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From: roger@mav.com (Roger Droz)
Newsgroups: comp.lang.perl
Subject: Re: Subroutine parameters
Message-ID: <1991Jun07.161213.22170@mav.com>
Date: 7 Jun 91 16:12:13 GMT
References: <9105312319.AA21073@no11sun.csd.uwo.ca>
Distribution: na
Organization: Maverick International Inc.
Lines: 88

In article <9105312319.AA21073@no11sun.csd.uwo.ca> clipper@csd.uwo.ca (Khun Yee Fung) writes:
> I am using perl to write a program generator. This program has a few
> recursive subroutines using more than one array passed among them.
> To make sure the subroutines do not alter the content of the arrays, I
> use something like:
> 
>   @arg1 = @dimen1;
>   @arg2 = @dimen2;
>   &traverse($level);
> 
>   .
>   .
>   .
>   sub traverse {
>     local(@dimen1) = @arg1;
>     local(@dimen2) = @arg2;
>     local($level) = @_;
> 
>     .
>     .
>     .
>     @arg1 = @dimen1;
>     @arg2 = @dimen2;
>     &traverse($level);
>     .
>     .
>   }

Perl can easily pass arrays by reference.  If I were a caller who didn't
trust the routine I was calling, I'd make a copy of the array and pass the
copy by reference:

# Pass @foo and @bar by value.
@foo_copy = @foo;
@bar_copy = @bar;
&traverse($local, *foo_copy, *bar_copy);

sub traverse {
    local($level, *dimen1, *dimen2) = @_;
    local(@arg1, @arg2);
    .
    .
    .
    @arg1 = @dimen1;
    @arg2 = @dimen2;
    &traverse($level, *arg1, *arg2);
    .
    .
}
Not much improvement because you still have to remember to make copies
of the arguments.  But after all, it is the caller that's not trusting
the called routine to the point of sacrificing performance and passing
arrays by value.

If you trust the called routine to make its own local copy before it
modifies an array, you can:

&traverse($level, *foo, *bar);

sub traverse {
    local($level, *arg1, *arg2) = @_;
    local(@dimen1) = @arg1;
    local(@dimen2) = @arg2;
    .
    .
    .
    # work with @dimen1 and @dimen2
    .
    .
    &traverse($level, *dimen1, *dimen2);
    .
    .
}

I'm posting this instead of mailing it because I'd like to see what the
real wizzards have to say on this subject.  Personally, I'm greatful for
a language where call-by-reference is the rule and call-by-value the exception.
(I don't really want to start that religion war.  I could be that I just
got used to call-by-reference from too much FORTRAN in my impressionable
years.)
____________
               Roger Droz                  Domain: roger@mav.COM           
()       ()    Maverick International      UUCP: uw-beaver!gtisqr!roger
 (_______)     Mukilteo, WA 
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