Uninitialized string warning

RICH15ARD has asked for the wisdom of the Perl Monks concerning the following question:

I'm learning Perl through a book I'm reading and I'm doing the exercises in the chapters. This one asks me to write a program to accept input until a "." appears in a line by itself...then print, in reverse order, the input back. Here's my code:

#!Perl -w

use strict;
use diagnostics;

my (@lines);

while (<> and $_ ne ".\n") {
    push @lines,$_;
}
foreach (reverse @lines) {
    print;
}
[download]

I'm getting an error message and I don't know what it means. It says something about an uninitialized string in ne. <W uninitialized> An undefined value was used as if it were already declared.... What am I doing wrong?

There are some things we will never know... And we will never know that we don't know them...

20050511 Edit by castaway: Changed title from 'Newbie Question'

Comment on Uninitialized string warning Download Code

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Re: Uninitialized string warning by Sandy (Curate) on May 11, 2005 at 19:53 UTC
Hi Richard, I am not certain of the exact reasons, but I did have the same problem myself once, and it boils down to: The automatic initialization of $_ happens only if the while loop looks like `while (<>)` Anything else fouls it up. e.g. This works `# ctrl -c to exit (this is only example code!!!) while (<>) { print "\$_ = <$_>\n"; }` [download] e.g. This does not work `# ctrl -c to exit (this is only example code!!!) while (<> and 1) { print "\$_ = <$_>\n"; }` [download]	[reply] [d/l] [select]
Re^2: Uninitialized string warning by VSarkiss (Monsignor) on May 11, 2005 at 20:36 UTC
Actually, perldoc perlop, in the section on "I/O operators", is very explicit about it: Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;)loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously. (emphasis mine) Do not rebuke them with harsh words ... but rather lead them gently - with URLs - so that they may learn wisdom.	[reply]
Re: Uninitialized string warning by ikegami (Patriarch) on May 11, 2005 at 19:56 UTC
What the others failed explain is that `<>` only assigns it's value to $_ when it is the entirety of the `while`'s condition. When you added the `and` bit, it ceased to be special. In other words, `while (<>) {` is equivalent to `while (defined($_ = <>)) {` but `while (<> and ...) {` is equivalent to `while (<> and ...) {` Notice the lack of `$_ =` in the second case. That's why `while (defined($_ = <>) and $_ ne ".\n") {` is needed. Update: It seems that Sandy explained the same thing while I was writting my reply.	[reply] [d/l] [select]
Re^2: Uninitialized string warning by Anonymous Monk on May 11, 2005 at 21:55 UTC
Update: It seems that Sandy explained the same thing while I was writting my reply. I am curious. What is the purpose of this update?	[reply]
Re^3: Uninitialized string warning by ikegami (Patriarch) on May 11, 2005 at 22:06 UTC
It's a retraction of my "What the others failed explain" qualifier.	[reply]
Re: Uninitialized string warning by xorl (Deacon) on May 11, 2005 at 19:41 UTC
You don't want the and $_ ne ".\n" in the condition statment. Take that out and put something like: if ($_ eq ".\n") {last;} just before the push line. `#!/usr/bin/perl -w use strict; use diagnostics; my (@lines); while (<>) { if ($_ eq ".\n") { last;} push @lines,$_; } foreach (reverse @lines) { print; }` [download]	[reply] [d/l]
Re: Uninitialized string warning by Adrade (Pilgrim) on May 11, 2005 at 21:22 UTC
another one just for fun `unshift(@w,$q) while(($q = <>) && ($q ne ".\n")); print @w` [download] Best, -Adam	[reply] [d/l]
Re: Uninitialized string warning by phaylon (Curate) on May 11, 2005 at 21:05 UTC
Just for fun: `use warnings; use strict; my @lines; push @lines, $_ while do { $_ = <> } ne ".\n"; print reverse @lines;` [download] Ordinary morality is for ordinary people. -- Aleister Crowley	[reply] [d/l]
Re: Uninitialized string warning by Transient (Hermit) on May 11, 2005 at 19:45 UTC
Or you could change it to: `use strict; my (@lines); while (defined($_ = <>) and $_ ne ".\n") { push @lines,$_; } foreach (reverse @lines) { print; }` [download]	[reply] [d/l]