Qt World Summit: Submit your Presentation

Converting QList<QString> to QList<float>?

  • Hi all,

    In my code I call a function that reads a text file, then uses a RegEx to parse it and then it returns a QStringList of comma separated values that I'm searching for (example): "13:53:33,479, 323.05" - the first is a timestamp, the next is the ID of the value within the timestamp (i.e. at time 13:53:33 there were 479 values recorded) and the last is the specific value I'm looking for. This part is working great (though the regex seems to be slow!).

    I want to use the time/value pairs to graph the data in a QCustomPlot and I've been struggling to extract the value out and convert it to a list of floats for the graph. Using an answer from this forum, I have been able to separate the QStringList and create a QList with the three items:

    void MainWindow::graphSelected(QStringList _graphingData) {
        graphingData = _graphingData;
        QStringList a{graphingData};
            QList<QStringList> list;
            foreach (const QString &s, a) {
            qDebug() << data;

    which gives me this list (qDebug() << data): ("14:09:17", "843", " 396.824"), ("14:09:18", "342", " 396.824")...

    For the life of me, I can't seem to figure out how to then iterate through and pull out the third item ("396.824"), convert it from a list of strings to a list of floats.

    Any tips?

    Thanks and best regards!

  • Hi @MScottM,

    Converting to a list of floats should be pretty easy, something like:

    void graphSelected(const QStringList &_graphingData) {
        QList<QStringList> list;
        QList<float> floats;
        foreach (const QString &s, _graphingData) {
            const QStringList parts = s.split(",");
            Q_ASSERT(parts.size() >= 3);
        qDebug() << list;
        qDebug() << floats;

    This gives, for example:

    (("14:09:17", "843", " 396.824"), ("14:09:18", "342", " 396.824"))
    (396.824, 396.824)

    Where the first output line is a list of list of strings, whereas the second line is a list of floats with matching top-level indexes.

    Note also, I've added an a Q_ASSERT to say that we expect there will always be at least three items per record. This is fine for dev, but if this is not guaranteed, then drop the Q_ASSERT and do something like this instead:

    floats.append(parts.size() < 3 ? NAN : parts.at(2).toFloat());

    That will ensure that the two lists still maintain the same indexes when values are missing. Of course, you can detect / handle missing values lots of other ways too.

    Also, depending on your case, you might want to define a class or struct to hold the tuple (the three values on each row), and use a QList<MyRowType> instead of both QList<QStringList> and QList<float> together. But that depends on what you want to do once you've parsed the data.

    PS I see no need for regex anywhere here... if you explain what you're using it for, then we might be able to help there too.


  • Yes! thanks @Paul-Colby - It works - as usual the code was simple, I just didn't have the experience to figure it out.

Log in to reply