Perl IO

Read and Write Files
with Perl

Paolo Marcatili - Programmazione 09-10

Agenda

> Perl IO
> Open a File
> Write on Files
> Read from Files
> While loop

2

Perl IO

(IO means Input/Output)


Why IO?

Since now, Perl is

#! /usr/bin/perl -w
use strict; <- ALWAYYYYYSSSS!!!
my $string=”All work and no play makes Jack a
dull boyn";
for (my $i=1;$i<100;$i++){
print $string;
}
4

Why IO?

But if we want to do the same
with a user-submitted string?

5

Why IO?

But if we want to do the same
with a user-submitted string?

IO can do this!

6

IO types

Main Inputs
> Keyboard
> File
> Errors
Main outputs
> Display
> File

7

More than tomatoes

Let’s try it:

#! /usr/bin/perl -w
use strict; my $string=<STDIN>;
for (my $i=1;$i<100;$i++){
print $string;
}

8

Rationale

Read from and write to different media

STDIN means standard input (keyboard)
^
this is a handle
<SMTH> means
“read from the source corresponding to handle SMTH”

9

Handles

Handles are just streams “nicknames”
Some of them are fixed:
STDIN <-default is keyboard
STDOUT <-default is display
STDERR <-default is display
Some are user defined (files)

10

Open/Write a ﬁle


open

We have to create a handle for our ﬁle
open(OUT, “>”,”out.txt”) or die(“Error opening out.txt: $!”);
^
N.B. : it’s user deﬁned, you decide it
Tip
“<“,”out.txt” <- read from out.txt
“>”,”out.txt” <- write into out.txt
“>>”,”out.txt” <- append to out.txt

12

close

When ﬁnished we have to close it:
close OUT;

If you dont, Santa will bring no gift.

13

Print OUT

#! /usr/bin/perl -w
use strict;
open(OUT, ">”,”out.txt") || die("Error opening out.txt: $!");
print "type your claim:n";
my $string=<STDIN>;
for (my $i=1;$i<100;$i++){
print OUT $string;
}
close OUT;

Now let’s play with <,>,>> and ﬁle permissions
14

Read from Files


Read

open(IN, “<song.txt”) or die(“Error opening song.txt: $!”);
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;
print <IN>;

close IN;

16

Read with loops

Problems:
> It’s long
> File size unknown

solution:
Use loops

17

While loop


While

while (condition){
do something…

}

19

While - for differences

While For

> Undetermined > Determined
> No counter > Counter

20

While example
Approx. solution of x^2-2=0
(Newton’s method)

my $sol=0.5;
my $err=$sol**2-2;
while ($err>.1){
$sol-=($sol**2-2)/(2*$sol);
$err=$sol**2-2;
print “Error=$errn”;
}
21

Read with while

#! /usr/bin/perl -w
use strict;
open(MOD, "<IG.pdb") || die("Error opening
IG.pdb: $!");

while (my $line=<MOD>){
print substr($line,0,6)."n";
}
close MOD;

22

Redirect outputs


Redirections

#!/usr/bin/perl
open(STDOUT, ">foo.out") || die "Can't redirect
stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";

select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered

close(STDOUT);
close(STDERR);

24

@ARGV


Command Line Arguments

> Command line arguments in Perl are extremely easy.
> @ARGV is the array that holds all arguments passed in from
the command line.
> Example:
> % ./prog.pl arg1 arg2 arg3
> @ARGV would contain ('arg1', arg2', 'arg3)

> $#ARGV returns the number of command line arguments that
have been passed.
> Remember $#array is the size of the array!

26

Quick Program with @ARGV

> Simple program called log.pl that takes in a number
and prints the log base 2 of that number;

#!/usr/local/bin/perl -w
$log = log($ARGV[0]) / log(2);
print “The log base 2 of $ARGV[0] is $log.n”;

> Run the program as follows:
> % log.pl 8
> This will return the following:
> The log base 2 of 8 is 3.

27

$_

> Perl default scalar value that is used when a
variable is not explicitly speciﬁed.
> Can be used in
> For Loops
> File Handling
> Regular Expressions

28

$_ and For Loops

> Example using $_ in a for loop

@array = ( “Perl”, “C”, “Java” );
for(@array) {
print $_ . “is a language I known”;
}

> Output :
Perl is a language I know.
C is a language I know.
Java is a language I know.

29

$_ and File Handlers

> Example in using $_ when reading in a ﬁle;

while( <> ) {
chomp $_; # remove the newline char
@array = split/ /, $_; # split the line on white space
# and stores data in an array
}

> Note:
> The line read in from the ﬁle is automatically store in the
default scalar variable $_

30

Opendir, readdir


Opendir & readdir
> Just like open, but for dirs

# load all files of the "data/" folder into the @files array
opendir(DIR, ”$ARGV[0]");
@files = readdir(DIR);
closedir(DIR);

# build a unsorted list from the @files array:
print "<ul>";
foreach $file (@files) {
next if ($file eq "." or $file eq "..");
print "<li><a href="$file">$file</a></li>";
}
print "</ul>";
32

Perl IO

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