Beefy Boxes and Bandwidth Generously Provided by pair Networks
There's more than one way to do things

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
Thanks! Learn something every day! (well, lots today). But what is going on here? What (under the covers) makes this a special case? Why is Perl wrapping $_=<> in defined($_ =<>)? Another convenience feature? And where else does this happen?

Here's my output with a slightly expanded print line checking both the value and the defined status and just to make sure there was no monkey business with the special variable $_ I changed it to $line - same result as you, of course. (I had to switch " and ' to keep the bash shell from trying to resolve $_):

$ perl -e'print 0' | perl -e'while ($line = <>) { print "\$line=<$lin +e> \$line=<" . ($line ? "true" : "false") . "> defined=<" . (defined( +$line) ? "true" : "false") . ">\n"};' $line=<0> $line=<false> defined=<true>

It is also not anything special about the syntax $somevar = <> because I get the same result if I use readline(...):

$ perl -e'print 0' | perl -e'while ($line = readline(*STDIN)) { print +"\$line=<$line> \$line=<" . ($line ? "true" : "false") . "> defined=< +" . (defined($line) ? "true" : "false") . ">\n"};' $line=<0> $line=<false> defined=<true>

Or is it? Could it be that both while($somevar = readline(...)) and while($somevar = <...>), but only these, have special semantics? I noticed that if I wrap readline in a function of my own that the special behavior goes away. For example,

use strict; use warnings; #The following all output: # BEFORE # $line=<0> $line=<false> defined=<true> # AFTER # # while (my $line = readline(*STDIN)) { # while (my $line = <STDIN>) { # while (my $line = <>) { # # but wrapping readline in a sub produces only # BEFORE # AFTER print "BEFORE\n"; while (my $line = wrapped_readline(\*STDIN)) { print "\$line=<$line> \$line=<" . ($line ? "true" : "false") . "> defined=<" . (defined($line) ? "true" : "false") . ">\n"; } print "AFTER\n"; sub wrapped_readline { my $fh = shift @_; my $sLine = readline($$fh); #print "wrapped_readline: <$sLine>\n"; return $sLine; }

Thanks in advance, beth

Update: refined question a bit; fixed error in sub of $_ with $line; added examples using readline and a sub wrapping readline.

In reply to Re^3: "defined" in while loop Edit | Delete | Quote | Reply | Private Reply by ELISHEVA
in thread "defined" in while loop Edit | Delete | Quote | Reply | Private Reply by manish.rathi

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or How to display code and escape characters are good places to start.
Log In?

What's my password?
Create A New User
Domain Nodelet?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others scrutinizing the Monastery: (2)
As of 2023-10-02 04:26 GMT
Find Nodes?
    Voting Booth?

    No recent polls found