Description
1 Objectives
Having had first hand experience with low-level dynamic memory management, you are now ready
to explore and utilize high-level turnkey alternative from the C++ standard template library (STL),
which provide not only efficient memory management but also many other useful and efficient
services.
To that end, this assignment will give you practice with using the STL sequential container classes
such as list, vector, and string, and iterators.
2 Line Editors
In the early 1970s, before the advent of video displays and screen editors, electric typewriters
were used as display devices providing a continuous printed output of a user’s computer session.
Multiple users were supported on the same computer, each at their own terminal. Both computers
and printing terminals were very slow compared to today’s standards. Meanwhile, computer
programmers spent most of their work hours writing programs using line editors.
Typically, a line editor would prompt you for a command and then you would type in a command
telling it which line you wanted displayed or edited. If you wanted to insert a line, then you would
tell it that you wanted to insert a line at a particular line address and then enter that line. If you
wanted to delete a line, you would have to specify the address of that line. You would repeatedly
issue editing commands and then wait until the computer responded. To get a visual view of the
program you were editing, you would issue a printing command and then wait until the computer
responded. The wait times would add up considerably. During peak hours, programmers’ editing
commands could bring their editing sessions to a halt. Meanwhile, one popular line editor of the
time was ed under Unix; it worked in silent mode, demanding minimal input, generating minimal
output, and offering an extensive set of commands with forgiving syntax.
Today, line text editors are virtually useless, without practical applications. Nonetheless, the process of actually implementing a line text editor does provide not only an instructive programming
experience but also plenty of opportunity to practice using the STL sequential container classes
and iterators.
3 Your Task
Design and implement a line text editor, named LineEd1
, without using the new and delete
operators; specific requirements will be given later. We first specify the expected functionality of
LineEd, using present tense.
1Acronym for line-oriented text editor. Note that, although LineEd’s command set and syntax might look a
little like the commands of the edlin editor on DOS, or the commands of the mighty ed editor on Unix, LineEd
is just a toy line editor with very limited command set and functionality
1
4 LineEd
LineEd is a line-oriented text editor that allows users to open, edit, and save new or existing text
files.
Internally, LineEd always operates on a buffer, a place in memory where it stores a copy of the
file it is editing. In addition to a buffer, LineEd uses a clipboard, a place in memory where cut
and copied text are temporarily stored.
4.1 Staring LineEd
To start LineEd on a text file named a.txt , you type the following command in a Linux/-
Mac/Windows specific shell and then presses the return key, which is denoted by this symbol
.
LineEd a.txt
If, for example, the text file a.txt exists and has three lines, then LineEd reads the file contents
into its buffer, line by line, and responds as follows:
“a.txt” 3 lines
Entering command mode .
?
Notice that LineEd prompts with the ’?’ symbol to indicate that it is operating in command
mode.
When started on a nonexistent file, say, b.txt, LineEd creates an empty buffer and responds as
follows:
“b.txt” [New File ]
Entering command mode .
?
However, LineEd does not create the file b.txt unless and until a w command is entered.
Finally, when started without a filename, LineEd creates an empty buffer and responds as
follows:
“?” [New File ]
Entering command mode .
?
2
4.2 LineEd’s Operating Modes
LineEd has two distinct operating modes.
Command mode: LineEd displays a ’?’ prompt to indicate it is operating in command mode.
Once the return key is pressed in command mode, LineEd interprets the
input characters as a command line.
Input mode: The a (append) and i (insert) commands put LineEd in input mode.
LineEd interprets every input character as text, displaying no prompts
and recognizing no commands in this mode. You can now input as many
lines of text as you wish into the buffer, pressing the return key at the end
of each line.
To put LineEd back in command mode, you type a period (.) on an
otherwise blank line. This line is not considered part of the input text.
4.3 Command Line Syntax
LineEd command lines have a simple syntax structure:
[ command symbol ] [ line address 1 ] [, [ line address 2 ] ]
The square brackets [ ] indicate optional parts of a command; they are used for notational
purposes only and do not need to be included when entering a command. There may be any
number of whitespace characters appearing before and after the optional parts of a command.
Whether or not a command requires a line range, LineEd allows every command to be followed by
a line range. Otherwise, too many errors might ensue, resulting in an unpleasant editing session.
Allowing a line range after a command, which itself may or may not be present, LineEd can
operate silently behind the scenes, using default values for missing line addresses and command,
consuming minimal input, producing minimal output, and complaining only when necessary.
4.4 Command Line Addresses
The two line addresses following a command character specify a line range to which the command
is applied. A line address is either a line number, a dot character (.), or a dollar sign character
($), as indicated in Table 1 below:
Table 1
Line address Property constraints
$ The number of the last line in the buffer $ = buffer size
. The number of the current line in the buffer 1 ≤ .≤ $
a line number An integer n addressing the n
th line of the buffer 1 ≤ n ≤ $
3
4.6 Command Line Rules
1. The command symbol in a command line is not case-sensitive, making no difference if it
entered as uppercase or lowercase.
2. If the command symbol is omitted, then LineED defaults to the ’P’ (print) command.
3. If the buffer is empty, then the command symbol must be one of the I (insert), A (append),
or Q (quit) commands, or possibly the V (paste) command if clipboard is non-empty.
4. If the first line address option is omitted, then LineED defaults to the current line.
5. If the second line address option is omitted, then LineED defaults to the first line address.
6. If a line address is negative, then LineED defaults to the first line in the buffer.
7. If a line address exceeds the buffer size, then LineED defaults to the last line in the buffer.
8. Regardless of how a line range is determined, the first line address cannot exceed the second
line address; otherwise, LineEd will swap the two line addresses to ensure that the first
line address is always less than or equal to the second line address.
The rules for command lines are relaxed and forgiving, and are in place to provide a user friendly
interface and to minimize challenging the user with annoying syntax errors. Any command line
is allowed to end with a pair of optional line addresses, regardless of whether or not they are
required.
Note that the command line rules above are based on the values of the three entities expected in a
command line, namely, the command symbol, first and second line addresses, and are independent
of the syntax governing the entities and of the algorithm used to parse a command line into those
entities.
Table 3 below shows how a command line is interpreted; the symbol z represents any command
from Table 2; the symbols x and y each represent either a line number, a dot character (.), or a
dollar sign character ($) as shown in Table 1.
Table 3. Command Line Interpretation
Command Line Entered Command Line Interpreted
z z.,.
zx zx,x
z,y z.,y
zx, zx,x
zx,y zx,y
p.,.
x px,x
,y p.,y
x, px,x
x,y px,y
, p.,.
5
5 Sample Editing Session
The simplest way to start LineEd is to run it at the command line without supplying a file name:
11 $ ./LineEd
12 “?” [New File ]
13 Entering command mode .
14 ? p
15 File empty . Must use one of I,A,Q commands .
16 ? m
17 bad command : m
18 ? .
19 File empty . Must use one of I,A,Q commands .
20 ? $
21 File empty . Must use one of I,A,Q commands .
22 ? i enter input mode and type some lines
23
24 Entering input mode .
25 line 1
26 line 2
27 . end of input, back to command mode
28 Entering command mode .
29 ? -10,10 silly line addresses, same as p1,$ currently
30 1: line 1
31 2> line 2
32 ? g1 go to line 1, print it, and make it the current line
33 1> line 1
34 ? 1,$ Print all lines
35 1> line 1
36 2: line 2
37 ? a append lines to the end of the buffer
38
39 Entering input mode .
40 line 3
41 . end of input, back to command mode
42 Entering command mode .
43 ? 1,5 Print all lines
44 1: line 1
45 2: line 2
46 3> line 3
47 ? q quit
48 There are unsaved changes .
49 Do you wish to save the changes (y or n)? y
50 Enter the name of a file to write to: abc.txt
51 3 lines written to file “abc.txt”
52 bye
For your convenience, command lines are shown in red and my comments, which are not part of
the command, are shown in brick red.
Any references to a line address refers to a line of the text being edited, not to the numbers listed
outside the box in brick red color. On line 32, for example, the number 1 in command g1 refers
to line number 1 in the buffer.
To indicate the current line, LineEd uses the symbol ’>’ after the line number instead of a :, as
shown on lines 35 and 46.
6
Now, let’s reopen abc.txt in our line editor:
LineEd abc .txt
Our driver program looks like this:
1 # include
2 # include
3 # include < cstdlib > // for EXIT_FAILURE
4
5 using std :: string ;
6 using std :: cout ;
7 using std :: cerr ;
8 using std :: endl ;
9 # include ” editor .h”
10
11 // function prototypes
12 void testLineEd ( const string & filename );
13
14 int main (int argc , char * argv [])
15 {
16 if ( argc > 2) // too many arguments
17 {
18 cerr << ” Usage 1: ” << argv [0] << “\n”;
19 cerr << ” Usage 2: ” << argv [0] << ” filename \n”; 20 exit ( EXIT_FAILURE ); 21 } 22 23 string filename {}; // empty file name , in case argc ==1 24 if ( argc == 2) // only one argument , a file name expected 25 { 26 filename = argv [1]; 27 }; 28 29 // Normally , we ’d run the editor by uncommenting the following two lines 30 // LineEd ed( filename ); // create a LineED object 31 // ed.run (); // run the line editor 32 33 // test our line editor 34 testLineEd ( filename ); // normally we ’d comment out this line 35 36 return 0; 37 } As indicated in the comments, we would normally run our editor as shown in lines 30 and 31. At line 30, we would create an LineEd object passing it an input file name at construction. We would then have LineEd run at line 31 continuously until the user chooses to stop. To test our line editor, and to facilitate grading it, however, we are going create a simple test script wrapped in a call of the testLineEd(filename) function on line 34: 7 abc.txt line 1 line 2 line 3 Source code 38 void testLineEd ( const string & filename ) 39 { 40 LineEd ed( filename );// create a LineED object 41 ed. runCommand (“z1,$”); // bad command 42 ed. runCommand (“1,$p”); // bad command 43 ed. runCommand (“1$”); // bad command 44 ed. runCommand (” ,.p”); // bad command output 1 “abc.txt” 3 lines 2 Entering command mode . 3 bad command : z 4 Bad address 2: $p 5 Bad address 1: 1$ 6 Bad address 2: .p 45 46 ed. runCommand (“1,$”); // print all 47 48 // next the given invalid line range 49 // $,1 is first adjusted to 1,$, 50 // then the line range 1,$ is cut out 51 // from the buffer into the clipboard , 52 // leaving the buffer empty . 53 ed. runCommand (“x$ , 1”); 54 55 ed. runCommand (“p”);// not allowed on empty buffer 7 1: line 1 8 2: line 2 9 3> line 3
10 File empty . Must use one of I,A,Q,V commands .
56 ed. runCommand (“v”);// paste ino empty buffer
57 ed. runCommand (“1,$”); // print all
11 1: line 1
12 2: line 2
13 3> line 3
58
59 ed. runCommand (“i2”);// insert at line 2
60 ed. runCommand (“1,$”); // print all
14
15 Entering input mode .
16 Line 2.1
17 Line 2.2
18 Line 2.3
19 .
20 Entering command mode .
21 1: line 1
22 2: Line 2.1
23 3: Line 2.2
24 4> Line 2.3
25 5: line 2
26 6: line 3
8
61
62 ed. runCommand (“x4,5”); // cut lines 4 and 5
63 ed. runCommand (“v100,1”); // same as “v1”
64 ed. runCommand (“1,$”); // print all
27 1: Line 2.3
28 2> line 2
29 3: line 1
30 4: Line 2.1
31 5: Line 2.2
32 6: line 3
65
66 ed. runCommand (“w”);// save changes
33 Save changes to the file : abc .txt (y or n)? y
34 6 lines written to file “abc.txt”
abc.txt
Line 2.3
line 2
line 1
Line 2.1
Line 2.2
line 3
67
68 ed. runCommand (“1,$”); // print all
69 ed. runCommand (“c3,6”);// change 2. to two.
70 ed. runCommand (“1,$”); // print all
35 1: Line 2.3
36 2: line 2
37 3: line 1
38 4: Line 2.1
39 5: Line 2.2
40 6> line 3
41 change ? 2.
42 to? two .
43 1: Line 2.3
44 2: line 2
45 3: line 1
46 4: Line two.1
47 5> Line two.2
48 6: line 3
71
72 ed. runCommand (“.”); // same as p. ,.
73 ed. runCommand (“n”); // down one line
74 ed. runCommand (“n”); // down one line
75 ed. runCommand (“1,$”); // print all
76 ed. runCommand (“w”); // save buffer
49 5> Line two.2
50 6> line 3
51 EOF reached
52 1: Line 2.3
53 2: line 2
54 3: line 1
55 4: Line two.1
56 5: Line two.2
57 6> line 3
58 Save changes to the file : abc .txt (y or n)? y
59 6 lines written to file “abc.txt”
9
abc.txt
Line 2.3
line 2
line 1
Line two.1
Line two.2
line 3
77
78 ed. runCommand (“x3”); // cut line 3
79 ed. runCommand (“d1”); // del line 1
80 ed. runCommand (“1,$”); // print all
60 1> line 2
61 2: Line two.1
62 3: Line two.2
63 4: line 3
81
82 ed. runCommand (“d2,3”); // del line 2-3
83 ed. runCommand (“1,$”); // print all
84 ed. runCommand (“v1”); // paste at 1
85 ed. runCommand (“1,$”); // print all
64 1: line 2
65 2> line 3
66 1> line 1
67 2: line 2
68 3: line 3
86
87 ed. runCommand (“j2,$”); // join lines
88 ed. runCommand (“1,$”); // print all
69 1: line 1
70 2> line 2 line 3
89
90 ed. runCommand (“j1,$”);// join all
91 ed. runCommand (“1,$”);// print all
92 ed. runCommand (“w”);// save buffer
71 1> line 1 line 2 line 3
72 Save changes to the file : abc .txt (y or n)? y
73 1 lines written to file “abc.txt”
abc.txt
line 1 line 2 line 3
10
93
94 ed. runCommand (“a”); // append
95 ed. runCommand (“$,1”); // print all
96 ed. runCommand (“q”); // quit
74
75 Entering input mode .
76 Last line
77 .
78 Entering command mode .
79 1: line 1 line 2 line 3
80 2> Last line
81 There are unsaved changes .
82 Do you wish to save the changes (y or n)? y
83 Save changes to the file : abc .txt (y or n)? n
84 Enter the name of a file to write to: xyz.txt
85 2 lines written to file “xyz.txt”
86 bye
xyz.txt
line 1 line 2 line 3
Last line
11
6 The Buffer
Since the order in which text lines are inserted into text files is important, the author of LineEd
has to choose between the following STL sequence container classes as the underlying data
structure for its buffer: array vector, deque, forward list, list,
C++ arrays are quickly ruled out because of their fixed size, letting alone their lack of support
for insertion or deletion of elements.
Since line editing activity typically involves insertion and deletion operations anywhere in the
buffer, we are left with only two options: forward list, and list.
Since line editing frequently involves upward and downward movement of the current line , we
are left with one option: list.
list buffer ;
Note that the types string and list provide highly efficient turnkey alternative to
your Line and LineList classes in Assignment 1, respectively.
7 The Clipboard
The only desired operations on the clipboard are reading and overwriting its entire contents.
Hence, of the five sequential containers array, vector, deque, forward list, and list, the vector is
most appropriate for the desired operations.
vector clipboard ;
8 Programming Requirements
Implement two classes named LineEd and Command associated as follows:
LineEd Command
where the dotted arrow line from class LineEd to class Command indicates that a LineEd
object does not internally store a Command object. Instead, LineEd uses or depends on
Command as a local variable in a member function or in the parameter list of a member
function.
12
Command
– command line : string Stores supplied command line
– status : bool Stores validity of this command
– symbol : string Stores the command symbol
– address1: string Stores address 1
– address2: string Stores address 2
+ Command(command line : const string&) : Ctor, sets and parses command line
+ parse(command line : const string&) : void Resets and parses command line
+ getSymbol() : string Returns the command symbol
+ getAddress1() : string Returns address 1
+ getAddress2() : string Returns address 2
+ getStatus() : bool Returns whether command line syntax
is valid
+ setStatus(status : bool) : void Sets status of this command
Note that class Command merely parses a given command line into a command symbol,
and two line addresses and never interprets these entities; in fact, apart from the syntax of
a command line, it is not even aware of the meaning of the entities within the command
line. Hence, the status member reflects the validity of the command line syntax as a
whole, as opposed to whether the command symbol is valid. In short, the class leaves
interpretation of the parsed entities to the clients of the class.
The attributes involved in modeling a LineEd object clearly include its buffer, clipboard,
current line, associated file name, whether the buffer contains unsaved contents, etc.
Feel free to introduce private members (data and functions) to facilitate your modeling of
the line editor.
The public interface of class LineEd must include a public constructor taking a file name as
a parameter, a public member function running a given single command line, and a public
member function running a continuous editing session.
Usage of LineEd’s public members
LineEd editor ( filename ); // create a LineEd object
editor . runCommand (“p1,$”); // run a single command
editor .run (); // start an editing session
The private interface of class LineEd must include several member functions, each implementing one of the commands listed in Table 2. Again, feel free to introduce your own
private member functions to facilitate your implementation of LineEd.
You are not allowed to use the new and delete operators in this assignment; the idea is to
recognize that it is possible to write substantial C++ programs without getting involved
and entangled with dynamic memory management.
You are not allowed to use global variables and C-style raw arrays.
13
9 Suggestions
Analyze the tasks at hand, using pen and paper, and ideally away from your computer!
Prepare an action plan for each task.
Avoid writing code in large chunks thinking that you can defer testing to after completions
of your code.
You might want to start working on class Command first because it is independent of and
simpler in functionality than class LineEd. Test as you write code.
You need to have an action plan on how to parse a command line. Extracting the command
symbol from a command line is rather straightforward as it can appear, if present, only at
the beginning of the command line. However, parsing the line range part of a command
line might be a little tricky, because a line range may have missing parts.
You might find it easier to parse a command line after trimming out all whitespace characters
in it. Since a command line is only a few characters long, it can be more efficient to directly
transfer all non-white space characters from the command line to another C++ string. In
any event, be sure to explore the facilities in the header, including its popular
family of find member functions.
Learn about list iterators and about iterator operations advance, distance, begin, end,
prev, and next in the header.
Introduce functionality to your LineEd class one function at a time, and test as you go,
one function at a time.
To do anything useful during an editing session, you need a non-empty buffer. So, consider
implementing member functions such as insert, append, and print before the others. For
example, to append to the end of the buffer your code might include elements similar to
those in the following incomplete code fragment.
string line ;
while ( getline (cin , line ))
{
buffer . push_back ( line );
// other housekeeping code
}
// make sure that the current line address is set to the last line appended
14
10 Deliverables
1. Header files: Command.h and LineEd.h
2. Implementation files: Command.cpp, LineEd.cpp, driver.cpp
3. A README.txt text file (as described in the course outline).
11 Evaluation Criteria
Evaluation Criteria
Functionality Testing correctness of execution of your program, Proper implementation of all specified requirements, Efficiency
60%
OOP style Encapsulating only the necessary data inside your objects, Information hiding, Proper use of C++ constructs and facilities
10%
Documentation Description of purpose of program, Javadoc comment style for
all methods and fields, comments on non-trivial pieces of code in
submitted programs
10%
Presentation Format, clarity, completeness of output, user friendly interface 10%
Code readability Meaningful identifiers, indentation, spacing, localizing variables 10%
