Question

I am using compare function to sort QList
and MyClass has n different attribute(s) like address, age, firstname, lastname etc

SortKey is something like this.

typedef QPair<QString, Qt::SortOrder> SortKeyPair;         
//pair of attr name (i.e. gender and order of sorting asc|desc)


static bool compare( MyClass *o1, MyClass *o2)
{
    //sortKey is global static var.
    if (sortKey.second == Qt::AscendingOrder) {
        if (o1->dataField(sortKey.first) < o2->dataField(sortKey.first)) return true;
        else return false;
    } else {
        if (o1->dataField(sortKey.first) > o2->dataField(sortKey.first)) return true;
        else return false;
    }
    return false;
}

Sorting by single attr. works fine with qSort.
But I need list sorted with multiple attr. like ‘sort by lastname, then by age, then by gender’

I need a sort function to sort by multiple sortKey (which is not working !)

How can we sort with multiple sort keys?
Is there any data structure available that supports this ? (like map always keep things sorted according to key given)

Answer 1

Just define your predicate in the logical way, using the major key for comparison and using the minor key as a fallback when the major is inconclusive:

if (o1->dataField( firstSortKey ) < o2->dataField( firstSortKey ))
    return true;
else if (o1->dataField( firstSortKey) > o2->dataField( firstSortKey ))
    return false;
else if ( o1->dataField( secondSortKey ) < o2->dataField( secondSortKey ) )
    return true;
else
    return false;

or, for the general case:

bool compare_on( int key_index, ...) {
  if (o1->dataField( key[key_index] ) < o2->dataField( key[key_index] ))
    return true;
  else if (o1->dataField( key[key_index] ) > o2->dataField( key[key_index] ))
    return false;
  else if ( key_index == max_key_index )
    return false;
  else
    return compare_on( key_index + 1 );