Qt/C++ - Lesson 062. Comparator for sorting QList

QList, sort, сортировка

When working with simple types, you can use standard sorting tools. If you want to sort complex objects, by a set of parameters, then you need to write a special comparator, which will perform a comparison on the required parameters.

Let's write a small comparator that will sort objects of class QPointF . Sorting rules will be simple. First of all, the sorting should be done on the X coordinate, and if the X coordinates are the same, then we sort by the Y coordinate.

That is, before sorting the list can look like this:

(QPointF(12.4,15.4), QPointF(14.1,7.5), QPointF(7.2,5.2), QPointF(4.2,18.5), QPointF(4.2,1.2), QPointF(-19.5,12.4), QPointF(1.1,1.2))

After sorting the list will look like this:

(QPointF(-19.5,12.4), QPointF(1.1,1.2), QPointF(4.2,1.2), QPointF(4.2,18.5), QPointF(7.2,5.2), QPointF(12.4,15.4), QPointF(14.1,7.5))

Comparator

Let us write the comparator as a static function:

// The comparator must be a static function,
// Otherwise it will not be possible to pass it as an argument
static bool compare(const QPointF& first, const QPointF& second)
{
    if (first.x() < second.x())
    {
        return true;
    }
    else if (first.x() > second.x())
    {
        return false;
    }
    else
    {
        if (first.y() < second.y())
        {
            return true;
        }
        else
        {
            return false;
        }
    }
 
}

Using the comparator

// Create and fill the list
QList<QPointF> pointsList;
pointsList << QPointF(12.4, 15.4)
           << QPointF(14.1, 7.5)
           << QPointF(7.2, 5.2)
           << QPointF(4.2, 18.5)
           << QPointF(4.2, 1.2)
           << QPointF(-19.5, 12.4)
           << QPointF(1.1, 1.2);
 
// Let's check the content of the list
qDebug() << pointsList;
 
// sort
std::sort(pointsList.begin(), pointsList.end(), compare);
 
// check result
qDebug() << pointsList;

The result will be similar to the one that was given at the beginning.