G
grocery_stocker
How are references and aliases in perl different than references in
aliases in C++?
aliases in C++?
J
Jim Gibson
grocery_stocker said:
Some differences (not exhaustive):
Aliases in Perl are like references in C++: a different way of
accessing the same object.
References in Perl are like pointers in C/C++: indirect access to an
object. However, in Perl you cannot do arithmetic on references the way
you can for C/C++ pointers (e.g. *(p++) = getchar()
In Perl, references are scalars, not separate pointer types as in C/C++.
In C/C++, arrays are really pointers to the first element in a
contiguous set of elements. In Perl, arrays are separate objects. A
reference to an array in Perl is different than a reference to one of
the members of the array.
G
grocery_stocker
Some differences (not exhaustive):
Aliases in Perl are like references in C++: a different way of
accessing the same object.
References in Perl are like pointers in C/C++: indirect access to an
object. However, in Perl you cannot do arithmetic on references the way
you can for C/C++ pointers (e.g. *(p++) = getchar()
In Perl, references are scalars, not separate pointer types as in C/C++.
In C/C++, arrays are really pointers to the first element in a
contiguous set of elements. In Perl, arrays are separate objects. A
reference to an array in Perl is different than a reference to one of
the members of the array.
Everytime I ask a question on the newsgroup, i keep on thinking "I'm
sure things would have been a lot easier if I would have taken more
than 6 week of FORTRAN." I don't care what anyone says. Learning to
program on your own. mastering the core concepts without formal
schooling, and then actually making it as a programmer takes a certain
level of skill and internal drive. Not everyone has it.
I think I only know a few people with no more than a high school
education that are doing the same kind of work, for the exact same
pay, as a person with an advanced degree in the sciences.
J
Jürgen Exner
Another very major difference: in C pointers are just (virtual) memory
addresses and the programmer is forced to implement his own memory
management for objects that are addressed by pointers.
In Perl any memory management for objects referred to by references is
done automatically by the system.
jue
addresses and the programmer is forced to implement his own memory
management for objects that are addressed by pointers.
In Perl any memory management for objects referred to by references is
done automatically by the system.
jue
A
acehreli
That is a common misconception.
An array is the contiguous set of elements that you mention; there is
no extra pointer. True, the name of the array decays to a pointer to
first element in certain contexts; but still, there is no extra
pointer to make an array.
That is exactly the case in C++ as well. Here is a program with a
function that takes an array as a separate object (not as a pointer to
its first element):
#include <assert.h>
typedef int TwoIntArray[2];
void as_array(TwoIntArray & array)
{
// Here, the type is int[2]
assert(sizeof(array) == sizeof(TwoIntArray));
}
void as_pointer(int * p)
{
// Here, the type is int*
assert(sizeof(p) == sizeof(int*));
}
int main()
{
int array[2] = { 7, 42 };
as_array(array);
// Decays to pointer to first member
as_pointer(array);
}
Ali
G
grocery_stocker
And going off on a tanget, given something like
#!/usr/bin/perl -w
# global array definition
my @array = ("a","b","c");
sub print_array {
foreach my $element (@array) {
$element .= "9";
print $element . "\n";
}
}
for ($i = 0; $i < 3; $i++) {
&print_array();
}
How would I prevent $elment from modifying @array?
A
A. Sinan Unur
use warnings;
is in general preferable to -w.
You forgot:
use strict;
Useless comment (it is also wrong).
my @array = qw( a b c ); # easier on the eyes
First off, omit the & unless you know and desire its specific effect in
this case.
$element is not modifying anything. $element is an alias to the current
element of @array. The statement you wrote
$element .= "9";
is modifying the contents of @array in each iteration of the loop.
The easiest way to avoid modifying the contents of @array would be for
you not to modify the contents of the array.
The name print_array is simply bad. The subroutine does not just print
the contents of the array but prints some modification of the elements
of the array.
Depending on what you actually want to do, there are many different ways
of writing such a subroutine (each of which is infinitely better than
what you wrote).
Some examples below. Some of these are sillier than the others.
#!/usr/bin/perl
use strict;
use warnings;
my @array = qw( a b c d e f g h i j k l );
my %hash = @array;
print "${_}9\n" for @array;
print map { "${_}9\n" } @array;
print_with_suffix( 9 => \@array );
print_with_suffix2( 9 => @array, \@array, \%hash );
sub print_with_suffix {
my ($suffix, $array_ref) = @_;
return unless ref $array_ref eq 'ARRAY';
print "${_}$suffix\n" for @$array_ref;
}
sub print_with_suffix2 {
my $suffix = shift;
for my $arg ( @_ ) {
if ( ref $arg eq 'ARRAY' ) {
print_with_suffix( $suffix, $arg );
}
elsif ( ref $arg eq 'HASH' ) {
print_with_suffix( $suffix, [ values %$arg ] );
}
else {
print "$arg$suffix\n";
}
}
}
__END__
--
A. Sinan Unur <[email protected]>
(remove .invalid and reverse each component for email address)
comp.lang.perl.misc guidelines on the WWW:
http://www.rehabitation.com/clpmisc/
J
Jürgen Exner
grocery_stocker said:
$element doesn't modify anything. If at all it is modified by the
assignment and because it is an alias to to each @array element those
are modified, too.
Solution: just create a copy:
my @array = ("a","b","c");
[...]
foreach (@array) {\
my $element = $_ # create copy instead of aliasing
$element .= "9";
jue
A
A. Sinan Unur
[ snip Jim's explanation ]
[ Do *NOT* quote sigs ]
I did do some FORTRAN programming almost 20 years ago. I am not sure
what you are getting at though.
Well, I am reminded of
http://www.catb.org/~esr/faqs/hacker-howto.html#believe5
Are you referring to yourself here?
I am not sure what "making it as a programmer" means above. On the other
hand, almost everyday at work is an opportunity for me to run into
someone who thinks he/she has made it as a programmer. I am not sure I
agree with those people's self-assessments.
The only thing that shows me is that the person with the advanced degree
in the sciences has chosen not to work in the field in which he/she
earned the degree.
Clearly, once one has a certain mental capability, whether one chooses
to invest time in an advanced degree is a matter of preference. Another
person with even superior mental capacity may choose not to "waste" five
to seven years toiling on a project which is of interest to only a few
people and which, as a norm, do not generate huge monetary returns on
that investment. This is why I do not put much stock in letters before
or after a person's name.
Achieving that goal also takes a certain level of skill and drive.
If I were you, I would not be so quick to pat myself on the back for
this particular reason until I were able to compete with Physics Ph.D.'s
in the fields in which they earned their degrees.
You can be proud of your achievements without resorting to this silly
argument.
Switching back to discussing Perl ... now.
Sinan
--
A. Sinan Unur <[email protected]>
(remove .invalid and reverse each component for email address)
comp.lang.perl.misc guidelines on the WWW:
http://www.rehabitation.com/clpmisc/
[ Do *NOT* quote sigs ]
I did do some FORTRAN programming almost 20 years ago. I am not sure
what you are getting at though.
Well, I am reminded of
http://www.catb.org/~esr/faqs/hacker-howto.html#believe5
Are you referring to yourself here?
I am not sure what "making it as a programmer" means above. On the other
hand, almost everyday at work is an opportunity for me to run into
someone who thinks he/she has made it as a programmer. I am not sure I
agree with those people's self-assessments.
The only thing that shows me is that the person with the advanced degree
in the sciences has chosen not to work in the field in which he/she
earned the degree.
Clearly, once one has a certain mental capability, whether one chooses
to invest time in an advanced degree is a matter of preference. Another
person with even superior mental capacity may choose not to "waste" five
to seven years toiling on a project which is of interest to only a few
people and which, as a norm, do not generate huge monetary returns on
that investment. This is why I do not put much stock in letters before
or after a person's name.
Achieving that goal also takes a certain level of skill and drive.
If I were you, I would not be so quick to pat myself on the back for
this particular reason until I were able to compete with Physics Ph.D.'s
in the fields in which they earned their degrees.
You can be proud of your achievements without resorting to this silly
argument.
Switching back to discussing Perl ... now.
Sinan
--
A. Sinan Unur <[email protected]>
(remove .invalid and reverse each component for email address)
comp.lang.perl.misc guidelines on the WWW:
http://www.rehabitation.com/clpmisc/
G
grocery_stocker
Well, FORTRAN was my first formal introduction to structured
programming. Is 6 weeks enough to actually learn how to think
logically?
No. I think some people that come mind are certain former Netscape and
FreeBSD engineers.
Yes. We now go back to our regular discussion on Perl.