do while loop isn't a really loop

appleii has asked for the wisdom of the Perl Monks concerning the following question:

do while loop isn't a really loop? so the loop control statements next, last, or redo cannot be used to leave or restart the block, so the code below will not be complied:

do
{
    last;
} while (1);
[download]

But this code will be OK:

do
{
    next if (not defined $_);
    print $_, "\n";
    next;
} for (1 .. 10);
[download]

What's the differentiation between them? And I wonder why the 'do for loop' can work.

Update:

It's a bug or a speciality?

Comment on do while loop isn't a really loop Select or Download Code

Replies are listed 'Best First'.
Re: do while loop isn't a really loop? by keszler (Priest) on Nov 12, 2009 at 03:03 UTC
`> perldoc -f do ... "do BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. See perlsyn for alternative strategies.` [download]	[reply] [d/l]
Re: do while loop isn't a really loop? by ikegami (Patriarch) on Nov 12, 2009 at 03:03 UTC
`next`, `last` and `redo` can only be used to leave or restart a loop block. You can't use them in blockless loops like `EXPR while EXPR;` and `EXPR for EXPR;`. Remember, `do` is just another function. `next`, `last` and `redo` can't be used to leave or restart a blockless `while` loop (`EXPR while EXPR;`). Remember, `do` is just another function. Update: My initial explanation was incorrect. They work with blockless foreach and counting loops (`EXPR for EXPR;`). `>perl -wle"print('x'),last while 1; print('y');" x Can't "last" outside a loop block at -e line 1. >perl -wle"print('x'),last for qw(a b c); print('y');" x y` [download] `>perl -wle"do { print('x'); last } while 1; print('y');" x Can't "last" outside a loop block at -e line 1. >perl -wle"do { print('x'); last } for qw(a b c); print('y');" x y` [download] That's rather inconsistent.	[reply] [d/l] [select]
Re^2: do while loop isn't a really loop? by appleii (Novice) on Nov 12, 2009 at 03:44 UTC
It's a perl's bug?	[reply]
Re^3: do while loop isn't a really loop? by ikegami (Patriarch) on Nov 12, 2009 at 04:49 UTC
That `redo`, `next` and `last` control blockless `for` loops? perlsyn could be clearer. That `redo`, `next` and `last` don't control blockless `while` loops? perlsyn is extremely clear on that, but the documentation for `redo`, `next` and `last` could be clearer.	[reply] [d/l] [select]