The "no strict refs" problem revisited.

zentara has asked for the wisdom of the Perl Monks concerning the following question:

This might seem simple to alot of you old hash veterans, but I keep hitting this error even though I know it lurks there. I'm often finding myself, using hashes to hold identifiers to objects, especially in Tk, where I want to create a large set of non-anonymous objects in loops. So almost everytime I try to set up my loops, I run into the "Can't use string ("1") as a HASH ref while "strict refs" in use at....." error.

So I've read all the stuff on auto-vivication, and similar posts. What I end up doing is falling back on a little trick I've stumbled upon, and just resort to using it to get around the error. My question is:

Is it correct to think about it this way, or am I just lucky? :-)

My idea is you can't create "depth" in a hash, by assigning a value to it. The code below shows the basic idea.

Usually I have a base item as a hash, like $box{$X}{$Y}{$z}. Now the problem comes when you want to attach data to it, if the "depth" for what you want to assign a value to dosn't exist yet, it gives an error; but if you "artificially" create the depth, even if it's undef, it all works well.

So in the code, I want to add a {'text'} key to the hash, but it won't let me, until I create a fake key {'rect'} at that level, then all goes well. But then my "base" turns from $box{$X}{$Y}{$z} to $box{$X}{$Y}{$z}{'rect'}. This is just an annoyance, but it always seems to work. Does anyone see pitfalls in this approach.?


#!/usr/bin/perl
use warnings;
use strict;

# demonstrates strict hash refs 
# Commonly seen error 
# Can't use string ("1") as a HASH ref while "strict refs" in use at .
+/z line  

my %box;

my $x = 10;
my $y = 10;
my $z = 1;

foreach my $X ( 0 .. $x ) {
    foreach my $Y ( 0 .. $y ) {

    # you cannot create a new hash depth by assigning 
    # a value to it 

       $box{$X}{$Y}{$z} =  1;              #dosn't work and causes err
+or below 
#      $box{$X}{$Y}{$z}{'rect'} = 1 ;      #works 
#      $box{$X}{$Y}{$z}{'rect'} = undef ;  #works 

       $box{$X}{$Y}{$z}{'text'} = 2;     #this gives strict refs error
+ 

    }
}
[download]

I'm not really a human, but I play one on earth. flash japh

Comment on The "no strict refs" problem revisited. Download Code

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Re: The "no strict refs" problem revisited. by Joost (Canon) on May 21, 2004 at 22:21 UTC
`$box{$X}{$Y}{$z} = 1; # DOES work # $box{$X}{$Y}{$z}{'rect'} = 1 ; # 'works' because it's comme +nted out # $box{$X}{$Y}{$z}{'rect'} = undef ; # see above $box{$X}{$Y}{$z}{'text'} = 2; # refs error` [download] The reason the last line gives an error is that it does the equivalent of `my $z = 1; $z->{'text'} = 2;` [download] The 2 lines above it don't work if you take away the # at the begining of the lines. A hash of hashes can be automatically created, but perl will not convert a scalar value in it to a hash unless it's undefined. By setting $box{$X}{$Y}{$z} = 1, you prevent $box{$X}{$Y}{$z} from being a hash. Personally I always find that if you write this as `$box{$X}->{$Y}->{$z} = 1; $box{$X}->{$Y}->{$z}->{'text'} = 2;` [download] it's much more clear what's going on, though it's more typing. Update: Try doing this instead: `$box{$X}{$Y}{$z} = undef;` [download] if you really want to create the 'depth' of that HoH. Update2: Though you really don't need it: `$box{$X}{$Y}{$z}{'text'} = 2;` [download] on it's own (without the preceding 3 lines) works perfectly. See also perlref "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: The "no strict refs" problem revisited. by jonadab (Parson) on May 22, 2004 at 01:31 UTC
"Can't use string ("1") as a HASH ref while "strict refs" in use at....." error. This sounds to me like something's not quite right in the code -- unless you really wanted to assign a value to a key in the %1 hash. If that's not what you intended, the strictures are helping you, by pointing out that your code doesn't really do what you want. `$box{$X}{$Y}{$z} = 1; #dosn't work and causes error below` What do you mean, "doesn't work"? This code works just fine, if by "work" you mean to assign the value 1 to the scalar value `$box{$X}{$Y}{$z}` (which could otherwise be written as `${${$box{$X}}{$Y}}{$z}` or `$box{$X}->{$Y}->{$z}` and which is the value for key $z within the hash that is the value for key $Y within the hash that is the value for key $X within %box). What did you think this line would do, if not assign a value of the number 1? That's what it does. `# $box{$X}{$Y}{$z}{'rect'} = 1 ; #works` I'm not certain this works the way you think it does. You've just assigned a numeric value to $box{$X}{$Y}{$z}, but now you're trying to treat that value (that numeric value, the one that you just assigned, the number 1) as a hash (or a hash reference, technically). 1 is not a hash (nor a hash reference), so this won't work -- straightforwardly. It could work as a symbolic reference, but in that case you'd be assigning to $1{rect} and $1{text}, which I suspect is not what you wanted to do. If you want `$box{$X}{$Y}{$z}` to be a hashref, why on earth are you making it a number? A number is not a hash reference. If you want a hashref there, put a hashref there: `$box{$X}{$Y}{$z} = {}; # Assign anonymous hashref. # Or, alternately... %{$box{$X}{$Y}{$z}} = (); # Assign empty list to hash.` [download] The undef trick the other poster mentions will also work, but for slightly less straightforward reasons. It should (hopefully) be obvious why assigning an empty hashref will provide you with a hashref in which you can subsequently create values. Also, assigning a hashref, even if it's empty, will cause the value to be defined (since it will contain the hashref you assigned); assigning undef merely causes it to exist (where "exists" means only that the key is in the hash but doesn't imply that a value is defined for it). If you don't understand this paragraph, don't worry about it for now; if you aren't testing for existence, you probably don't really need to know about it. I think that assigning an empty hashref is what you want to do (though I'm not sure why), but I could be misunderstanding. If so, please explain in more detail what it is you actually want to do, so we can help better. `;$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}` `split//,".rekcah lreP rehtona tsuJ";$\=$;[-1]->();print`	[reply] [d/l] [select]
Re: The "no strict refs" problem revisited. by zentara (Cardinal) on May 22, 2004 at 13:02 UTC
*By setting $box{$X}{$Y}{$z} = 1, you prevent $box{$X}{$Y}{$z} from being a hash.* Ah, that's the part which eluded. Thanks. I'm not really a human, but I play one on earth. flash japh	[reply]