Deprecated: Function ereg() is deprecated in /rscs/rs/websites/lmpx.com/code/nav/common.inc on line 82

From:	rich.japh@gmail.com	Date:	Sat Jun 27 21:39:19 2009
Subject:	Re: help on example from "mastering algorithm with perl" on binarysearch

Thank you.

Its very clear now.
-----Original Message-----
From: "John W. Krahn" <jwkrahn@shaw.ca>

Date: Sat, 27 Jun 2009 10:46:28
To: Perl Beginners<beginners@perl.org>
Subject: Re: help on example from "mastering algorithm with perl" on binary
search


rich lee wrote:
> Hello everyone,

Hello,

> I am trying to read "mastering algorithm with perl" and below example has me
> bit stuck.
> I understand everything except these 2 lines
>
>
> $low = $try + 1, next if $array->[$try] lt $word;
> $high = $try -1, next if $array->[$try] gt $word;

Those two lines are short for:

if ( $array->[ $try ] lt $word ) {
$low = $try + 1;
next;
}
if ( $array->[ $try ] gt $word ) {
$high = $try - 1;
next;
}


> I understand what it's doing but I am not sure how they are being ran.
> For example, Let's say in one particular turn, $array->[$try] is greater
> than $word...
> I am trying to understand how it skips this line ---> $low = $try + 1, next
> if $array->[$try] lt $word;

If $array->[$try] is greater than $word then it cannot be less than
$word, or equal to $word at the same time.


> when I look at above 2 lines, it looks like they should be processed
> sequentially. Meaning $low = $try + 1 should be processed everytime...and
> $high=$try-1 should be
> processed everytime....

No, only "if $array->[$try] lt $word" is processed every time.


> In reality(after looking at the explanation), it should only lower or raise
> when $array->[$try] is either less than(then it should raise the bottom) and
> greater( should lower the top)
> I think I am just not understanding the syntax on above 2 lines.. can
> someone please describe it for me(or am i missing the precedence of the
> relationship of the line??
>
>
> $index = binary_search( \@array, $word )
>
> sub binary_search {
> my ($array, $word) = @_;
> my ($low, $high) = ( 0, @$array - 1 );
>
> while ( $low <= $high ) {
> my $try = int( ($low+$high)/2 );
> $low = $try + 1, next if $array->[$try] lt $word;
> $high = $try -1, next if $array->[$try] gt $word;
>
> return $try; #we've found the word!
> }
> return; #the word isn't there
> }



John
--
Those people who think they know everything are a great
annoyance to those of us who do. -- Isaac Asimov

--
To unsubscribe, e-mail: beginners-unsubscribe@perl.org
For additional commands, e-mail: beginners-help@perl.org
http://learn.perl.org/