Hi Lee,
On Sun, 23 Jun 2013 04:43:58 +0200
lee <l...@yun.yagibdah.de> wrote:
> James Alton <jamesalton...@gmail.com> writes:
>
> > lee,
> >
> > You have a post statement if and then a code block. You can only use one of
> > two forms:
> >
> > print "test" if $color eq "blue"; #no parenthesis required
> > if($color eq "blue"){print "test";}
>
> And I can't have 'last if $color eq "blue" print "test\n";'? That would
> be something quite natural and a logical thing to do ...
Do you want something like:
if ($color eq "blue")
{
last;
}
else
{
print "test\n";
}
Because I cannot parse the meaning of your other statement.
>
> > As far as last, http://perldoc.perl.org/functions/last.html
> > The example given:
> >
> > LINE: while (<STDIN>) {
> > last LINE if /^$/; # exit when done with header
> > #...
> > }
> >
> > What they're saying here is that it breaks out of a loop. Its not like a
> > return where it breaks out of a subroutine, it just continues beyond the
> > loop.
>
> Yes, I looked at that and found it rather confusing. Now I guess it
> breaks out of the current (innermost) loop unless a label is being used
> which would allow to break out of whatever loop the label refers to,
> like:
>
>
> BREAK: while($loop_1) {
> # do something with $color and $loop_2
> while($loop_2) {
> last BREAK if $color eq "blue";
> print "not blue\n";
> # do something with $color and $loop_2
> }
> }
> print "broken\n";
>
Yes, Perl Best Practices recommends to always use a label with "last", "redo"
or "next" to avoid problems where you add or remove more inner loops. I agree
with it here:
http://perl-begin.org/tutorials/bad-elements/#flow-stmts-without-labels
Yesterday, I introduced a bug or two in my C code (which were luckily caught by
the tests) because I removed a loop and as result a few C "continue" statements
(which is equivalent to Perl 5’s "next") terminated the more previously outer
loop. It is possible that the C compilers of
http://en.wikipedia.org/wiki/GNU_Compiler_Collection and
http://en.wikipedia.org/wiki/Clang have extensions for doing that similar to
what Perl has, but I couldn't find them yet.
>
> The really confusing part is/was what the 'last' statement actually does
> when used in conjunction with an 'if'. Apparently it re-defines the
> meaning of 'if' --- or there are multiple versions of 'if' in perl:
No, it does not.
> One
> version is the "usual" one like 'if(condition) { foo; } else { bar; }'.
> The other one is some sort of appendix which is logically prepended
> to the statement and, unfortunately, lacks the ability to do something
> besides the statement it is appended to. Maybe '$color eq "blue" ?
> print "test" : print "red\n" if $color eq "red";' ...
>
Actually, it is appended instead of prefixed, and you can do any single
statement inside that, like:
$x = $y if COND();
print "Hello\n" if COND();
etc.
See http://perldoc.perl.org/perlsyn.html#Statement-Modifiers
Also, please don't use the ternary conditional operator for side-effects, see:
http://perl-begin.org/tutorials/bad-elements/#ternary_operator_instead_of_if_else
> So how about:
>
>
> {
> print "123\n";
> print "456\n";
> } if $color eq "blue";
>
>
> That doesn't compile, so this preappended 'if' is like implemented only
> half way in both directions :/
A trailing if applies only to one statement (by design).
You can use:
do {
print "123\n";
print "456\n";
} if $color eq "blue";
but in this case I believe a normal conditional with the condition at the
beginning would be preferable.
Regards,
Shlomi Fish
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