Palindrome Goodie #83

merged 4 commits into from May 3, 2012


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3 participants

bm1362 commented May 1, 2012

Very simple, displays whether or not a string is a palindrome. Queried like so:

'is (query) a[n] a palindrome[?]' and 'isPalindrome (query)'

Results look like: '(query) is a palindrome!'

I'm new to Perl so please let me know if I did something stupid.



There are a few problems, so I'll show you what I would do and then walk through it.

triggers any => "palindrome";

handle query_clean => sub {

    #Remove the trigger text from the query.
    return unless /^is \s+ (\S+) \s+ an? \s* palindrome\??$/ix;

    #Check to see if it is a palindrome.
    return ($1 eq scalar reverse $1) ? "$1 is a palindrome." : "$1 is not a palindrome";

First thing I am doing differently is with that trigger. regexp triggers are actually a lot slower than words, so we try to use words wherever possible.
Then in the handler I use a regexp to make sure I have proper input. In regular expressions, () are capturing groups, so the (\S+) captures the word we plan to use (\S is anything but space, so a single word.)
At the end, I use a ternary operation to give the result.

bm1362 commented May 2, 2012

Awesome, thanks! Looks good, I just have a few clarifications:

From what I understand, $_ is a local variable scoped within closures; evaluating a regex will use $_ as a parameter implicitly, correct? And $1 is representing the 1st capturing group? ( whatever matches \S+)

My code was a bit verbose due to my first time using Perl, your code is much better!


As I understand it, $_ is set automatically by for loops, but DDG sets it explicitly for the handler subroutine. /foo/; and s/foo/bar/; are the same as $_ =~ ..., and you are also correct on $1.
I could be wrong about $_ though, I am also a perl noob :)

bm1362 commented May 2, 2012

I looked it up last night, $_ is the default variable for functions in Perl.

In the sub, $_ contains the value of the arguments passed in. It is also passed to functions with arguments during a call with no arguments. So, invoking regex without explicitly passing a source uses $_.

Easy enough but not very intuitive IMO. Perl kinda feels like assembly, the $1-$9 variables etc. I'm headed into to work in a bit, I'll implement the improvements. Thanks again!

bm1362 commented on 470ce3c May 2, 2012

Thanks to crazedpsyc for refactoring!

@crazedpsyc crazedpsyc merged commit 470ce3c into duckduckgo:master May 3, 2012

@bm1362 Hey there, when you get a chance, would you mind sending an email to please? Or if you don't mind, can I have your email address? (We have some goodies for our contributors!) Thanks!

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