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Access elements and Length of an array when I have only the reference

rpaskudniak
User

Jul 6, 2010, 4:10 PM

Post #1 of 5 (731 views)

Access elements and Length of an array when I have only the reference	Can't Post

Greetings.

At the risk of reinventing the wheel -the CPAN module Text::ParseWords - (Thanks, BillKSmith), I would like to know how to do this: I want to pass an array reference to a subroutine and:
- Determine the number of items in the array
- Get a value from inside that array e.g. $my_array[3]
- Diddle the array e.g shift, push, change a value. The code I would like to run would look something like:

Code

my @one_list = (1, 2, 3, 4); 
print "@one_list\n"; 
append_one(\@one_list);   # Notice: Passing array reference 
print "@one_list\n"; 
 
sub append_one 
{ 
  my @my_list = @_;    #This is definitely messed up code 
 
  printf("Before push: Top entry in array is %d\n", $#my_list); 
  push (@my_list, 5); 
  printf("After push: Top entry in array is %d\n", $#my_list); 
}

Y'all know, of course, that the code presented above will NOT work as desired; in the subroutine, @my_list is an array of one entry - the array reference that was passed. Pushing the five only appends the 5 to that tiny array, leaving the original array untouched. (The Before and After values are 0 and 1.) I am guessing at *-dereferencing of a scalar and using that in an array context but I would need a syntax for that.

How can I accomplish my nefarious designs? Angelic

Note: Even if I already knew how to access the values in the array, I would not scan up until an entry is undefined. In Perl, an array might have many entries undefined before the last one.

Thanks much!
--------------------
-- Rasputin Paskudniak (In perpetual pursuit of undomesticated, semi-aquatic avians)

(This post was edited by rpaskudniak on Jul 6, 2010, 4:13 PM)

BillKSmith
Veteran

Jul 6, 2010, 8:12 PM

Post #2 of 5 (725 views)

Re: [rpaskudniak] Access elements and Length of an array when I have only the reference [In reply to]	Can't Post

Perl provides several solutions to this problem. (ref perldoc perlref). Here is one.

Code

  
 
sub append_one { 
#   my @my_list = @_;    #This is definitely messed up code 
    (my $my_list) = @_; 
    printf("Before push: Top entry in array is %d\n", $#my_list); 
#   push (@my_list, 5); 
    push (@{$my_list}, 5); 
    printf("After push: Top entry in array is %d\n", $#my_list); 
}

There is a simple rule that often produces awkward code, but always works. Anywhere that you would normally use a symbol, you can replace that symbol with a reference surrounded by {}. My change to your push statement is an application of this rule.
Good Luck,
Bill

rpaskudniak
User

Jul 7, 2010, 8:50 PM

Post #3 of 5 (669 views)

Re: [rpaskudniak] Access elements and Length of an array when I have only the reference [In reply to]	Can't Post

Thanks, Bill.

Your solution was real close but it took some experimentation to get it quite right. While the perldoc didn't quite address this, I managed to glean something from it and your code. Here's the code that worked; note that I have removed some of the parentheses you had used.

Code

#!/usr/bin/perl -w #-d 
# 
# testglob.pl 
# 
use strict; 
use diagnostics; 
 
my @one_list = (1, 2, 3, 4); 
print "Whole list before call: @one_list\n"; 
append_one(\@one_list); 
print "Whole list after call: @one_list\n"; 
 
sub append_one 
{ 
  my $list_ref = shift (@_); 
 
  #printf("Before push: Top entry in array is %d\n", $#($list_ref)); 
  for (my $lc = 0; defined($list_ref->[$lc]); $lc++) 
  { printf("Entry[%d]=%d\n", $lc, $list_ref->[$lc]); } 
 
  print "Pushing one more value on the array\n"; 
  push (@$list_ref, 5); 
 
  #printf("After push: Top entry in array is %d\n", $#($list_ref)); 
  for (my $lc = 0; defined($list_ref->[$lc]); $lc++) 
  { printf("Entry[%d]=%d\n", $lc, $list_ref->[$lc]); } 
}

This produced:

Quote

Jake@Maxwell ~/scripts$ ./testglob.pl
Whole list before call: 1 2 3 4
Entry[0]=1
Entry[1]=2
Entry[2]=3
Entry[3]=4
Pushing one more value on the array
Entry[0]=1
Entry[1]=2
Entry[2]=3
Entry[3]=4
Entry[4]=5
Whole list after call: 1 2 3 4 5

OK, this was a great start. However, the first part of my question remains unanswered. You can see the commented printfs in append_one(), where I am trying to obtain the equivalent of $#array from the array reference. I have tried many permutations of $, #, {}, () and all earn me a syntax error. Short of forcing the caller to explicitly pass $#array to the subroutine, how can I access the $# of the array, given only the reference? I used the "defined()" operator here just to get by, but that is a rigged test. I already described why I don't want to rely on this as a loop terminator in general.

This is getting to be FUN! Thanks much!
--------------------
-- Rasputin Paskudniak (In perpetual pursuit of undomesticated, semi-aquatic avians)

BillKSmith
Veteran

Jul 8, 2010, 4:27 AM

Post #4 of 5 (658 views)

Re: [rpaskudniak] Access elements and Length of an array when I have only the reference [In reply to]	Can't Post

Use my rule. Replace the symbol "array" with "{$array_ref}"

Code

  
 
$#{$array_ref}

Good Luck,
Bill

rpaskudniak
User

Jul 8, 2010, 8:46 PM

Post #5 of 5 (644 views)

Re: [BillKSmith] Access elements and Length of an array when I have only the reference [In reply to]	Can't Post

Bill,

This one worked exactly as you now prescribed it. Apparently, the {} around the $array_ref reference is a manner of dereference operator, sufficient for the $# prefix to get at the desired value. HMMmm.. Before I posted this thread, I had been looking into globs for my solution. (Note the name of the source file - testglob.pl.) The {$array_ref} seems almost like the glob. I had been wondering if I would need to dig into the symbol table. But that's for another thread...

Thanks extremely for the help. I can now go back to what I was trying to do when I posted the thread Split when there is an escaped delimiter within a field. I will resume that thread to tell y'all how I plan to take care of that issue.
--------------------
-- Rasputin Paskudniak (In perpetual pursuit of undomesticated, semi-aquatic avians)

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