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Re^4: File Reading and Closing confusion

by repellent (Priest)
on Aug 09, 2010 at 03:49 UTC ( #853722=note: print w/ replies, xml ) Need Help??


in reply to Re^3: File Reading and Closing confusion
in thread File Reading and Closing confusion

I was referring to being wary when $line=<FH> is used on its own. To illustrate:

$ perl -e 'print "with_ending\nno_ending"' > a.txt $ perl -MData::Dumper open(FH, "<", "a.txt") or die $!; my $line1 = <FH>; my $line2 = <FH>; my $line3 = <FH>; close FH; $Data::Dumper::Useqq = 1; print Dumper [$line1, $line2, $line3]; __END__ $VAR1 = [ "with_ending\n", "no_end", undef ];

We see that there is a possibility that a line read ($line3 in this case) may return undef. That's what I meant: we need to be wary that a line read will not always return defined.


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Re^5: File Reading and Closing confusion
by shmem (Canon) on Aug 09, 2010 at 09:30 UTC
    I was referring to being wary when $line=<FH> is used on its own.

    Yes, but while takes care of that - and we/you were talking about a while loop, weren't you?

    1.    while (<>) {
    ...
    
    which is Perl short-hand for the more explicitly written version:
    1.    LINE: while (defined($line = <ARGV>)) {
    ...
    

    But of course,

    We see that there is a possibility that a line read ($line3 in this case) may return undef.

    - if you read past EOF, for instance. Then readline returns undef:

    open(FH, "<", "a.txt") or die $!; my $line1 = <FH>; my $line2 = <FH>; print "eof!\n" if eof FH; my $line3 = <FH>; close FH; __END__ eof!
    That's what I meant: we need to be wary that a line read will not always return defined.

    That's exactly the condition when both the while($line = <FH>) { } and while(defined($line = <FH>)) { } loops terminate, so there's no difference in a while loop.

        Yes, but while takes care of that - and we/you were talking about a while loop, weren't you?

      Yes and no. I was talking about:
      while (<FH>) { ... }

      VS
      $line = <FH>; # on its own / not as while condition

      Given the OP's prelude, I really thought that the OP was referring to these two cases. Sorry for the confusion! :)

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