Beefy Boxes and Bandwidth Generously Provided by pair Networks
Do you know where your variables are?
 
PerlMonks  

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
I see your motivation, and I get the train of thought that led to your initial assumption -- a hash element is an lvalue, and there's an intuitive sense that a loop variable is being used as an lvalue -- elements of the list are being assigned to it on each iteration.

But that intuitive sense seems to be wrong in a subtle way. Many of us have relied on the documented behavior that results in things like:

my @array = qw/1 2 3 4/; foreach my $elem ( @array ) { $elem *= 5; } print join " ",@array,$/; __OUTPUT__ 5 10 15 20
As pointed out by "perldoc perlsyn": 'the "foreach" loop index variable is an implicit alias for each item in the list that you're looping over.' Another passage from perlsyn, very close afterwards, points out: '"foreach" probably won't do what you expect if VAR is a tied or other special variable. Don't do that...'

Next, we'd have to look closely at how perl stores variables internally (The Damian's excellent book "Object Oriented Perl" has one of the nicest explanations), and try to work out how different a hash element is from a normal scalar. Well, it's late, and I won't try that just now... The point is that the loop variable is not being used as an lvalue -- it's something else with special properties.

update: Regarding perl implementation issues, I would suppose that the compiler has a very rigid sense of for-loop syntax: the thing after "for" (or "foreach") must be a scalar; if there's a curly brace after that, it probably interprets this as the start of a block (not a hash index), and reports the syntax error because there's no list being provided for the iteration. Of course, the error is reported if you're using an array element (square-brackets) as well.


In reply to Re: Surpised by foreach iterator limitation by graff
in thread Surpised by foreach iterator limitation by shotgunefx

Title:
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?
    Username:
    Password:

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

    How do I use this? | Other CB clients
    Other Users?
    Others taking refuge in the Monastery: (3)
    As of 2020-02-23 08:09 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?
      What numbers are you going to focus on primarily in 2020?










      Results (102 votes). Check out past polls.

      Notices?