newbie needs help [references and values]

[John]

Dear listers,


I have just started on perl and I have a question about if the thing I
copy is copy the value itself or I am copying the refence.

Can you help me with that?

The code is writen below.

My question is if I (as I understand) copy the values of @a into @b
and then I change the values for @a (on the subroutines) and print
then back. Why @b is altered?

If this is the normal behavior,

Why $st1 and $st2 do look to have the same behavior?

What I am missing?

Thanks for your advise.
John

#the code################################

use strict;

my $st1="abcdefabcdef";

my @a = ( [qw(1 2 3 4 5 6)],
[qw(2 4 6 8 10 12)],
[qw(3 5 7 9 13 15)]
);

my @b = @a;

print $a[1][2],"\n"; # before to do anything PRINT 6
print $a[1][3],"\n"; # before to do anything PRINT 8

pe(\@a); # call the module

print $a[1][2],"\n"; # this value has changed PRINT 88
print $a[1][3],"\n"; # this has changed too PRINT 55
print $b[1][3],"\n"; # but why this has changed is is reffered to
# another array??? WHY PRINT 55?????
# if this is the normal behavior... read down

print "\n";

my $st2= $st1;

$st2 =~ tr/abcdef/123456/;

print "$st1\n"; # why in this case we change one and the
print "$st2\n"; # other remains as it was?


sub pe{
my ($inner1)= @_;

$inner1->[1][2]=88;

per($inner1);
}
sub per{
my ($inner2)=@_;

$inner2->[1][3]=55;
}


and the result is:
6
8
88
55
55

abcdefabcdef
123456123456

 

[Andreas Kahari]

Dear listers,


I have just started on perl and I have a question about if the thing I
copy is copy the value itself or I am copying the refence. [cut]
use strict;

my $st1="abcdefabcdef";

my @a = ( [qw(1 2 3 4 5 6)],
[qw(2 4 6 8 10 12)],
[qw(3 5 7 9 13 15)]
);

Note that @a is an array of references to arrays of numbers.
When doing "@b = @a", you copy the references to those arrays
over to @b.

That should answer most of your questions.


Andreas

 

[Anno Siegel]

Dear listers,


I have just started on perl and I have a question about if the thing I
copy is copy the value itself or I am copying the refence.

Can you help me with that?

The code is writen below.

My question is if I (as I understand) copy the values of @a into @b
and then I change the values for @a (on the subroutines) and print
then back. Why @b is altered?

If this is the normal behavior,
Yes.

Why $st1 and $st2 do look to have the same behavior?

You mean "seem to have a different behavior".

What I am missing?

Thanks for your advise.
John

#the code################################

use strict;

my $st1="abcdefabcdef";

my @a = ( [qw(1 2 3 4 5 6)],
[qw(2 4 6 8 10 12)],
[qw(3 5 7 9 13 15)]
);

The elements of @a are array references.

my @b = @a;

Here you copy the references from array @a to @b. The actual (anonymous)
arrays the references are pointing to are the same for both @a and @b.
So, if you change one (through @a below), the other changes too.

print $a[1][2],"\n"; # before to do anything PRINT 6
print $a[1][3],"\n"; # before to do anything PRINT 8

pe(\@a); # call the module

You mean "call the routine". There is no module involved.

Using a subroutine to make changes to @a is just obfuscation. It would
have been clearer to put the code right here:

$a[1]->[2] = 88;
$a[1]->[3] = 55;

print $a[1][2],"\n"; # this value has changed PRINT 88
print $a[1][3],"\n"; # this has changed too PRINT 55
print $b[1][3],"\n"; # but why this has changed is is reffered to
# another array??? WHY PRINT 55?????

Because both $a[1] and $b[1] point to the same anonymous array, which
you have changed.

# if this is the normal behavior... read down

print "\n";

my $st2= $st1;

$st2 =~ tr/abcdef/123456/;

print "$st1\n"; # why in this case we change one and the
print "$st2\n"; # other remains as it was?

Here no references are involved. $st1 and $st2 point to two different
scalars.

[snippage]

Anno

 

[Glenn Jackman]

I have just started on perl and I have a question about if the thing I
copy is copy the value itself or I am copying the refence.

Read http://www.stonehenge.com/merlyn/UnixReview/col30.html