Perl Global variable uninitialized

Question

I'm new to perl so please bear with me.

I have script that is parsing a CSV file. To make things easier to debug I am using a state machine FSA::Rules (works great love it).

Every thing is going well only now I need to make my logs make sense, as part of this I need to record line numbers so my program looks some thing like this.

my $line = '';
my $lineCount = 0;
sub do {
    ...
    #CSV opened 
    ...
    #State machine stuff happens here
    readLine;
     if ($line =~ m/.*Pattern*/){
         #do stuff
     }
}
sub readLine{
    $line = <CSV>;
    $lineCount ++;
}

But I get the following error

Use of uninitialized value $line in pattern match (m//) at

Any one know why $line would not be initialized? Thanks.

This isn't the code that produces the error. You never call subroutine do, and you shouldn't name a subroutine the same as a reserved word as, for one thing, it's very awkward to call. Please show the code that fails. Do you declare my $line again inside sub do? — Borodin
– Borodin, Commented Feb 1, 2012 at 23:06
The $line variable is not initialized because the contents of $line do not match the supplied regular expression. Please post all your code after adding use strict; and use warnings; after the #! line (which should be the first line of the script). — David Harris
– David Harris, Commented Feb 1, 2012 at 23:15
@David: I'm sorry? Variables don't get uninitialized if they fail to match a regex. And a #! line isn't necessary or even desirable (although strict and warnings are!) — Borodin
– Borodin, Commented Feb 1, 2012 at 23:22
How do you check for end of file? If you don't, for some strange reason, that is probably your error. — TLP
– TLP, Commented Feb 1, 2012 at 23:33
#! lines are not required in Unix, except as an optional mechanism to run your script from the command line without having to specify "/usr/bin/perl" in front of it. — mob
– mob, Commented Feb 1, 2012 at 23:57

mob · Accepted Answer · 2012-02-01 23:26:15Z

4

When you reach end of file, $line = <CSV> will assign the undefined value to $line. The usual idiom is to check whether the readline function (which is implicitly called by the <> operator) returned a good value or not before proceeding ...

 while (my $line = <CSV>) {
     # guaranteed that $line has a defined value
     ...
 }

but you with your sequence of calls, you are avoiding that check. Your current code also increments $lineCount even when <CSV> does not return a good value, which may not be what you want either.

answered Feb 1, 2012 at 23:26

mob

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1 Comment

Eric Strom Over a year ago

You should mention that when written as you have it, perl's optimizer will rewrite the while loop into while (defined(my $line = <CSV>)) {...} which is where the defined test comes from, since a while loop would normally only test for a true value.

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