<STDIN> not initializing $

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

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Re: <STDIN> not initializing $_ by pc88mxer (Vicar) on Apr 30, 2008 at 23:05 UTC
Yes, `while (<STDIN>) { ... }` is special in that it: Reads a line from `STDIN` Assigns the line to `$_` Exits the loop if `STDIN` was at EOF before the read Invoking `<STDIN>` by itself just reads a line but doesn't automatically assign it to `$_`. Note that this is different from merely testing whether or not `$_` is true as is shown by this example: `$/ = "0"; while (<DATA>) { print "got $_ and it ", ($_ ? "is" : "is not"), " true\n"; } __END__ 00000` [download] which emits: `got 0 and it is not true got 0 and it is not true ...` [download]	[reply] [d/l] [select]
Re: <STDIN> not initializing $_ by oko1 (Deacon) on May 01, 2008 at 00:41 UTC
The part that you may be missing is that '$_' is not created by <STDIN> - it's created as a result of the 'while' loop, in the same way that it would be created if you'd used a 'for' loop. `while (<STDIN>){ # Loops over <STDIN> as long as it returns ' +true' print; } while ($_ = <STDIN>){ # Exact equivalent of the above but "spelled + out" explicitly print $_; # Ditto for this line - exactly equvalent to + a simple 'print;' } for (1,2,3){ # Loop over specified list print; # Print out each item (which has been assign +ed to $_) in turn } for $_ (1,2,3){ # Exact equivalent of the above but "spelled + out" explicitly print $_; # Same story }` [download] In general, whenever you see a variable being omitted (e.g., 's/foo/bar/' instead of '$xyz =~ s/foo/bar/'), '$_' is being used. It's part of the standard Perl idiom, and you need to get familiar with it. At the very least, you'll need it to read other people's code. -- Human history becomes more and more a race between education and catastrophe. -- HG Wells	[reply] [d/l]
Re^2: <STDIN> not initializing $_ by ikegami (Patriarch) on May 01, 2008 at 01:02 UTC
Not quite. `while (<STDIN>)` is equivalent to `while ($_ = <STDIN>)` but when spelled out explicitly, they're both `while (defined($_ = <STDIN>))` `>perl -MO=Deparse -e"while (<STDIN>) {}" while (defined($_ = <STDIN>)) { (); } -e syntax OK >perl -MO=Deparse -e"while ($_ = <STDIN>) {}" while (defined($_ = <STDIN>)) { (); } -e syntax OK` [download]	[reply] [d/l] [select]
Re^2: <STDIN> not initializing $_ by ysth (Canon) on May 01, 2008 at 04:17 UTC
Note that the implicit "$_ = " in a while/until expression is only added for <>, readline, or glob operations. There's an implicit defined() also, as ikegami notes, but that's broader. It is added when the while/until condition is a scalar assignment (itself possibly implicit as noted above) from a <>, readline, glob, readdir, or each operation. -- Online Fortune Cookie Search	[reply]
Re^3: <STDIN> not initializing $_ by carol (Beadle) on May 03, 2008 at 11:44 UTC
Thank you all, monks, for your insights regarding my question. I'm currently in the process of reading "Programming Perl". Today I discovered the book explicitly explains `<>` and `$_`. It also confirms your answers, especially ysth's. The first part of `Line Input (Angle) Operator' in `Input Operators' in Chapter 2 covers the subject thoroughly. A short quote: If and only if the line input operator is the only thing inside the conditional of a `while` loop, the value is automatically assigned to the special variable `$_`. After that some useful examples follow.	[reply] [d/l] [select]
Re^3: <STDIN> not initializing $_ by ikegami (Patriarch) on May 01, 2008 at 04:32 UTC
Update: Ignore this post. I read "and a" instead of "from a". It is added when the while/until condition is a scalar assignment As far as I can tell, that's not true. The following prints nothing: `perl -le"while ($a=0) { print 'ysth is right'; last }"` [download] Tested with Perl 5.6.0, 5.6.1, 5.8.0, 5.8.8, 5.10.0.	[reply] [d/l]
Re^4: <STDIN> not initializing $_ by stiller (Friar) on May 01, 2008 at 05:39 UTC
Re^4: <STDIN> not initializing $_ by mscharrer (Hermit) on May 01, 2008 at 07:33 UTC
Re: <STDIN> not initializing $_ by wade (Pilgrim) on Apr 30, 2008 at 23:22 UTC
Yeah, and since $_ can get silently overwritten (especially as you add code and forget that you were counting on $_) I really hate to use it. Instead, I assign everything to my own variables: `use strict; use warnings; while (1) { my $foo = <STDIN>; $foo =~ m/./ and print "success.\n"; # ... (more code) ... }` [download] I'd also tend not to use a `while(1)` loop but that's a whole other Oprah. -- Wade	[reply] [d/l] [select]
Re^2: <STDIN> not initializing $_ by oko1 (Deacon) on May 01, 2008 at 00:30 UTC
> Yeah, and since $_ can get silently overwritten [...] Just about as silently as every other variable in Perl. No matter what your own preferences may be, '$_' is an important and integral part of Perl - and there's no reason to give misleading advice to someone who is trying to learn about it. > [...] I really hate to use it. Say 'goodbye' to grep and map, then. In fact, bid 'good night' to half the functionality of most Perl functions. -- Human history becomes more and more a race between education and catastrophe. -- HG Wells	[reply]
Re^3: <STDIN> not initializing $_ by wade (Pilgrim) on May 01, 2008 at 05:10 UTC
I'm just saying that I like to assign values to variables explicitly when possible. I think it's clearer and a little less error-prone but that's just me. -- Wade	[reply]
Re: <STDIN> not initializing $_ by FunkyMonk (Bishop) on May 04, 2008 at 23:45 UTC
I'm late coming in with this, sorry:( If `while (<STDIN>)` does something you don't expect, the Perl documentation should be your first port of call. You can... google for perldoc while search for while at http://perldoc.perl.org Use `perldoc perlsyn` from the command line (unfortunately you need to know that `while`'s described in perlsyn) If you're using ActivePerl on windows, Perl's documentation can be accessed via Start -> Programs -> ActivePerl -> Documentation. Unless I state otherwise, all my code runs with strict and warnings	[reply] [d/l] [select]