PV178: Programming for the CLI Environment
Seminar: Week 4
Michal Ordelt, Tomáš Pochop
Institute of Computer Science and Faculty of Informatics
Masaryk University
March 25, 2007
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Regular expressions
i 1 using System . Text. Regu la rExpressions
2 2 private Regex rx;
3 3 rx = new Regex ( p a t t e r n ) ;
4 4 i f ( rx . l s M a t c h ( i n p u t T e x t ) ) {
5 5 \\some match found , do something
6 6 }
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Some regular expressions elements
Quantifiers:
+ One or more matches
* Zero, one or more matches
Character classes:
\d Decimal digit
\s Any whitespace
. Any character except \n
For more regular expressions language elements see MSDN.
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Q (?'groupname'pattern)
Captures the matched substring into a group groupname. The
string used for groupname must not contain any punctuation
and it cannot begin with a number.
B Class M a t c h C o l l e c t i o n represents the set of successful
matches found by iteratively applying a regular expression
pattern to the input string.
B M a t c h C o l l e c t i o n Regex.Matches(string i n p u t ) method
searches an input string for all occurrences of a regular
expression and returns all the successful matches as if Match
were called numerous times.
B Match. Groups ["groupname "] .Value returns the matched
substring for group groupname.
Regular expressions grouping example
i //capture subexpressions groups number and text
2 rx = new Regex(@" ( ? ' n u m b e r ' \ d + ) \ s + ( ? ' t e x t ' ˇ * ) " );
3 M atchCol lection matches = rx . Matches( someString );
4 //go through all matched elements
5 foreach (Match match J j i matches)!
e //access groups
7 string num = match . Groups [" number" ] . Value ;
s string t e x t = match . Groups [" t e x t " ] . Value ;
 }
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A simple .sub subtitle file contains a lot of lines in format:
{startframe}{endframe}Text to be shown. Write an application
that moves the subtitle forward or back by a specified amount of
time.
First implement class Line representing a single line of the subtitle
file. This class stores the startframe, endframe and text, and
provide the method Move ( i n t 64 seconds, double framerate)
which moves the startframe and endframe by the specified number
of second, at the specified framerate. The overriden method
ToStringO returns the line in the .sub format.
Hints:
Framerate means number of frames per second
The example .sub file has framerate 23.976
To convert double to Int64 use Convert .ToInt64(double)
Next implement class Subtitle : IEnumerable<Line>
representing the subtitle file. This class reads one by one all lines
from the input file, using regular expresions fills Line class and
returns it as an enumerator.
Move each line by the specified amount of seconds and write it to
the output file.
The input file, framerate, time and output file are set from
command line as arguments.
Hints:
To read text from file use TextReader class
Use encoding Encoding.GetEncoding(1250) for file I/O
Remember to close all streams, readers and writers.
To write to a file use the TextWriter class. Example:
i //write to the file file, out
2 F i l e S t r e a m fs = F i le . C r e a t e (" f i I e . o u t " );
3 T e x t W r i t e r w r ;
4 //assign the textwriter to the filestream
5 wr = new S t r e a m W r i t e r ( f s ,
e E n c o d i n g . G e t E n c o d i n g ( 1 2 5 0 ) ) ;
7 //write a line to the file
s w r . W r i t e L i n e ( " W r i t t e n ^ i n ^ f i l e " );
9 w r . C l o s e ( ) ; //close the writer
io f s . Close ( ) ; //close the stream
C # , like many object-oriented languages, handles errors and
abnormal conditions with exceptions. An exception is an object
that encapsulates information about an unusual program
occurrence.
It is important to distinguish between bugs, errors, and exceptions.
A bug is a programmer mistake that should be fixed before the
code is shipped. Exceptions are not a protection against bugs.
Although a bug might cause an exception to be thrown, you should
not rely on exceptions to handle your bugs. Rather, you should fix
the bug.
When your program encounters an exceptional circumstance, such
as running out of memory, it throws (or "raises") an exception.
When an exception is thrown, execution of the current function
halts and the stack is unwound until an appropriate exception
handler is found.
Exceptions
To signal an abnormal condition in a C # class, you throw an
exception. To do this, use the keyword throw. This code creates a
new instance of System.Exception and then throws it:
i E x c e p t i o n e x c e p t i o n ;
2 e x c e p t i o n = new E x c e p t i o n (" S o m e t h i n g ^ b a d ^ h a p p e n e d " )
3 throw e x c e p t i o n ;
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Exceptions handling
In C#, an exception handler is called a catch block and is created
with the catch keyword.
i try_{
2 //an exception can occur here
3 }
4 catch ( ExceptionType e ) {
5 //here we catch an ExceptionType exception
e //and all derived from ExceptionType too
* }
s f i n a l l y {
9 //this code is always executed
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Exception class: useful members
string Message
The error message that explains the reason for the exception.
string StackTrace
A string that describes the contents of the call stack
string ToStringO
The default implementation obtains the name of the class
that threw the current exception, the message, the result of
calling ToString on the inner exception, and the StackTrace.
You can create your own exception classes by deriving from the
Exception class. Example:
i //derive from the Exception class
2 public class MyException : Exception {
3 //constructor with message
4 public MyException ( s t r i n g message)
5 : base( message) {
e } //ctor with message and inner exception
7 public Exception ( s t r i n g message, Exception in
s : base( message , i n n e r ) {
}
10 }
Raise MyException:
i throw new MyException (" My^ except ion ^occured !" );
Exceptions task
Add exceptions handling to the Subtitle timing application and
cover all possible problems, that may occur (can't read from input
file, can't write to output file, wrong format of line, etc.). Try to
use your own exception too.
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