1. You can iterate with QAbstractItemModel.index(row, column) with row from 0 to QAbstractItemModel.rowCount() and column = column with the value to search for
2. Look at http://doc.qt.nokia.com/4.7/modelview.html under 2.2
3. When you only want to export the columns that are visible, then you have to check all columns if they are visible.
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