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Thread: Is it possible to use QString::contains(QRegExp) in a switch statement?

  1. #1
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    Post Is it possible to use QString::contains(QRegExp) in a switch statement?

    Say I'm reading a file, line by line, and I want to check if the line QString satisfies any QRegExp I have defined.

    I could use a bunch of if statements such as :
    Qt Code:
    1. while(!in.EOF)
    2. {
    3. line = in.readLine();
    4.  
    5. if(line.contains(RegExp1)){...}
    6. else if(line.contains(RegExp2){...}
    7. ...
    8. else if(line.contains(RegExpN){...}
    9. }
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    But I thought it would be neater to write it in the format of a switch. Can this be done. I've googled, but it gives no relevant results.

    I thought I could have done something like this:

    Qt Code:
    1. while(!in.EOF)
    2. {
    3. switch(true)
    4. {
    5. case line.contains(RegExp1): {...}
    6. ...
    7. case line.contains(RegExpN): {...}
    8. }
    9. }
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    Doesn't work though.

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    HI,

    in C++ the case argument MUST be an INTEGER (char, short, int, ...)
    A camel can go 14 days without drink,
    I can't!!!

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    If you are sure each line will satisfy only one regular expression, you can do:
    Qt Code:
    1. int mask = 0;
    2. mask |= line.contains(rx1) << 1;
    3. mask |= line.contains(rx2) << 2;
    4. mask |= line.contains(rx3) << 3;
    5. // etc.
    6. switch(mask) {
    7. case 1: ...
    8. case 2: ...
    9. case 4: ...
    10. };
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    Just be aware this code is suboptimal - every regular expression has to be evaluated. With a bunch of if statements only those are evaluated that are before the one that matches.

    If you are worried about your code looking like spaghetti, you can simplify the syntax with macro expansion.
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  4. The following user says thank you to wysota for this useful post:

    positive_4_life (2nd March 2011)

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    Qt Code:
    1. mask |= line.contains(rx1) << 1;
    2. mask |= line.contains(rx2) << 2;
    3. mask |= line.contains(rx3) << 3;
    4. // etc.
    5. switch(mask) {
    6. case 1: ...
    7. case 2: ...
    8. case 4: ...
    9. };
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    If you use this structure, you should also add a default clause to the switch, with an assertion failure or exception. This will protect you against the inevitable day when something in the code changes and more than one of the regular expressions succeeds.

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    Quote Originally Posted by jdiewald View Post
    If you use this structure, you should also add a default clause to the switch, with an assertion failure or exception. This will protect you against the inevitable day when something in the code changes and more than one of the regular expressions succeeds.
    He's reading the data from a (external) file. Adding assertions/exceptions can be quite dangerous. The programm should not crash every time the file is messed up.

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    Can you be more specific what you are parsing?
    I'm quite sure that this issue can be solved in better way.
    One of possibilities is union of regular expretions:
    Qt Code:
    1. QRegExp regExp("(regExp1)|(regExp2)|(regExp3)|(regExp4)")
    2.  
    3. while(1) // this (!in.EOF) was wrong!
    4. {
    5. line = in.readLine();
    6. if (line.isNull()) {
    7. break;
    8. }
    9. if(line.contains(regExp)){
    10. const QStringList result = regExp.capturedTexts();
    11. int i;
    12. for (i=0; i<result.count(); ++i) {
    13. if (!result[i].isEmpty()) {
    14. break; // or some action to take
    15. }
    16. }
    17. // process i value
    18. }
    19. }
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    Last edited by MarekR22; 3rd March 2011 at 10:41.

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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    @MarekR22: Could you explain why a while(1) should be better than a check of the end of the buffer/file/whatever?

    But another idea:

    Qt Code:
    1. // base class
    2. struct Matcher {
    3. virtual bool matches(line);
    4. virtual void doStuff();
    5. }
    6.  
    7. // impl
    8. struct Matcher1 : public Matcher {...}
    9. struct Matcher2 : public Matcher {...}
    10.  
    11. // algorithm
    12. // 1.) make a list of the matcher
    13.  
    14. foreach(const QString& line, lines) {
    15.  
    16. foreach(Matcher* matcher, list_of_matchers) {
    17.  
    18. if(matcher->matches(line)) {
    19. matcher->doStuff();
    20. break;
    21. }
    22. }
    23.  
    24. }
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  9. #8
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    Default Re: Is it possible to use QString::contains(QRegExp) in a switch statement?

    this is more like STL habit where this kind construction is obligatory since in STL istream::eof() returns true only when attempt of reading bound end of file happened.
    In Qt you there is no EOF method, but method QTextStream::atEnd and this is why I confused things (sorry). This different names indicates that it works in different way.

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