Beefy Boxes and Bandwidth Generously Provided by pair Networks
Don't ask to ask, just ask

Comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

Usually, while( EXPRESSION ) stops looping when the expression becomes false, but Perl has an exception to that rule: if the expression is just reading a line from a file (and perhaps assigning it to a variable), then it only stops looping when the expression is undefined (undefined is false of course, but there are certain values which are false but defined).

This works in conjunction with the fact that reading a line from a file that has reached its end will return undef.

Observe the difference:

$ echo -n "0" | perl -E'while(<>) { say "GOT" }' GOT $ echo -n "0" | perl -E'while(<> and 1) { say "GOT" }' $

The first one prints "GOT" even though it read a false value from STDIN. This is because the special "reading from a filehandle" version of while gets used. The second doesn't print "GOT". This is because the more complex expression forces while to use its standard behaviour.

There is another interesting aspect of the reading-from-filehandle version of while: if you don't assign the line to a variable, it gets assigned to $_. This doesn't happen when you read from a filehandle normally - only within while. Observe:

$ echo "Hello" | perl -E'while(<>){ print $_ }' Hello $ echo "Hello" | perl -E'<>; print $_' $

So anyway, while (<$fh>) works out as being equivalent to:

while (defined($_ = <$fh>))

... which (by design, not just by luck) happens to be the kind of loop most people usually want when they're reading from a file.

I suppose the other thing that is worth mentioning is the special variable $/ which is used by readline (an thus by <$fh>) to determine the current line ending. By default it's "\n" but you can set it to anything else you like, including undef.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: How does while works with filehandles? by tobyink
in thread How does while works with filehandles? by bhargava_hemant7

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

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and all is quiet...

    How do I use this? | Other CB clients
    Other Users?
    Others perusing the Monastery: (5)
    As of 2018-03-18 00:57 GMT
    Find Nodes?
      Voting Booth?
      When I think of a mole I think of:

      Results (228 votes). Check out past polls.