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

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
Hi Dragonchild, actually this isn't quite right

Yes, it is a statement modifier, but it's also a looping construct with its own scope.

In fact modifiers of all kinds do not have their own scope. They execute in the scope of their enclosing block. This can be seen by my $x=1 if $y;$x++; where usage of the the $x after the if does not raise an exception.

The reason it seems like they do when you use the do BLOCK pseudo-function is because in this case the block itself has its own scope. This is illustrated by the code below where the only variable usage that throws use strict; is the one declared inside of a do block.

Another interesting point with the do usage is that the block gets executed an extra time, ie before the first evaluation of the conditional. This can be seen by the output line Do's pre-evaluation:DO{}while

The compound statement forms of the modifiers are different in that they have their own scopes and scoping rules (for instance the for my $i (0..10) {} where the my's scope is actually the contents of the block.

use strict; use warnings; use Tie::Cycle; tie my $cycle, 'Tie::Cycle', [ 1..3,0,reverse (1..3),0 ]; $\=":stmt while\n"; print my $statement=$_ while ($_=$cycle)>0; print defined($statement) ? "Defined" : "Notdefined"; $_="Do's pre-evaluation"; $\=":do{}while\n"; do { print my $do = $_ } while ($_=$cycle)>0; eval 'print defined($do) ? "Defined" : "Notdefined";'; print $@ if $@; $\=":stmt while 0\n"; print my $while="Test" while 0; print defined($while) ? "Defined" : "Notdefined"; $\=":stmt if\n"; print my $if="Test" if 0; print defined($if) ? "Defined" : "Notdefined"; $\=":stmt for\n"; print my $for=$_ for 0; print defined($for) ? "Defined" : "Notdefined"; __DATA__ 1:stmt while 2:stmt while 3:stmt while Notdefined:stmt while Do's pre-evaluation:do{}while 3:do{}while 2:do{}while 1:do{}while Global symbol "$do" requires explicit package name at (eval 1) line 1. :do{}while Notdefined:stmt while 0 Notdefined:stmt if 0:stmt for Notdefined:stmt for
Notice that in the non do {} simple statement form, even when the condition is 0 and one might expect Perl to optimize it away the variable is created in the outside scope.

Yves
--
You are not ready to use symrefs unless you already know why they are bad. -- tadmc (CLPM)


In reply to Re:while modifier( and scoping) by demerphq
in thread while modifier by Anonymous Monk

Title:
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 or How to display code and escape characters are good places to start.
Log In?
Username:
Password:

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

How do I use this? | Other CB clients
Other Users?
Others romping around the Monastery: (2)
As of 2022-08-19 07:45 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    No recent polls found

    Notices?