Re^4: Perl vs C

[JavaFan]

Please explain what you think the difference is between a Perl array and a Perl List.

Arrays are variables, lists are values. It's the same difference as between $foo and 3.

I think this just some kind of nomenclature difference.. Not any real disagreement!

Differences in nomenclature are not something to trivialize - specially not when it's about important concepts.

A simple Perl list is similar to a C char ** array (array of pointers of pointers to strings)

No, it's not. First of all, a char is not a string, but a (small) integer. Depending on usage, a C char is equivalent to a Perl integer, or a Perl string consisting of exactly one character. Secondly, in C, array elements all have the same size - all elements are integers, characters or pointers for instance. (Pointers in your example). In Perl, values of a list can have different "sizes" - or rather, different types (mixed integers, references, objects, strings, etc) - as the "size" of a value isn't a useful concept in Perl (it is in perl, but not in Perl). Third, and this is the big one, elements of an array may change (both in C and in Perl). But in Perl, lists are unmutable (of course they can be changed by perl, or by using XS - but not from a Perl POV). They are as unmutable as the value 3.

[Marshall]

Yes, a char is a character. But char*, is a pointer to an character. And char** is a pointer to a pointer to a character (array).

Ok some examples:
in Perl:

#!usr/bin/perl -w
use warnings;
my @list = ("a1", "b23", "c45", "d1");
print join (" ",@list),"\n"; 
#prints: a1 b23 c45 d1

In Perl, the list "knows" how big it is.
This is not true in C! In C we have to do stuff to help things out! Like this:

include <stdio.h>
int main ()
{
   char *list[] = {"a1", "b23", "c45", "d1",'\0'};
   // list is an array of pointers to chars 
   char **mover;
  
   for (mover=list; *mover ;mover++)
       printf ("%s ",*mover);
   printf("\n"); 
}
// prints: a1 b23 c45 d1

For me, the difference is obvious! The Perl version is cool!
The alternative way in C would be to keep track of the number of items in the list array instead of a NULL value sentinel (and yes, there are more ways than than one I showed above). All ways are a pain in the rear compared with Perl!

Perl lists may be changed at will just like in C. I don't understand this comment: "lists are unmutable". Please explain, I just don't understand what you mean.

[JavaFan]

I don't understand this comment: "lists are unmutable". Please explain, I just don't understand what you mean.

Try to make pure Perl program that modifies a list. Come back when you either have such a program, or when you are convince this isn't possible.

[Marshall]

Here is a simple thing that removes things from a Perl list that do not match a criteria.

#!usr/bin/perl -w
use warnings;

my @list = ("a1", "b23", "c45", "d1", "b43", "b65");
print join (" ",@list),"\n"; 
#prints: a1 b23 c45 d1 b43 b65

@list = grep{!/^b/}@list;  #remove things that start with b
print join (" ",@list),"\n"; 
#prints: a1 c45 d1