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Chapter 9: Perl (continue). Advanced Perl Programming. Some materials are taken from Sams Teach Yourself Perl 5 in 21 Days, Second Edition. Announcement. Quiz on the next class. Sort alphabetically ( sort { $a cmp $b } @array; ) grades=(80,100,95); foreach $grade (sort @grades) {

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chapter 9 perl continue

Chapter 9: Perl (continue)

Advanced Perl Programming

Some materials are taken from Sams Teach Yourself Perl 5 in 21 Days, Second Edition

announcement
Announcement
  • Quiz on the next class.
  • Sort alphabetically (sort { $a cmp $b } @array;)

grades=(80,100,95);

foreach $grade (sort @grades) {

print "$grade\n"

} # output 100, 80, 95

  • Sort numerically

foreach $grade (sort {$a<=>$b} @grades) {

print "$grade\n"

} # output 80, 95, 100

  • Associative arrays

%fruit = ("apples“ => 6, "cherries“ => 8, "oranges“ => 11);

$d = 2*$fruit{“apples”} +3* $fruit{“cherries”};

advanced perl programming objectives
Advanced Perl ProgrammingObjectives

After studying this lesson, you should be able to:

  • Pattern Matches
  • File operations: open, read/write, close
  • Use Command-Line Arguments as Values
  • Define and use subroutines
regular expressions
Regular Expressions
  • A regular expression (pattern) is a sequence of characters to be searched for in a character string. In Perl, patterns are normally enclosed in slash characters: /def/
  • Match operator (=~)

$result = $var =~ /def/;

The result of the =~ operation is one of the following:

    • A nonzero value, or true, if the pattern is found in the string
    • 0, or false, if the pattern is not matched
pattern matches
Pattern Matches
  • The !~ operator is similar to =~, except that it checks whether a pattern is not matched.
  • $result = $var !~ /abc/;
  • A program that illustrates the use of the matching operator.

#!/usr/bin/perl

print ("Ask me a question politely:\n");

$question = <STDIN>;

if ($question =~ /please/) { #true if the value stored in $question

# contains the word please

print ("Thank you for being polite!\n");

} else {

print ("That was not very polite!\n");

}

regular expressions and
Regular Expressions* and ?
  • The * special character matches zero or more occurrences of the preceding character.

/de*f/ matches df, def, deef, and so on.

/[eE]*/ matches the empty string as well as any combination of E or e in any order.

  • The ? character matches zero or one occurrence of the preceding character.

/de?f/ matches either df or def

regular expressions1
Regular Expressions [ ]
  • [ ] special characters enable you to define patterns that match one of a group of alternatives.
    • For example: /d[eE]f/ matches def or dEf
  • Combine [ ] with + to match a sequence of characters of any length.
    • For example: d[eE]+f/ matches all of the following: def, dEf, deef, dEef, dEEEeeeEef
regular expressions escape for special characters
Regular Expressions: Escape for Special Characters
  • If you want your pattern to include a character that is normally treated as a special character, precede the character with a backslash \.
  • For example:
    • To check for one or more occurrences of * in a string, use the following pattern: /\*+/
    • To include a backslash in a pattern, specify two backslashes: /\\+/
regular expressions excluding
Regular Expressions: Excluding
  • When the ^ character appears as the first character after the [, it indicates that the pattern is to match any character except the ones displayed between the [ and ].
  • For example, the pattern /d[^eE]f/ matches any pattern that satisfies the following criteria:
    • The first character is d.
    • The second character is anything other than e or E.
    • The last character is f.
regular expressions and1
Regular Expressions^ and $
  • The pattern anchors ^ ensures that the pattern is matched only at the beginning of a string.

/^def/ matches def only if these are the first three characters in the string.

  • The pattern anchors $ ensures that the pattern is matched only at the end of a string.

/def$/ matches def only if these are the last three characters in the string.

  • What does /^def$/ mean?
regular expressions b and b
Regular Expressions\b and \B
  • The \b pattern anchor specifies that the pattern must be on a word boundary.
    • /\bdef/ matches only if def is the beginning of a word. This means that def and defghi match but abcdef does not.
    • You can also use \b to indicate the end of a word. For example, /def\b/ matches def and abcdef, but not defghi.
  • The \B pattern anchor is the opposite of \b. \B matches only if the pattern is contained in a word.
    • /\Bdef/ matches abcdef, but not def.
    • /def\B/ matches defghi.
    • /\Bdef\B/ matches cdefg or abcdefghi, but not def, defghi, or abcdef
  • The \b and \B pattern anchors enable you to search for words in an input line without having to break up the line using split.
regular expressions usages
Regular ExpressionsUsages
  • Conditional Matches using Regular Expressions

$dataVariable =~ / please/ ;

  • Substitutions using Regular Expressions

s/pattern/replacement/options;

$pet =~ s/\bcat\b/dog/ig ; # i: case insensitive

$userinput =~ s/\n//g ; # remove the new line characters

$string =~ s/^\s+// ;# strip leading spaces from a string

$string =~ s/\s+$// ; # strip trailing spaces from a string

advanced perl programming objectives1
Advanced Perl ProgrammingObjectives
  • Pattern Matches
  • File operations: open, read/write, close
  • Use Command-Line Arguments as Values
  • Define and use subroutines
how perl accesses disk files
How Perl Accesses Disk Files
  • Perl uses filehandles to reference files
  • A filehandle is the name for an I/O connection between your Perl program and the operating system, and it can be used inside your program to open, read, write, and close the file
  • Nearly all program functions are written to return a value that indicates whether the function was carried out successfully
file open
File Open

Syntax: open (filehandle, filename);

  • filehandle represents the name you want to use in your Perl program to refer to the file.
  • filename represents the location of the file on your machine.
  • If open returns a nonzero value, the file has been opened successfully.
  • If open returns 0, an error has occurred.

if (open(MYFILE, “test.txt")) {

# here\'s what to do if the file opened

}

file open mode
File Open Mode

Read mode: (default mode) open (FILE, "test.txt");

Enables the program to read the existing contents of the file but does not enable it to write into the file

Write mode: open (FILE, “>test.txt");

Destroys the current contents of the file and overwrites them with the output supplied by the program

Append mode: open (FILE, “>>test.txt");

Appends output supplied by the program to the existing contents of the file

close a file
Close a File
  • Syntax: close (filehandle);

close requires one argument: the file handle representing the file you want to close.

  • Once closed, you cannot read from it or write to it without invoking open again.
read from a file
Read from a File
  • Syntax: $var = <filehandle>;
  • A program that reads lines from a file and prints them on the screen.

#!/usr/local/bin/perl

if ( open(MYFILE, “test.txt") ) {

$line = <MYFILE>; # read a line from file

while ($line ne "") { # loop until the end of file

print ($line); # print on the screen

$line = <MYFILE>; # read a line from file

}

}

close(MYFILE);

write to a file
Write to a File
  • Syntax: print filehandle contents;
  • A program that opens two files and copies one into another.

#!/usr/local/bin/perl

open(INFILE, “test.txt") # open in read mode

open(OUTFILE, ">test.txt.bak"); # open in write mode

$line = <INFILE>;

while ($line ne "") {

print OUTFILE ($line); # write into a file

$line = <INFILE>;

}

close(INFILE);

close(OUTFILE);

a comprehensive example
A Comprehensive Example
  • A program that tests whether a file exists before opening it for writing.

#!/usr/local/bin/perl

unless (open(INFILE, "infile")) {

die ("Input file cannot be opened.\n");

}

if (-e "outfile") { die ("Output file already exists.\n");}

unless (open(OUTFILE, ">outfile")) {

die ("Output file cannot be opened.\n");

}

$line = <INFILE>;

while ($line ne "") {

print OUTFILE ($line);

$line = <INFILE>;

}

close(INFILE);

close(OUTFILE);

advanced perl programming objectives2
Advanced Perl ProgrammingObjectives
  • Pattern Matches
  • File operations: open, read/write, close
  • Use Command-Line Arguments as Values
  • Define and use subroutines
command line arguments
Command Line Arguments
  • Perl stores the command-line arguments in @ARGV.
  • $ARGV[0] contains the first argument, $ARGV[1] contains the second argument, etc
  • An Example, input.pl:

#!/usr/bin/perl

$numArgs = $#ARGV + 1;

print “You gave $numArgs command-line arguments.\n";

foreach $argnum (0 .. $#ARGV) {

print "$ARGV[$argnum], ";

}

Running results:

$ Input.pl 1 2 3 4

You gave me 4 command-line arguments.

1, 2, 3, 4

advanced perl programming objectives3
Advanced Perl ProgrammingObjectives
  • Pattern Matches
  • File operations: open, read/write, close
  • Use Command-Line Arguments as Values
  • Define and use subroutines
subroutines
Subroutines
  • In Perl, a subroutine is a separate body of code designed to perform a particular task. A Perl program executes this body of code by calling or invoking the subroutine; the act of invoking a subroutine is called a subroutine invocation.
  • Subroutines serve two useful purposes:
    • break down your program into smaller parts, making it easier to read and understand.
    • enable you to use one piece of code to perform the same task multiple times, eliminating needless duplication
  • Definition

sub subname { statement_block }

  • Invocation

&subname;

subroutines1
Subroutines

A program that uses a subroutine.

#!/usr/local/bin/perl

$total = 0;

&getnumbers;

foreach $number (@numbers) {

$total += $number;

}

print ("the total is $total\n");

sub getnumbers {

$line = <STDIN>;

$line =~ s/^\s+|\s*\n$//g; # removes the leading and trailing white # space (including the trailing newline)

# from the input line.

@numbers = split(/\s+/, $line);

}

summery
Summery
  • Regular Expressions
    • Conditional matches: =~. !~
    • Substitutions: s/pattern/replacement/options;
  • File operations:
    • Open: open(filehandle, filename)
    • Read: $line = <filehandle>;
    • Write: print filehandle $line;
    • Close: close(filehandle);
  • Use Command-Line Arguments as Values: @ARGV
  • Define and use subroutines
    • Define: sub subname { statement_block }
    • Invocation: &subname;
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