Re: Pointers and References

"I am still trying to wrap my head around ..."

You're using terms — pointer, address, memory location — which I suspect you've picked up from one or more other languages. You are then attempting to apply those terms to Perl, assuming they have the same meaning. I think this may be the source of your problems.

I'd suggest the first thing to do would be to look at perlintro; in particular, the "Perl variable types" section. At the end of that section you'll find the gentlest of introductions to references with a list of other links to more information; I'd suggest checking out perlreftut first.

Consider the following:

$ perl -E '
    my $x = 5;
    say $x;
    my $y = \$x;
    say $y;
    say $$y;
    $$y += 3;
    say $x;
'
5
SCALAR(0x60008a1c8)
5
8
[download]

$x has the value 5.
$y is assigned a reference to $x.
$y has the value SCALAR(0x60008a1c8).
Dereferencing $y (with $$y) gives you back $x.
Incrementing $$y by 3 is the same as incrementing $x by 3.
$x now has the value 8.

You can reference and dereference to great depths if you want; as in this exaggerated example:

$ perl -E 'my $x = 5; say $x; my $y = \\\\$x; $$$$$y += 3; say $x'
5
8
[download]

The construct \(...), where ... is some list, evaluates to a list of references to each element of the list:

$ perl -E 'say for \(qw{1 2 3})'
SCALAR(0x60008a730)
SCALAR(0x60008a7d8)
SCALAR(0x60008a748)
[download]

You can take references to other data types:

$ perl -E 'my @x = qw{1 2 3}; say for @x; my $y = \@x; say $y'
1
2
3
ARRAY(0x60008a8e8)
[download]

and dereference them:

$ perl -E 'my @x = qw{1 2 3}; say for @x; my $y = \@x; say $y; say for
+ @$y'
1
2
3
ARRAY(0x60008a828)
1
2
3
[download]

You can take references to references:

$ perl -E 'my @x = qw{1 2 3}; say for @x; my $y = \\@x; say $y'
1
2
3
REF(0x600003e80)
[download]

and dereference them one level at a time:

$ perl -E 'my @x = qw{1 2 3}; say for @x; my $y = \\@x; say $y; say $$
+y; say for @$$y'
1
2
3
REF(0x600003e80)
ARRAY(0x60008a868)
1
2
3
[download]

I suggest you play around with examples like these to get a better understanding of how all of this works.

Also note that I didn't use, or indeed need, terms such as pointer, address or memory location.

You used strict and warnings in your OP which is very good. I suggest you do the same with oneliners. Here's a common alias I use; you might want to set up something similar for yourself (although, perhaps, one a little less involved).

$ alias perle
alias perle='perl -Mstrict -Mwarnings -Mautodie=:all -MCarp::Always -E
+'
[download]

That will pick up things like this:

$ perl -E '$x =5'

$ perle '$x =5'
Global symbol "$x" requires explicit package name (did you forget to d
+eclare "my $x"?) ...

$ perl -E 'my $x = 5; say @$x'

$ perle 'my $x = 5; say @$x'
Can't use string ("5") as an ARRAY ref while "strict refs" in use ...
[download]

— Ken

Comment on Re: Pointers and References Select or Download Code

Replies are listed 'Best First'.
Re^2: Pointers and References by Leudwinus (Scribe) on Nov 25, 2020 at 17:36 UTC
Hi Ken, You're using terms — pointer, address, memory location — which I suspect you've picked up from one or more other languages. You are then attempting to apply those terms to Perl, assuming they have the same meaning. I think this may be the source of your problems. Guilty as charged! I was trying to replicate in Perl the following program from a C tutorial on functions and pointers: `#include <stdio.h> int sum_and_diff (int a, int b, int res) { int sum; sum = a + b; res = a – b; return sum; } void main (void) { int b = 2; int diff; printf ("The sum of 5 and %d is %d\n", b, sum_and_diff (5, b, &diff)); printf ("The difference of 5 and %d is %d\n", b, diff); }` [download] Perhaps I just need to focus on one language at a time! And thank you for the detailed example and explanation! You can reference and dereference to great depths if you want; as in this exaggerated example I'm just reading this now but came to the same conclusion earlier today when I was reading some of the other responses to this thread. I suggest you play around with examples like these to get a better understanding of how all of this works. Agreed. What I take away from your examples is that when I try to print or output a variable that contains a reference, I can clearly see what that is in reference to. For example: `SCALAR(0x60008a730) --> reference to a scalar ARRAY(0x60008a828) --> reference to an array REF(0x600003e80) --> reference to a reference` [download] This helps you understand how to dereference it: `$$x # dereference reference to scalar (if $x is a scalar) @$x # dereference reference to array (if $x is reference to an array) @$$x # dereference reference to reference of an array` [download] I hope I got that last bit right! Also note that I didn't use, or indeed need, terms such as pointer, address or memory location. Duly noted! And thanks for the `alias` tip. I will have to park that one away for the time being because as useful as Perl one-liners are, I don't think I'm quite ready to use them that frequently. Gratias tibi ago Leudwinus	[reply] [d/l] [select]
Re^3: Pointers and References by kcott (Archbishop) on Nov 25, 2020 at 19:51 UTC
"And thanks for the `alias` tip. I will have to park that one away for the time being because as useful as Perl one-liners are, I don't think I'm quite ready to use them that frequently." I have aliases set up such that they're always available, regardless of frequency of usage. In `~/.bashrc`, I have: `... if [ -f "${HOME}/.bash_aliases" ]; then . "${HOME}/.bash_aliases" fi ...` [download] And `~/.bash_aliases` has lines like this: `... alias vi='vim' alias view='vim -R' ... alias perle='perl -Mstrict -Mwarnings -Mautodie=:all -MCarp::Always -E +' alias perlb='perl -MO=Deparse,-p -e' ... alias apache_up='/usr/sbin/apachectl start' alias apache_down='/usr/sbin/apachectl stop' ...` [download] You may need to adjust to suit whatever shell you're using; however, the basic principle should be applicable to any UNIX-like system. I use this with Linux for $work; with Cygwin for personal, home use; and, up until a year or so ago, with macOS (formerly Mac OS X). — Ken	[reply] [d/l] [select]


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