Perl, why can't I do this?

[MrNewbie]

Here's a piece of my perl code, I'm supposed to be reading a number from a file, incrementing it, and writing it back to the file.


if(open(FILE, "+>file.txt")){
my $var = <FILE>;
print "Here, var = $var\n";
$var++;
print "Here, var = $vart\n";
seek(FILE, 0, 2);
print FILE "$var";
close FILE;
}
else{
print "Error opening file";
}

With the

print "Here, var = $var\n";
$var++;
print "Here, var = $vart\n";

The first use of $var causes an error due to the use of an uninitialized variable.
What am I doing wrong? I just can't see it.
Thanks

[klamath]

I think if you have a file open for output, you can't read from it; vice-versa for writing to a file opened for input.

[MrNewbie]

Isn't opening it like this:
open(FILE, "+>file.txt");

Opening for reading and writing?

[iDxMan]

Why don't you make 2 passes at it.


#!/usr/bin/perl

$file = "foo";

open(FILE,"<$file") or die "Error opening $file\n$!\n";
$var = <FILE>;
close(FILE);

$var =~ tr/0-9.//cd;

++$var;

open(FILE,">$file") or die "$!";
print FILE $var;
close(FILE);

[MrNewbie]

I'm going to have too, thanks. I'm confused about what this does though.
$var =~ tr/0-9.//cd;

Drops anything that isn't a number?

[takshaka]

Originally posted by MrNewbie:
if(open(FILE, "+>file.txt")){

perldoc -f open (http://www.perldoc.com/perl5.6.1/pod/func/open.html)
As the docs say, this clobbers the file every time. <FILE> returns undef on an empty file, thus the warning.

If you want to open in read/write mode and automatically create the file if it doesn't exist, you'll need to use sysopen() with the O_RDWR and O_CREAT flags.

perldoc -f sysopen (http://www.perldoc.com/perl5.6.1/pod/func/sysopen.html)
perldoc perlopentut (http://www.perldoc.com/perl5.6.1/pod/perlopentut.html)


print "Here, var = $vart\n";


Typo.

use strict;


seek(FILE, 0, 2);


That sets the filehandle's position to end-of-file. There must be a way to set it to the beginning of a file. If only someone would write down how to do that...

perldoc -f seek (http://www.perldoc.com/perl5.6.1/pod/func/seek.html)

[iDxMan]

Originally posted by MrNewbie:
<STRONG>I'm going to have too, thanks. I'm confused about what this does though.
$var =~ tr/0-9.//cd;

Drops anything that isn't a number?</STRONG>

Just translates anything that's not a number or period out of the string. There for sanity and then some...

-r