"Use of uninitialized value" even though the value is defined

mandrake00 has asked for the wisdom of the Perl Monks concerning the following question:

For the code:
$path = "/main/lvs/"; open(CELLFILE,"./instList.in") || die "Can't open cell list file: $!"; + while (<CELLFILE>) { $cell = $_; chomp($cell); system("rm ./instLvsOut/passfail.tmp"); open(PASSFAIL,">>./instLvsOut/passfail.tmp"); system("ls $path | grep $cell.>./instLvsOut/passfail.tmp"); print "$cell:\n"; @fp0 =<PASSFAIL>; $fp = $fp0[0]; chomp($fp); system("cat ./instLvsOut/passfail.tmp"); print "fp is $fp\n"; }
[download]

I am getting the following error:
Use of uninitialized value in concatenation (.) or string at instLvsReport.pl line 14
I have already defined $fp by commands "@fp0 =<PASSFAIL>;" and "$fp = $fp0[0];". The command "system("cat ./instLvsOut/passfail.tmp");" properly prints out the content of <PASSFAIL>. Any idea why still I get this error? Please help.

Comment on "Use of uninitialized value" even though the value is defined Download Code

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Re: "Use of uninitialized value" even though the value is defined by moritz (Cardinal) on Apr 01, 2011 at 09:52 UTC
Please always `use strict; use warnings;` and declare your variables. You don't check the success of the second open call. If the open() fails, there will be no lines in @fp0 (same if the file exists but is empty), and $fp will be undef. You can also use autodie to prevent such errors. Perl 6 - second systems done right	[reply] [d/l]
Re^2: "Use of uninitialized value" even though the value is defined by mandrake00 (Initiate) on Apr 01, 2011 at 10:12 UTC
Thanks. Still @fp0 =<PASSFAIL> does not seem to accept the content of PASSFAIL. Any idea why?	[reply]
Re^3: "Use of uninitialized value" even though the value is defined by jwkrahn (Abbot) on Apr 01, 2011 at 11:04 UTC
`open(PASSFAIL,">>./instLvsOut/passfail.tmp")` [download] Because you are opening the file for output only and you can't read from that filehandle.	[reply] [d/l]
Re^3: "Use of uninitialized value" even though the value is defined by mandrake00 (Initiate) on Apr 01, 2011 at 10:24 UTC
To check whether the passfail.tmp file is empty, I tried the command system("cat ./instLvsOut/passfail.tmp"); just before the line @fp0 =<PASSFAIL>. It displayed the content of the file properly. It was not empty. Any idea why this code does not accept values in @fp0 =<PASSFAIL>	[reply]
Re: "Use of uninitialized value" even though the value is defined by jpl (Monk) on Apr 01, 2011 at 11:05 UTC
You opened PASSFAIL for output. If you want to read from it, you must open it for reading. Do so after invoking system(), or the file will not yet exist, and the open will fail. As others have noted, it's always wise to check the results of open() and log an error message if it fails.	[reply]
Re^2: "Use of uninitialized value" even though the value is defined by mandrake00 (Initiate) on Apr 01, 2011 at 11:23 UTC
Thank You! This really works! :)	[reply]
Re: "Use of uninitialized value" even though the value is defined by Anonymous Monk on Apr 01, 2011 at 10:01 UTC
Where in the code do you turn on warnings?	[reply]
Re^2: "Use of uninitialized value" even though the value is defined by sumeetgrover (Monk) on Apr 01, 2011 at 10:03 UTC
You can write the following code below the shebang line: `use strict; use warnings;` [download] Hope this helps! Sumeet Grover.	[reply] [d/l]
Re^3: "Use of uninitialized value" even though the value is defined by Anonymous Monk on Apr 01, 2011 at 10:06 UTC
Why, when the code is not strict/warnings compliant?	[reply]
Re^2: "Use of uninitialized value" even though the value is defined by mandrake00 (Initiate) on Apr 01, 2011 at 10:26 UTC
I am using -w option at the first line.	[reply]