程序代写代做代考 ada COMP284 Scripting Languages – Handouts

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COMP284 Scripting Languages – Handouts

COMP284 Scripting Languages
Lecture 7: Perl (Part 6)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

1 Input/Output
Filehandles
Open
Close
Read
Select
Print
Here documents

2 Arguments and Options
Invocation Arguments
Options

COMP284 Scripting Languages Lecture 7 Slide L7 – 1

Input/Output Filehandles

I/O Connections

• Perl programs interact with their environment via I/O connections
• A filehandle is the name in a Perl program for such an I/O connection,

given by a Perl identifier
Beware: Despite the terminology, no files might be involved

• There are six pre-defined filehandles
STDIN Standard Input, for user input, typically the keyboard

STDOUT Standard Output, for user output, typically the terminal

STDERR Standard Error, for error output,
typically defaults to the terminal

DATA Input from data stored after __END__ at the end of a
Perl program

ARGV Iterates over command-line filenames in @ARGV

ARGVOUT Points to the currently open output file when doing edit-
in-place processing with -i
perl -pi -e ’s/cat/dog/’ file

COMP284 Scripting Languages Lecture 7 Slide L7 – 2

Input/Output Filehandles

I/O Connections

Except for the six predefined I/O connections, all other I/O connections

• need to be opened before they can be used
open filehandle, mode, expr

• should be closed once no longer needed
close filehandle

• can be used to read from

• can be used to write to
print filehandle list

printf filehandle list

• can be selected as default output
select filehandle

COMP284 Scripting Languages Lecture 7 Slide L7 – 3

Input/Output Filehandles

I/O Connections

Example:

open INPUT , “<", "oldtext.txt" or die "Cannot open file"; open OUTPUT , ">“, “newtext.txt”;

while () {

s!(\d+) degrees Fahrenheit!

sprintf(“%d” ,(($1 -32)*5/9)+0.5).” degrees Celsius”!e;

print OUTPUT;

}

close(INPUT );

close(OUTPUT );

oldtext.txt:

105 degrees Fahrenheit is quite warm

newtext.txt:

41 degrees Celcius is quite warm

COMP284 Scripting Languages Lecture 7 Slide L7 – 4

Input/Output Open

Opening a filehandle

open filehandle, expr

open filehandle, mode, expr

• Opens an I/O connection specified by mode and expr and associates it
with filehandle

• expr specifies a file or command
• mode is one of the following

Mode Operation Create Truncate

< read file > write file yes yes
>> append file yes

+< read/write file +> read/write file yes yes
+>> read/append file yes

|- write to command yes
-! read from command yes

COMP284 Scripting Languages Lecture 7 Slide L7 – 5

Input/Output Close

Closing a filehandle

close

close filehandle

• Flushes the I/O buffer and closes the I/O connection associated with
filehandle

• Returns true if those operations succeed
• Closes the currently selected filehandle if the argument is omitted

COMP284 Scripting Languages Lecture 7 Slide L7 – 6

Input/Output Read

Reading

• In a scalar context, returns a string consisting of all characters from
filehandle up to the next occurrence of $/
(the input record separator)

• In a list context, returns a list of strings representing the whole content
of filehandle separated into string using $/ as a separator
(Default value of $/: newline \n)

1 open INPUT , “<", "oldtext.txt" or die "Cannot open file"; 2 $first_line = ;
3 while ($other_line = ) { … }
4 close INPUT;
5
6 open LS, ” -|”, “ls -1”;
7 @files = ;
8 close LS;
9 foreach $file (@files) { … }

COMP284 Scripting Languages Lecture 7 Slide L7 – 7

Input/Output Select

Selecting a filehandle as default output

select

select filehandle

• If filehandle is supplied, sets the new current default filehandle for
output
; write or print without a filehandle default to filehandle
; References to variables related to output will refer to filehandle

• Returns the currently selected filehandle

COMP284 Scripting Languages Lecture 7 Slide L7 – 8

Input/Output Print

Printing

print filehandle list

print filehandle

print list

print

• Print a string or a list of strings to filehandle
• If filehandle is omitted, prints to the last selected filehandle
• If list is omitted, prints $_
• The current value of $, (if any) is printed between each list item

(Default: undef)

• The current value of $\ (if any) is printed after the entire list has
been printed
(Default: undef)

COMP284 Scripting Languages Lecture 7 Slide L7 – 9

Input/Output Print

Printing: Formatting

sprintf(format, list)

• Returns a string formatted by the usual printf conventions of the C
library function sprintf (but does not by itself print anything)

sprintf “(%10.3f)” 1234.5678

format a floating-point number with minimum width 10 and precision 3
and put the result in parentheses:

( 1234.568)

See http://perldoc.perl.org/functions/sprintf.html for
further details

COMP284 Scripting Languages Lecture 7 Slide L7 – 10

http://perldoc.perl.org/functions/sprintf.html

Input/Output Print

Printing: Formatting

printf filehandle format, list

printf format, list

• Equivalent to
print filehandle sprintf(format, list)

except that $\ (the output record separator) is not appended

COMP284 Scripting Languages Lecture 7 Slide L7 – 11

Input/Output Print

Printing: Formatting

Format strings can be stored in variables and can be constructed
on-the-fly:

@list = qw(wilma dino pebbles );

$format = “The items are:\n”. (“%10s\n” x @list );

printf $format , @list;

Output:

The items are:

wilma

dino

pebbles

(The code above uses the ‘quote word’ function qw()
to generate a list of words.
See http://perlmeme.org/howtos/perlfunc/qw_function.html
for details)

COMP284 Scripting Languages Lecture 7 Slide L7 – 12

http://perlmeme.org/howtos/perlfunc/qw_function.html

Input/Output Here documents

Here documents

• A here document is a way of specifying multi-line strings in a scripting
or programming language

• The basic syntax is
<

$title

$title

Lots of HTML markup here

END

The double-quotes in “END”
indicate that everything be-
tween the opening “END” and
the closing END should be
treated like a double-quoted
string

Content -type: text/html

My HTML document

My HTML document

Lots of HTML markup here

COMP284 Scripting Languages Lecture 7 Slide L7 – 14

Input/Output Here documents

Here documents: Single-quotes

$title = “My HTML document”

print <<’END’; Content -type: text/html

$title

END

The single-quotes in ’END’ indicate that everything between ’END’ and
END should be treated like a single-quoted string
; no variable interpolation is applied
; $title will not be expanded
Content -type: text/html

$title

END

COMP284 Scripting Languages Lecture 7 Slide L7 – 15

Input/Output Here documents

Here documents: Backticks

$command = “ls”;

print <<‘END ‘; $command -1 END The backticks in ‘END‘ tell Perl to run the here document as a shell script (with the here document treated like a double-quoted string) handouts.aux handouts.log handouts.pdf handouts.tex COMP284 Scripting Languages Lecture 7 Slide L7 – 16 Input/Output Here documents Here documents: Variables Here documents can be assigned to variables and manipulated using string operations $header = <<"HEADER"; Content -type: text/html

$title

HEADER

$body = <<"BODY";

$title

Lots of HTML markup here

BODY

$html = $header.$body;

print $html;

COMP284 Scripting Languages Lecture 7 Slide L7 – 17

Arguments and Options Invocation Arguments

Invocation Arguments

• Another way to provide input to a Perl program are
invocation arguments (command-line arguments)

./ perl_program arg1 arg2 arg3

• The invocation arguments given to a Perl program are stored in the
special array @ARGV

perl_program1:

print “Number of arguments: ” ,$#ARGV+1,”\n”;

for ($index =0; $index <= $#ARGV; $index ++) { print "Argument $index: $ARGV[$index],"\n"; } ./ perl_program1 ada ’bob’ 2 Output: Number of arguments: 3 Argument 0: ada Argument 1: bob Argument 2: 2 COMP284 Scripting Languages Lecture 7 Slide L7 – 18 Arguments and Options Options Options • There are various Perl modules that make it easier to process command-line options –scale=5 –debug –file=’image.png’ • One such module is Getopt::Long: http://perldoc.perl.org/Getopt/Long.html • The module provides the GetOptions function • GetOptions parses the command line arguments that are present in @ARGV according to an option specification • Arguments that do not fit to the option specification remain in @ARGV • GetOptions returns true if @ARGV can be processed successfully COMP284 Scripting Languages Lecture 7 Slide L7 – 19 http://perldoc.perl.org/Getopt/Long.html Arguments and Options Options Options: Example perl_program2: use Getopt ::Long; my $file = "photo.jpg"; my $scale = 2; my $debug = 0; $result = GetOptions ("debug" => \$debug , # flag

“scale=i” => \$scale , # numeric

“file=s” => \$file); # string

print “Debug: $debug; Scale: $scale; File: $file\n”;

print “Number of arguments: ” ,$#ARGV+1,”\n”;

print “Arguments: “,join(“,”,@ARGV), “\n”;

./ perl_program2 –scale =5 –file=’image.png’ arg1 arg2

Debug: 0; Scale: 5; File: image.png

Number of arguments: 2

Arguments: arg1 , arg2

COMP284 Scripting Languages Lecture 7 Slide L7 – 20

Arguments and Options Options

Revision

Read

• Chapter 5: Input and Output

of

R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
O’Reilly, 2011.

• http://perldoc.perl.org/perlop.html#I%2fO-Operators
• http://perldoc.perl.org/perlop.html#Quote-Like-Operators
• http://perldoc.perl.org/Getopt/Long.html

COMP284 Scripting Languages Lecture 7 Slide L7 – 21

http://perldoc.perl.org/perlop.html#I%2fO-Operators
http://perldoc.perl.org/perlop.html#Quote-Like-Operators
http://perldoc.perl.org/Getopt/Long.html

Lecture 7
Input/Output
Filehandles
Open
Close
Read
Select
Print
Here documents

Arguments and Options
Invocation Arguments
Options

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