Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. General and Desktop
  4. How to search for a specific character in a QString
Forum Updated to NodeBB v4.3 + New Features

How to search for a specific character in a QString

Scheduled Pinned Locked Moved Solved General and Desktop
30 Posts 9 Posters 43.3k Views 5 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #2

    Hi,

    What about QString::contains ?

    Interested in AI ? www.idiap.ch
    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

    tomyT 1 Reply Last reply
    5
    • ArtmeticA Offline
      ArtmeticA Offline
      Artmetic
      wrote on last edited by
      #3

      or

      bool ClassName::containLetter(QChar letter, QString myString)
      {
      foreach(QChar char, myString)
      if(letter== char)
      return true;
      return false;
      }

      1 Reply Last reply
      0
      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #4

        @Artmetic why reinvent the wheel when the function is already available ?

        Interested in AI ? www.idiap.ch
        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

        1 Reply Last reply
        3
        • ArtmeticA Offline
          ArtmeticA Offline
          Artmetic
          wrote on last edited by
          #5

          Yes it is avaible, but everything depends of the problem. The use of your solution is probably optimalized and faster, but of course it depends :)

          1 Reply Last reply
          0
          • tomyT tomy

            Hi,

            Consider we have a QString and we want to search for a character in it.

            What method is best for it and what does it return when that character is found or not please?

            jsulmJ Offline
            jsulmJ Offline
            jsulm
            Lifetime Qt Champion
            wrote on last edited by
            #6

            @tomy Taking a short look at Qt documentation you would be able to answer your question by yourself...

            https://forum.qt.io/topic/113070/qt-code-of-conduct

            1 Reply Last reply
            0
            • SGaistS SGaist

              Hi,

              What about QString::contains ?

              tomyT Offline
              tomyT Offline
              tomy
              wrote on last edited by tomy
              #7

              @SGaist
              Good solution, but what if we want to know "the index of that QChar" if exists?

              @jsulm
              Good thanks. How should I look for a specific method while apparently there are thousands methods on Qt documentations? For example I want the index of a specific char in a string (if that exists), what is an optimized way to look for it?

              PS: Of course I can use a simple for loop but when there is method already, why increase lines of code by loops!

              J.HilkJ jsulmJ 2 Replies Last reply
              0
              • tomyT tomy

                @SGaist
                Good solution, but what if we want to know "the index of that QChar" if exists?

                @jsulm
                Good thanks. How should I look for a specific method while apparently there are thousands methods on Qt documentations? For example I want the index of a specific char in a string (if that exists), what is an optimized way to look for it?

                PS: Of course I can use a simple for loop but when there is method already, why increase lines of code by loops!

                J.HilkJ Offline
                J.HilkJ Offline
                J.Hilk
                Moderators
                wrote on last edited by
                #8

                @tomy

                the answer would be

                int QString::indexOf(const QString &str, int from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
                

                ;)

                straight from the docu, 10 seconds search. Once you have some kind of routine, the docu is realy fast and good to navigate.


                Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                Q: What's that?
                A: It's blue light.
                Q: What does it do?
                A: It turns blue.

                1 Reply Last reply
                1
                • tomyT Offline
                  tomyT Offline
                  tomy
                  wrote on last edited by
                  #9

                  Thanks, but it does not work OK always. For example consider these:

                  double d = 12000000;
                  QString s = QString::number(d);
                   int point = s.indexOf('.');
                   lineEdit -> setText(QString::number(point));
                  

                  The result is 1!
                  Using this one too:

                  double d = 12000000;
                  QString s;
                  QTextStream ss(&s);
                  ss << d;
                  int point = s.indexOf('.')
                  lineEdit -> setText(QString::number(point));
                  

                  The result is 1 again!

                  matthew.kuiashM 1 Reply Last reply
                  0
                  • tomyT tomy

                    Thanks, but it does not work OK always. For example consider these:

                    double d = 12000000;
                    QString s = QString::number(d);
                     int point = s.indexOf('.');
                     lineEdit -> setText(QString::number(point));
                    

                    The result is 1!
                    Using this one too:

                    double d = 12000000;
                    QString s;
                    QTextStream ss(&s);
                    ss << d;
                    int point = s.indexOf('.')
                    lineEdit -> setText(QString::number(point));
                    

                    The result is 1 again!

                    matthew.kuiashM Offline
                    matthew.kuiashM Offline
                    matthew.kuiash
                    wrote on last edited by
                    #10

                    @tomy Dude, the answer "1" is spot on accurate.

                    If you display the result of QString::number(12000000) you'll get "1.2e+07" == 1.2 * 10^7 == 1.2 * 10000000 == 12000000.

                    Therefore the '.' is the second character e.g. index '1'.

                    Try QString::number(d, 'f', 0). That should work as it will output 12000000 and s.indexof('.') will return -1 (if that's what you want).

                    The legendary cellist Pablo Casals was asked why he continued to practice at age 90. "Because I think I'm making progress," he replied.

                    1 Reply Last reply
                    1
                    • tomyT Offline
                      tomyT Offline
                      tomy
                      wrote on last edited by
                      #11

                      There will not be any dot in other numbers, say, 12.21! Because we have used '0' as the precision!

                      matthew.kuiashM 1 Reply Last reply
                      0
                      • tomyT tomy

                        There will not be any dot in other numbers, say, 12.21! Because we have used '0' as the precision!

                        matthew.kuiashM Offline
                        matthew.kuiashM Offline
                        matthew.kuiash
                        wrote on last edited by matthew.kuiash
                        #12

                        @tomy Precisely. Your example was of an integer number, albeit packed into a double (there is adequate precision for your example)

                        If you ask for extra precision you will get extra '0's after the decimal point. I don't know if this is what you want.

                        What is it you are trying to do?! If you are justing searching for characters use indexOf. Is this is something to do with figuring out how many digits are required to display a number? In which case use math functions such as floor, ceiling and log10. If you want to know about the fractional parts use X-floor(X).

                        The example you gave does work. Just not the way you want it too and that was not what you originally asked for.

                        The legendary cellist Pablo Casals was asked why he continued to practice at age 90. "Because I think I'm making progress," he replied.

                        tomyT 1 Reply Last reply
                        1
                        • SGaistS Offline
                          SGaistS Offline
                          SGaist
                          Lifetime Qt Champion
                          wrote on last edited by
                          #13

                          Also, why not use a QDoubleSpinBox to show your numbers ?

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                          1 Reply Last reply
                          1
                          • tomyT tomy

                            @SGaist
                            Good solution, but what if we want to know "the index of that QChar" if exists?

                            @jsulm
                            Good thanks. How should I look for a specific method while apparently there are thousands methods on Qt documentations? For example I want the index of a specific char in a string (if that exists), what is an optimized way to look for it?

                            PS: Of course I can use a simple for loop but when there is method already, why increase lines of code by loops!

                            jsulmJ Offline
                            jsulmJ Offline
                            jsulm
                            Lifetime Qt Champion
                            wrote on last edited by
                            #14

                            @tomy How? Well, you know it is QString so take a look at its documentation. QString doesn't have "thousands" of methods.

                            https://forum.qt.io/topic/113070/qt-code-of-conduct

                            1 Reply Last reply
                            0
                            • matthew.kuiashM matthew.kuiash

                              @tomy Precisely. Your example was of an integer number, albeit packed into a double (there is adequate precision for your example)

                              If you ask for extra precision you will get extra '0's after the decimal point. I don't know if this is what you want.

                              What is it you are trying to do?! If you are justing searching for characters use indexOf. Is this is something to do with figuring out how many digits are required to display a number? In which case use math functions such as floor, ceiling and log10. If you want to know about the fractional parts use X-floor(X).

                              The example you gave does work. Just not the way you want it too and that was not what you originally asked for.

                              tomyT Offline
                              tomyT Offline
                              tomy
                              wrote on last edited by
                              #15

                              @matthew.kuiash

                              What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

                              I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

                              @SGaist:
                              Very good info, thanks.
                              But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
                              QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

                              matthew.kuiashM Z 2 Replies Last reply
                              0
                              • VRoninV Offline
                                VRoninV Offline
                                VRonin
                                wrote on last edited by VRonin
                                #16
                                double d = 12000000.0;
                                QString s = QString::number(d,'f',std::numeric_limits<decltype(d)>::max_digits10);
                                if(s.contains('.')){
                                int i=s.size()-1;
                                for(;s.at(i)=='0';--i){}
                                s=s.left(i+ (s.at(i)=='.' ? 0:1));
                                }
                                

                                Edit:

                                alternative:

                                // needs #include <cmath>
                                double d = 12000000.0;
                                double junk;
                                int precision=0;
                                for(double tester=d;!qFuzzyIsNull(std::modf(tester,&junk));++precision,tester*=10.0){}
                                QString s = QString::number(d,'f',precision);
                                

                                "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                ~Napoleon Bonaparte

                                On a crusade to banish setIndexWidget() from the holy land of Qt

                                tomyT 1 Reply Last reply
                                3
                                • VRoninV VRonin
                                  double d = 12000000.0;
                                  QString s = QString::number(d,'f',std::numeric_limits<decltype(d)>::max_digits10);
                                  if(s.contains('.')){
                                  int i=s.size()-1;
                                  for(;s.at(i)=='0';--i){}
                                  s=s.left(i+ (s.at(i)=='.' ? 0:1));
                                  }
                                  

                                  Edit:

                                  alternative:

                                  // needs #include <cmath>
                                  double d = 12000000.0;
                                  double junk;
                                  int precision=0;
                                  for(double tester=d;!qFuzzyIsNull(std::modf(tester,&junk));++precision,tester*=10.0){}
                                  QString s = QString::number(d,'f',precision);
                                  
                                  tomyT Offline
                                  tomyT Offline
                                  tomy
                                  wrote on last edited by tomy
                                  #17

                                  @VRonin
                                  Thank you.

                                  Both have flaws. But also both have high level of precision.
                                  I think the flaws belong to the way numbers are stored in registers. And I think there is no completely (100%) accurate calculator in the world because of that.

                                  I chose the latter. Thanks for your help. I'm appreciative.

                                  1 Reply Last reply
                                  0
                                  • tomyT tomy

                                    @matthew.kuiash

                                    What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

                                    I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

                                    @SGaist:
                                    Very good info, thanks.
                                    But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
                                    QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

                                    matthew.kuiashM Offline
                                    matthew.kuiashM Offline
                                    matthew.kuiash
                                    wrote on last edited by
                                    #18

                                    @tomy OK. You have plenty to go on here but I'll just caution you that the number of digits after the decimal point can be extremely high for doubles.

                                    The legendary cellist Pablo Casals was asked why he continued to practice at age 90. "Because I think I'm making progress," he replied.

                                    tomyT 1 Reply Last reply
                                    0
                                    • matthew.kuiashM matthew.kuiash

                                      @tomy OK. You have plenty to go on here but I'll just caution you that the number of digits after the decimal point can be extremely high for doubles.

                                      tomyT Offline
                                      tomyT Offline
                                      tomy
                                      wrote on last edited by tomy
                                      #19

                                      @matthew.kuiash
                                      I have used as high precision as possible for doubles using VRonin's code.
                                      I think I'm at the end. The differences between what we expect and what is shown using the code may happen for low of high precisions because of the difference of the way we and the machine calculate expressions.

                                      1 Reply Last reply
                                      0
                                      • tomyT tomy

                                        @matthew.kuiash

                                        What I need is showing a double result. It may be a very small negative/positive number (say, -0.000000001234) or a very big positive/negative number (say, +2500000000036540). It's range I should cover by code.

                                        I only need the 'number of' digits after the decimal point. Then I will use another method to use 'that number' precision for the output.

                                        @SGaist:
                                        Very good info, thanks.
                                        But the lineEdit there, is used to "show" the result of a calculation. (My calculator works fine but I just want to make it more precise).
                                        QDoubleSpinBox has two buttons and works in essence like an editable cadre, while I just need "to show" the result (and also save the current result for the next expressions).

                                        Z Offline
                                        Z Offline
                                        zapprox
                                        wrote on last edited by
                                        #20

                                        Hi @tomy. QDoubleSpinBox can be used as you mentioned . First, to hide buttons call QAbstractSpinBox::setButtonSymbols(ButtonSymbols bs) and second call QAbstractSpinBox::setReadOnly(bool r).
                                        Have a nice day.

                                        VRoninV tomyT 2 Replies Last reply
                                        0
                                        • Z zapprox

                                          Hi @tomy. QDoubleSpinBox can be used as you mentioned . First, to hide buttons call QAbstractSpinBox::setButtonSymbols(ButtonSymbols bs) and second call QAbstractSpinBox::setReadOnly(bool r).
                                          Have a nice day.

                                          VRoninV Offline
                                          VRoninV Offline
                                          VRonin
                                          wrote on last edited by
                                          #21

                                          QDoubleSpinBox can be used as you mentioned

                                          You still have to hard code the precision (number of decimals) shown so it does not solve the problem

                                          The differences between what we expect and what is shown using the code may happen for low of high precisions

                                          The maximum number of digits that can be represented in text is is std::numeric_limits<double>::digits10, you can't get meaningful results if you go beyond this limit

                                          "La mort n'est rien, mais vivre vaincu et sans gloire, c'est mourir tous les jours"
                                          ~Napoleon Bonaparte

                                          On a crusade to banish setIndexWidget() from the holy land of Qt

                                          1 Reply Last reply
                                          1

                                          • Login

                                          • Login or register to search.
                                          • First post
                                            Last post
                                          0
                                          • Categories
                                          • Recent
                                          • Tags
                                          • Popular
                                          • Users
                                          • Groups
                                          • Search
                                          • Get Qt Extensions
                                          • Unsolved