Thank you.
Its very clear now.
-----Original Message-----
From: "John W. Krahn" <jwkr...@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
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annoyance to those of us who do. -- Isaac Asimov
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