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Thread: String similarity check

  1. #1
    Join Date
    Jan 2012
    Dortmund, Germany
    Thanked 10 Times in 8 Posts
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    Default String similarity check

    Is there any ready-made string similarity check in Qt?
    I've read about Levenshtein distance or soundex, is there anything like this built in?

    According to this thread (sorry, German language) I've implemented a counting of small substrings that gives me quite reasonable results for Names (which I need to compare) when I take substrings of n=2 chars and an 80% threshold.

    I'd like to know if there is a better routine fast enough that I can just take and use.

    This is what I use right now (corrections on style and speed are welcome!)

    Qt Code:
    1. bool isSimilar(QString a, QString b, qreal percentage=80, int n = 2, Qt::CaseSensitivity caseSense= Qt::CaseInsensitive);
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    Qt Code:
    1. bool MainWindow::isSimilar(QString a, QString b, qreal percentage, int n, Qt::CaseSensitivity caseSense)
    2. //Iterates substrings in groups of n chars from a und finds these in b.
    3. //The number of hits is then divided by the length of the shorter string.
    4. //To properly take word beginnings and endings into account
    5. //spaces are being inserted before and after the strings.
    6. {
    7. if (a.isEmpty()||b.isEmpty()) return false;
    8. qreal hits=0;
    9. a=QString(" ").repeated(n-1)+a+QString(" ").repeated(n-1);
    10. b=QString(" ").repeated(n-1)+b+QString(" ").repeated(n-1);
    11. QString part;
    12. for (int i=0;i<a.count()-(n-1);i++)
    13. {
    14. part=a.mid(i,n);
    15. if (b.contains(part,caseSense)) hits++;
    16. }
    17. if (a.length()<b.length()) return (percentage < (100*hits/(a.length()-(n-1))));
    18. else return (percentage < (100*hits/(b.length()-(n-1))));
    19. }
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    For the name "Markus Bertram" I get these results:
    • Bertram, Markus - 93,3
    • Markus E. Bertram - 100
    • Marcus Emil Bertram - 86,7
    • marc bertram - 84,6 (case-insensitive)
    • Martin Bertram - 73,3 (false)
    • Martin Bergmann - 46,7 (false)
    Last edited by sedi; 23rd June 2012 at 16:46. Reason: spelling corrections

  2. #2
    Join Date
    Sep 2011
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    Default Re: String similarity check

    don't know anything off hand.

    your current implementation should at least include this change

    Qt Code:
    1. const QString spaces = QString(" ").repeated(n-1);
    2. a = spaces + a + spaces;
    3. // etc
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    If you have a problem, CUT and PASTE your code. Do not retype or simplify it. Give a COMPLETE and COMPILABLE example of your problem. Otherwise we are all guessing the problem from a fabrication where relevant details are often missing.

  3. The following user says thank you to amleto for this useful post:

    sedi (23rd June 2012)

  4. #3
    Join Date
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    Warsaw, Poland
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    Default Re: String similarity check

    The most basic approaches I know are Hamming distance and Edit Distance (aka Levenshtein Distance). There are other string metrics available, though:

    Edit distance is quite easy to implement efficiently using QtConcurrent or OpenCL..
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.

  5. The following user says thank you to wysota for this useful post:

    sedi (23rd June 2012)

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