Newsgroups: comp.lang.perl
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!convex!usenet
From: Tom Christiansen <tchrist@convex.COM>
Subject: Re: two perl questions
Message-ID: <1991Jun08.060025.21067@convex.com>
Sender: usenet@convex.com (news access account)
Nntp-Posting-Host: pixel.convex.com
Reply-To: tchrist@convex.COM (Tom Christiansen)
Organization: CONVEX Software Development, Richardson, TX
References: <JBRYANS.91Jun6175843@beach.csulb.edu> <1991Jun7.163638.7834@iWarp.intel.com> <$&bHr-=p@cs.psu.edu>
Date: Sat, 08 Jun 1991 06:00:25 GMT
Lines: 127

Felix Lee wrote, quoting Randal:
:>Array can be forced by putting it into a list, or by assigning into
:>an array.
:
:"putting it into a list" is not quite enough.  Perl has the real
:problem that lists are indistinguishable from the comma operator.
:	$x = (5, 10);
:	print "$x\n";
:	$x = @x = (5, 10);
:	print "$x\n";

Quite so; that's is why I said ``put it in a list AND SUBSCRIPT IT.''

:What if you want the last word from a split()?  This:
:	$x = split(' ', 'a b c');
:returns the number of split items.  This:
:	@x = split(' ', 'a b c');
:	$x = $x[$#x];
:works, but requires a temporary array.

This requires no *named* temporary array:

    $x = (reverse split(' ','a b c'))[0];

Although to pull out the last piece of something, I'd use a regexp anyway:

    ($x) = 'a b c' =~ /(\S+)$/;
    $x = ('a b c' =~ /(\S+)$/)[0];

:Do you think this works?
:	$x = (0, split(' ', 'a b c'));

Yes.  Don't you?  Shouldn't it?

:How about this?
:	sub i1 { @_; }
:	$x = &i1(split(' ', 'a b c'));
:And this?
:	sub i2 { return @_; }
:	$x = &i2(split(' ', 'a b c'));
:Do you understand the difference between &i1 and &i2?  I certainly
:don't.

So, &i1 yields 3, whereas &i2 yield 'c'.  I won't claim to really
understand it, but here are some clues.  If you compile with -D1024,
you'll find that the assignments generate the same code, but the
two subroutines are very different: that 'return' incurs a good deal
of overhead, just enough to make it do what I'm assuming you want it to.

SUB main'i1 = {
    C_TYPE = EXPR
    C_ADDR = 0x800957c8
    C_NEXT = 0x0
    C_LINE = 8 (0x800957c8)
    C_OPT = CFT_EVAL
    C_FLAGS = (COND,TERM)
    C_EXPR = {
        OP_TYPE = ARRAY
        OP_LEN = 1
        [1]ARG_TYPE = STAB (unevaluated)
        [1]ARG_STAB = {
            STAB_NAME = main'_
        }
    }
    AC_STAB = NULL
    AC_EXPR = NULL
}

But here's the 2nd.  

SUB main'i2 = {
    C_TYPE = BLOCK
    C_ADDR = 0x80095b88
    C_NEXT = 0x0
    C_LINE = 9 (0x80095b88)
    C_LABEL = "_SUB_"
    C_OPT = CFT_FALSE
    C_FLAGS = (TERM)
    C_EXPR = NULL
    CC_TRUE = {
        C_TYPE = EXPR
        C_ADDR = 0x80095948
        C_NEXT = 0x0
        C_LINE = 9 (0x80095948)
        C_OPT = CFT_EVAL
        C_FLAGS = (COND,TERM)
        C_EXPR = {
            OP_TYPE = RETURN
            OP_LEN = 2
            [1]ARG_TYPE = WORD (unevaluated)
            [1]ARG_STAB = {}
            [2]ARG_TYPE = EXPR
            [2]ARG_FLAGS = (ARYOK)
            [2]ARG_ARG = {
                OP_TYPE = ARRAY
                OP_LEN = 1
                OP_FLAGS = (LISTISH)
                [1]ARG_TYPE = STAB (unevaluated)
                [1]ARG_STAB = {
                    STAB_NAME = main'_
                }
            }
        }
        AC_STAB = NULL
        AC_EXPR = NULL
    }
    CC_ALT = NULL
}

Notice in particular how the ARG_FLAGS in &i2 are ARYOK, and also
&i2's OP_FLAGS are LISTISH.  You don't see that in &i1.  

But I'm not particularly happy with this.  Larry, you must say
that this behavior is at the very least unexpected, eh?

:Let's hear it for experimental programming.

	Perl programming is an *empirical* science.  :-)

		Larry Wall in <10226@jpl-devvax.JPL.NASA.GOV>


--tom

--
Tom Christiansen		tchrist@convex.com	convex!tchrist
		"So much mail, so little time."