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How can I allocate the $rownum value in reverse order in QGIS?

I have a created a grid layer to use in an atlas and the ID allocated is from top left to bottom right - which is what I want if I want to create a $rownum attribute to use as the map number in my atlas (e.g. 1 - 78)

But I have selected only those grid polygons that have relevant data beneath them for my maps, so I have created a grid layer that is a subset of the original. In this subset grid, the ID numbers have been reversed - that is, they go from bottom right to top left so if I create the $rownum field the numbers will be in that reverse [new] order. (e.g. 45 - 1).

So I want to allocate that $rownum attribute in reverse order so that the atlas maps read from top left to bottom right.

Can anyone help?

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Internally you cannot modify Row Numbers. Your calculation into a new field is the number of rows - $rownum:

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This will reverse the $rownum into a field. $rownum (row number) objects cannot be modified as they are handled by the driver/database. In the case of shapefiles they are transient as edited features move to the end and then get compressed back to 0 based, unique and ascending on save edits.

  • that looks as if it will change one row id. For the sake of the example in my question I pulled 45 out of the air. I actually have 118 grid polygons. I'm not keen to do this individually for each grid. – Johanna Mar 12 '15 at 0:51
  • I guess what I'm looking for is an 'auto increment' function that I can use on a new field in the table. – Johanna Mar 12 '15 at 0:59
  • The whole point of the field calculation is that it does it for the field on every row in the table. If you have 118 polygons then it's 118 - $rownum. There are ways to auto increment on create but that requires some programming. – Michael Stimson Mar 12 '15 at 1:03
  • Give this a try, it should solve your issue. If it doesn't you can always just rollback the edits. – Nathan W Mar 12 '15 at 1:06
  • Thank you, it didn't look like it was going to work, but it did. – Johanna Mar 12 '15 at 2:11

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