Read and Write Fileswith Perl

Paolo Marcatili - Programmazione 08-09

2

Agenda

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

Paolo Marcatili - Programmazione 08-09

Perl IO

(IO means Input/Output)

Paolo Marcatili - Programmazione 08-09

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Why IO?

Since now, Perl is

#! /usr/bin/perl -wuse strict; <- ALWAYYYYYSSSS!!!my $string="I love tomatoes!\n";for (my $i=1;$i<100;$i++){ print $string;}

Paolo Marcatili - Programmazione 08-09

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Why IO?

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

Paolo Marcatili - Programmazione 08-09

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Why IO?

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

IO can do this!

Paolo Marcatili - Programmazione 08-09

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IO types

Paolo Marcatili - Programmazione 08-09

Main Inputs> Keyboard> File> ErrorsMain outputs> Display> File

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More than tomatoes

Let’s try it:

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

Paolo Marcatili - Programmazione 08-09

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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”

Paolo Marcatili - Programmazione 08-09

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Handles

Handles are just streams “nicknames”Some of them are fixed:STDIN <-default is keyboardSTDOUT <-default is displaySTDERR <-default is displaySome are user defined (files)

Paolo Marcatili - Programmazione 08-09

Open/Write a file

Paolo Marcatili - Programmazione 08-09

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open

We have to create a handle for our fileopen(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

Paolo Marcatili - Programmazione 08-09

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close

When finished we have to close it:close OUT;

If you dont, Santa will bring no gift.

Paolo Marcatili - Programmazione 08-09

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Print OUT

#! /usr/bin/perl -wuse 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

Paolo Marcatili - Programmazione 08-09

Read from Files

Paolo Marcatili - Programmazione 08-09

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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;

Paolo Marcatili - Programmazione 08-09

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Read with loops

Problems:> It’s long> File size unknown

solution:Use loops

Paolo Marcatili - Programmazione 08-09

While loop

Paolo Marcatili - Programmazione 08-09

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While

while (condition){ do something…

}

Paolo Marcatili - Programmazione 08-09

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While - for differences

While

> Undetermined> No counter

Paolo Marcatili - Programmazione 08-09

For

> Determined> Counter

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While example

Approx. solution of x^2-2=0

my $err=1;my $sol=0;while ($err>.1){$sol+=.0001;$err=$sol**2-2;}

Paolo Marcatili - Programmazione 08-09

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Read with while

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

IG.pdb: $!");

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

Paolo Marcatili - Programmazione 08-09

Redirect outputs

Paolo Marcatili - Programmazione 08-09

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Redirections

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

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

close(STDOUT);close(STDERR);