Uri Guttman wrote:
>>>>>> "BL" == Ben Lavery <ben.lav...@gmail.com> writes:
>
>
> BL> #Here, using a hash looks much cleaner than
iterating through an array
>
> hashes are almost always better for token lookups than scanning
> arrays. don't doubt yourself in this area.
>
>
Jim Gibson wrote:
> On 3/8/11 Tue Mar 8, 2011 2:51 PM, "Ben Lavery" <ben.lav...@gmail.com>
> scribbled:
>
>>
>> I'm wondering if I need to convert my hash of valid words into an
array, then
>> iterate through each entry with something like:
>> if($word_list[i] =~ m/$temp_word/){
>> push(@all_combinations, $temp_word);
>> }
>
> You can use the built-in grep function to compare each of the words
in your
> word list to your specified pattern, used as a regular expression, and
> return all that match. In that case, the period is fine, as it will match
> any character:
>
> my @matches = grep( /^$temp_word$/i, @word_list );
> push( @all_combinations, @matches );
>
>
Uri and Jim have hit upon one of my major stumbling blocks with learning
Perl. There seems to be a difference of opinion on the proper times to
use hashes vs. arrays/lists...and how best to use them. For those that
have heard of this book, I'm currently digesting this piece of information:
Effective Perl Programming, 2nd Ed., Item 9: "Know the difference
between lists and arrays."
I have this suspicion that it relates to how C/C++ programmers write C
programs in Perl, rather than Perl programs. For instance, someone in
the "C mindset" might be tempted to write the above 'grep' as (following
code is a bad idea, I know...):
foreach ( @word_list ) {
if ( /^$temp_word$/i ) {
push( @all_combinations, ( $_ ));
}
}
I understand that this is very inefficient in Perl, and (please correct
me if I'm wrong) that Perl is optimized to search and manipulate hashes
and lists as a whole, rather than executing code over each item
one-at-a-time. So, I get the idea that I should treat a list of items
as a LIST (as a whole), rather than the conventional thinking of
enumerating each item of a list into an array. Please tell me, gurus,
if I'm on the right track! ;-)
So, my question becomes, why is hash manipulation more efficient than
list manipulation?? Uri states that hashes are "better for for token
lookups than scanning arrays". But each key in the hash has an unused
associated value (1) in this case, which increases the amount of data
stored in memory. Maybe that's inconsequential. But, if we "grep a
list" (as Jim suggests) instead of "scan an array", shouldn't the list
manipulation be more efficient (or at least equivalently efficient) to a
hash lookup?
Thanks in advance for your sharing your thoughts...
Brian
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