This document provides an overview of input/output (IO) operations in Perl, including how to open and read/write to files. It discusses opening files and assigning them to handles, as well as using open, print, and close to write to a file. It describes using while loops to read from files line by line to avoid reading the entire file at once. The document also mentions redirecting program outputs, using command line arguments via the @ARGV array, and the $_ variable. Finally, it briefly introduces opendir and readdir for reading the contents of directories.

1. Read and Write Files
with Perl
Paolo Marcatili - Programmazione 09-10

2. Agenda
> Perl IO
> Open a File
> Write on Files
> Read from Files
> While loop
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3. Perl IO
(IO means Input/Output)
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4. 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;
}
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5. Why IO?
But if we want to do the same
with a user-submitted string?
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6. Why IO?
But if we want to do the same
with a user-submitted string?
IO can do this!
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7. IO types
Main Inputs
> Keyboard
> File
> Errors
Main outputs
> Display
> File
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8. 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;
}
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9. 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”
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10. 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)
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11. Open/Write a file
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12. 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
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13. close
When finished we have to close it:
close OUT;
If you dont, Santa will bring no gift.
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14. 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
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15. Read from Files
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16. 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;
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17. Read with loops
Problems:
> It’s long
> File size unknown
solution:
Use loops
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18. While loop
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19. While
while (condition){
do something…
}
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20. While - for differences
While For
> Undetermined > Determined
> No counter > Counter
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21. 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”;
}
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22. 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;
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23. Redirect outputs
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24. 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);
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25. @ARGV
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26. 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!
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27. 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.
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28. $_
> Perl default scalar value that is used when a
variable is not explicitly specified.
> Can be used in
> For Loops
> File Handling
> Regular Expressions
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29. $_ 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.
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30. $_ 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 $_
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31. Opendir, readdir
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32. Opendir & readdir
> Just like open, but for dirs
# load all files of the "data/" folder into the @files array
opendir(DIR, ”$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>";
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