146 * 1 abcdef
147 2 ghi
148 3 jklm
149 4 jkj
150 5 jklasj
151 6 jklajs ; lkfj
152 7 qiowuioj
10
153 8 lkjw ; lkj
154 9 qwklejklj ;c
155 10 jkaljsoiu
156 12345678901234567890
157 Enter command : a
158
159 *
160 12345678901234567890
161 * 1 abcdef
162 2 ghi
163 3 jklm
164 4 jkj
165 5 jklasj
166 6 jklajs ; lkfj
167 7 qiowuioj
168 8 lkjw ; lkj
169 9 qwklejklj ;c
170 10 jkaljsoiu
171 12345678901234567890
172 Enter command : q
173 >>>
5.5.2 Part #5.2 - Upward/Downward Movement and Scrolling the Window: move_up() and move_down()
As I said previously, I personally found this part to be the hardest (although that might be because I had to come up
with all of the below rules by myself), and there will be test cases in parts #6 and #7 that won’t involve upward/downward
cursor movement, meaning that you can still get those test cases correct even if you do not finish this part.
When no scrolling occurs, up/down movement is simple: move the cursor up or down. When scrolling occurs, however,
then the rules are more complicated. The following rules dictate how scrolling up and down should work. I based these rules
off of Sublime Text.
❼ Scrolling up: Scrolling up can only occur when the user tries to move the cursor up while the cursor is
at the top of the window. Moreover, if the cursor is already at the first line of the file, then scrolling cannot occur;
otherwise, scrolling should proceed, with the cursor staying at the top of the window.
❼ Scrolling down. Scrolling down can only occur when the user tries to move the cursor down while the
cursor is at the last line of the file. This difference means that you can view past the end of the file when scrolling
down (although you cannot edit past the end of the file), but you can’t view before the start of the file when scrolling
up. Here are four scenarios.
1. If the cursor is not at the last line of the file but is at the bottom of the window, then scroll down (without moving
the cursor).
2. If the cursor is at the last line of the file but not at the top of the window, then scroll down but move the cursor
up. This will simulate the effect of scrolling down while keeping the cursor at the same position relative to the
file’s contents.
3. If the cursor is at the last line of the file and at the top of the window, then scrolling down should be prohibited.
In other words, although the user can scroll past the end of the file, they cannot cause the last line of the file to
completely disappear. (Recall from part #2 that the Editor initializer rejects empty files, so you need not worry
about that case.)
Below is an example of how your code should behave after you have completed this part.
Input file:
1 Line 1
2 Line 2
3 Line 3
4 Line 4
5 Line 5
6 Line 6
7 Line 7
8 Line 8
9 Line 9
10 Line 10
11 Line 11
12 Line 12
Interpreter interaction:
11
1 >>> e = Editor (’ text_files / lines . txt ’, ’ ignore ’)
2 >>> e. run ()
3 *
4 12345678901234567890
5 * 1 Line 1
6 2 Line 2
7 3 Line 3
8 4 Line 4
9 5 Line 5
10 6 Line 6
11 7 Line 7
12 8 Line 8
13 9 Line 9
14 10 Line 10
15 12345678901234567890
16 Enter command : w
17
18 *
19 12345678901234567890
20 * 1 Line 1
21 2 Line 2
22 3 Line 3
23 4 Line 4
24 5 Line 5
25 6 Line 6
26 7 Line 7
27 8 Line 8
28 9 Line 9
29 10 Line 10
30 12345678901234567890
31 Enter command : d
32
33 *
34 12345678901234567890
35 * 1 Line 1
36 2 Line 2
37 3 Line 3
38 4 Line 4
39 5 Line 5
40 6 Line 6
41 7 Line 7
42 8 Line 8
43 9 Line 9
44 10 Line 10
45 12345678901234567890
46 Enter command : w
47
48 *
49 12345678901234567890
50 * 1 Line 1
51 2 Line 2
52 3 Line 3
53 4 Line 4
54 5 Line 5
55 6 Line 6
56 7 Line 7
57 8 Line 8
58 9 Line 9
59 10 Line 10
60 12345678901234567890
61 Enter command : s
62
63 *
64 12345678901234567890
65 1 Line 1
66 * 2 Line 2
67 3 Line 3
68 4 Line 4
69 5 Line 5
70 6 Line 6
71 7 Line 7
72 8 Line 8
73 9 Line 9
74 10 Line 10
12
75 12345678901234567890
76 Enter command : w
77
78 *
79 12345678901234567890
80 * 1 Line 1
81 2 Line 2
82 3 Line 3
83 4 Line 4
84 5 Line 5
85 6 Line 6
86 7 Line 7
87 8 Line 8
88 9 Line 9
89 10 Line 10
90 12345678901234567890
91 Enter command : s
92
93 *
94 12345678901234567890
95 1 Line 1
96 * 2 Line 2
97 3 Line 3
98 4 Line 4
99 5 Line 5
100 6 Line 6
101 7 Line 7
102 8 Line 8
103 9 Line 9
104 10 Line 10
105 12345678901234567890
106 Enter command : s
107
108 *
109 12345678901234567890
110 1 Line 1
111 2 Line 2
112 * 3 Line 3
113 4 Line 4
114 5 Line 5
115 6 Line 6
116 7 Line 7
117 8 Line 8
118 9 Line 9
119 10 Line 10
120 12345678901234567890
121 Enter command : s
122
123
124
125 [... omitting some parts in order to save electronic trees ...]
126
127
128
129 *
130 12345678901234567890
131 1 Line 1
132 2 Line 2
133 3 Line 3
134 4 Line 4
135 5 Line 5
136 6 Line 6
137 7 Line 7
138 * 8 Line 8
139 9 Line 9
140 10 Line 10
141 12345678901234567890
142 Enter command : s
143
144 *
145 12345678901234567890
146 1 Line 1
147 2 Line 2
148 3 Line 3
13
149 4 Line 4
150 5 Line 5
151 6 Line 6
152 7 Line 7
153 8 Line 8
154 * 9 Line 9
155 10 Line 10
156 12345678901234567890
157 Enter command : s
158
159
*
160 12345678901234567890
161 1 Line 1
162 2 Line 2
163 3 Line 3
164 4 Line 4
165 5 Line 5
166 6 Line 6
167 7 Line 7
168 8 Line 8
169 9 Line 9
170 * 10 Line 10
171 12345678901234567890
172 Enter command : s
173
174
*
175 12345678901234567890
176 2 Line 2
177 3 Line 3
178 4 Line 4
179 5 Line 5
180 6 Line 6
181 7 L
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