--- Bryan R Harris <[EMAIL PROTECTED]> wrote:
>
> Thanks Ovid, Timothy...
>
> You mention checking out perlref in the perldocs-- I'm familiar with
> "perldoc -f keyword", but how would I find information on these things if I
> didn't know the keyword "perlref"? (Apparently I'm the only legitimate
> "beginner" here! =)
>
> Oh, and one more question. =)
>
> $lines[0] = [ "dog", "cat" ];
> print "$lines[0][0,1]\n";
>
> This prints "cat". Shouldn't it print "dogcat"?
Timothy answered your question by describing and array slice, but I thought you might
want to also
know why the above works the way that it does. The following uses an array slice to
print what
you want:
print @{$lines[0]}[0,1];
However, if you just have this:
print $lines[0][0,1];
Perl winds up seeing a list inside of the second set of square brackets and when it
sees a list,
it evaluates every item in turn, from left to right, and the entire expression is the
result of
the last evaluation. So in this case, the zero evaluates to zero, the one evaluates
to one and
this reduces to:
print $lines[0][1];
However, this behavior should not be relied upon as it's difficult to keep straight:
$lines[0] = [ "dog", "cat" ];
Why doesn't that just assign "cat" to the first element of @lines? I'm not sure, but
I'm guessing
it is because the square brackets are performing a subtle double-duty. In the first
example:
print $lines[0][0,1];
the square brackets are being used to indicate an array element. In the second
example:
$lines[0] = [ "dog", "cat" ];
The square brackets are being used to create a reference to an anonymous array and
constructs the
anonymous array from a list. Keeping things straight can be a major headache.
Further, you can
get strange warnings:
print $lines[0][0,2,1];
That works, but if you have warnings enabled, the 2 produces a "Useless use of a
constant in void
context ..." warning. Changing the two to a one or zero will eliminate the warning
(this might be
a bug in Perl, or it might be a hack to allow a "1" to be used at the end of a module
without
producing a spurious warning -- since all modules need to return a true value).
Note that the following, even though it contains a 2, does not produce a warning:
#!/usr/bin/perl -w
$lines[0] = [ qw/ dog cat bird / ];
print $lines[0][0,0,2];
That's because the 2, being the last item evaluated, is being used as the array index
and thus is
not being used in void context. Got all that? :)
Cheers,
Curtis "Ovid" Poe
=====
"Ovid" on http://www.perlmonks.org/
Someone asked me how to count to 10 in Perl:
push@A,$_ for reverse q.e...q.n.;for(@A){$_=unpack(q|c|,$_);@a=split//;
shift@a;shift@a if $a[$[]eq$[;$_=join q||,@a};print $_,$/for reverse @A
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