Re: Dynamically creating a hash for keys()

On Oct 19, 2004, at 08:50, Jason Foster wrote:

> Can anyone help me to understand why this code refuses to compile?  
> Even better, can anyone help fix it :)
>
>     %hash = qw( fred filntstone barney rubble tom delong );
>     print( keys( reverse( %hash ) ) );
>
> The error message...
>
>     Type of arg 1 to keys must be hash (not reverse) at ./killme.pl 
> line 4, near ") ) "
>
> ... was pretty confusing since it implies that "reverse" is a type?!

$ perldoc -f keys
        keys HASH
...
$ perldoc -f reverse
        reverse LIST
                In list context, returns a list value consisting of the 
ele-
                ments of LIST in the opposite order.  In scalar context, 
con-
                catenates the elements of LIST and returns a string 
value with
                all characters in the opposite order.

reverse returns a list. keys expects a hash.

Reversing a hash doesn't make sense, assuming you're thinking of 
reverse in the same sense as a list. The members of a hash are not 
stored in the order in which they were inserted; if you iterate a hash, 
the order in which you visit members is dependent only on the 
implementation of the hash. In other words, if order is important to 
you, don't use a hash.

On the other hand, you can *invert* the hash, swapping keys with 
values. I think that's in the perlfaq.

-1 for fun.

--
Craig S. Cottingham
[email protected]
OpenPGP key available from:
http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x7977F79C