Re: <STDIN> not initializing $

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

Comment on Re: <STDIN> not initializing $_ Download Code

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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 $_ by stiller (Friar) on May 01, 2008 at 05:39 UTC
An extra edge case: `perl -le'while($a = 0){print "$a ok\n"; last }' # perl -le'while($a = 1){print "$a ok\n"; last }' # prints 1 ok` [download] But the "implicit test for defined" is built into the `while( )` by language design, there is no extra magic about that in any context?	[reply] [d/l] [select]
Re^4: <STDIN> not initializing $_ by mscharrer (Hermit) on May 01, 2008 at 07:33 UTC
You missed the most important part of the quote: It is added when the while/until condition is a scalar assignment .. from a <>, readline, glob, readdir, or each* operation.* The implicit defined is not added for normal scalar assignments.	[reply]