Question

get longest matching substring from a list

Asked by: itcdr

how can I get the longest matching substring from a large list?

string = "david letterman kiefer sutherland"

list:
------
david
late show with david letterman
... (> 10,000 names)


so I want to match "david letterman" to show in the list "late show with david letterman" then take off "kiefer sutherland" for the episode name.

any ideas?

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Asked On
2007-07-02 at 14:10:05ID22671613
Topics

PHP Scripting Language

,

MySQL Server

,

Perl Programming Language

Participating Experts
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Points
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Comments
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Answers

 

by: JamesCsslPosted on 2007-07-02 at 14:39:39ID: 19407132

To get the longest string, you should probably sort your test strings from longest to shortest.

 

by: GEM100Posted on 2007-07-02 at 14:42:45ID: 19407158

Try using diff_match() function from here:

http://shaunwagner.com/user_files/php/diff_patch.txt

 

by: itcdrPosted on 2007-07-02 at 14:50:07ID: 19407203

trying diff_match

solution can contain perl, php or mysql as I use all 3 quite often. just sorting from longest to shortest doesn't solve my problem.

 

by: piramidaPosted on 2007-07-02 at 14:55:25ID: 19407233

Assuming your substring always starts at the beginning of the string you're searching for.
Assuming you have your list in an array of strings, say @strings, and the string you're searching for in $search

Try this:

// this function does the substring calculation
function common_substring_size($string, $search)
{
    // find length of longest common substring first
    $L = array();
    $length1 = strlen($string);
    $length2 = strlen($search);
    $longest = 0;
 
    for ($i = $length1; $i >= 0; $i--)
    {
      $j = 0
      while ($j < $length2 && $i + $j < $length1 && $string[$i + $j] == $search[$j]) $j++
       $longest = max($longest, $j)
     }

     return $longest;
}

// the rest of the code
for( $i = 0; $i <= count($strings); $i ++) {
    $lengths[$i] = common_substring_size($str,$search)
}
$longest_total = 0;
for( $i = 0; $i <= count($lengths); $i++) {
    if ($longest_total < $lengths[i]) {
      $longest_total = $lengths[i]
      $selected_index = $i
    }
}

// Insert a check if $longest_total is above your threshhold value for a non-match, say 5 symbols, if you want...
$leftover = substr($search, $longest_total])

// Now you have the rest of the search string in $leftover
// and the matching entry index in $selected_index
// But be warned, it will not be instant, especially with a large set of strings.

 

by: ozoPosted on 2007-07-02 at 14:57:38ID: 19407241

If
kiefer sutherland
were in the list, would that count as a longer match?
how about
d letterman kiefer s
or
letterman sutherland
Will you be doing this on enough different strings with the same list to make it
worth it to preprocess the list?

 

by: itcdrPosted on 2007-07-02 at 15:05:39ID: 19407295

the show name should be at the begining. so if "kiefer sutherland" were in the list it shouldn't match, but if "david letterman kiefer" were then it would be better than just matching "david letterman"

this will be done across thousands of names, so yes if pre-processing is faster than we should do it.

 

by: GEM100Posted on 2007-07-02 at 15:11:02ID: 19407330

So, in this case, how about just searching for full string, if no results then substr that string removing one character from the end and do search again, and so on. Instead of removing 1 character, you may try removing the whole word from the end. This should do.

 

by: piramidaPosted on 2007-07-02 at 15:18:22ID: 19407364

itcdr, if the performance is really important and you will be searching over the same set of strings many times, consider building suffix trees (http://en.wikipedia.org/wiki/Suffix_tree) for the string set, making the longest match search almost linear (at the expense of clarity and memory usage).

 

by: itcdrPosted on 2007-07-02 at 15:28:30ID: 19407405

thanks. trying all the suggestions. here's an example of one that is having trouble.

string="justice episode name"

in the list we have tons that have "justice" in it:

hollywood_justice
in_justice
justice_2006 - *should match this one*
justice_league_unlimited
...

 

by: ozoPosted on 2007-07-02 at 15:36:36ID: 19407443

Why should it match that one?

 

by: itcdrPosted on 2007-07-02 at 15:47:16ID: 19407497

string="justice episode name"

1) look for any that contain "justice","episode", or "name"

2) look for ones that match the closest at the begining or have the most words in common. now left with

justice_2006
or
justice_league_unlimited

3) dates aren't too important plus the string doesn't contain "league" or "unlimited" so the winner is "justice_2006"


basically I have about 8,000 titles containing show name and episode name or number in no standard format. these need to filed under one of 12,000 show names. and new titles will come in each day. does that make sense?

 

by: steelseth12Posted on 2007-07-02 at 15:52:07ID: 19407520

The only thing i can think of exept from redesigning your database is doing a fulltext search and sort the results by relevancy and limit 1

something like ..


SELECT *, MATCH(columnname) AGAINST("david letterman kiefer sutherland" IN BOOLEAN MODE) as score FROM tablename  WHERE MATCH(columnname) AGAINST("david letterman kiefer sutherland bla bla" IN BOOLEAN MODE) ORDER BY score DESC LIMIT 1

See
http://dev.mysql.com/doc/refman/5.0/en/fulltext-search.html
http://dev.mysql.com/doc/refman/5.0/en/fulltext-boolean.html

 

by: ozoPosted on 2008-06-22 at 11:50:35ID: 21841726

justice_2006
or
justice_league_unlimited

3) dates aren't too important plus the string doesn't contain "league" or "unlimited" so the winner is "justice_2006"

Does that mean that if there is a tie for the length of the match, we should prefer the shorter string?

20120131-EE-VQP-002

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