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=== Files and I/O ===

=== Files and I/O ===

You can open a file for input or output using the open() function. It's documented in extravagant detail in perlfunc and perlopentut, but in short:

You can open a file for input or output using the [http://perldoc.perl.org/functions/open.html open()] function. It's documented in extravagant detail in [http://perldoc.perl.org/perlfunc.html perlfunc] and [http://perldoc.perl.org/perlopentut.html perlopentut], but in short:

     open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";

     open(my $in, "<", "input.txt") or die "Can't open input.txt: $!";

     open(my $log, ">>", "my.log") or die "Can't open my.log: $!";

     open(my $log, ">>", "my.log") or die "Can't open my.log: $!";

You can read from an open filehandle using the <> operator. In scalar context it reads a single line from the filehandle, and in list context it reads the whole file in, assigning each line to an element of the list:

You can read from an open filehandle using the '''<>''' operator. In scalar context it reads a single line from the filehandle, and in list context it reads the whole file in, assigning each line to an element of the list:

     my $line = <$in>;

     my $line = <$in>;

Reading in the whole file at one time is called slurping. It can be useful but it may be a memory hog. Most text file processing can be done a line at a time with Perl's looping constructs.

Reading in the whole file at one time is called slurping. It can be useful but it may be a memory hog. Most text file processing can be done a line at a time with Perl's looping constructs.

The <> operator is most often seen in a while loop:

The '''<>''' operator is most often seen in a '''while''' loop:

     while (<$in>) { # assigns each line in turn to $_

     while (<$in>) { # assigns each line in turn to $_

     }

     }

We've already seen how to print to standard output using print(). However, print() can also take an optional first argument specifying which filehandle to print to:

We've already seen how to print to standard output using '''[http://perldoc.perl.org/functions/print.html print()]'''. However, '''[http://perldoc.perl.org/functions/print.html print()]''' can also take an optional first argument specifying which filehandle to print to:

     print STDERR "This is your final warning.\n";

     print STDERR "This is your final warning.\n";

     print $log $logmessage;

     print $log $logmessage;

When you're done with your filehandles, you should close() them (though to be honest, Perl will clean up after you if you forget):

When you're done with your filehandles, you should '''[http://perldoc.perl.org/functions/close.html close()]''' them (though to be honest, Perl will clean up after you if you forget):

     close $in or die "$in: $!";

     close $in or die "$in: $!";