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I make a new column in my attribute table and it has a default value (for example 0). I want to (probably using the Field Calculator) have the result that row 1 has the value 1, row 2=2 and so on, just like a numbered index.
How do I do this in the Field Calculator (including the use of $rownum which I think I need for switching to the line)?
Just put $rownum (QGIS 2) or @row_number (QGIS 3) as the expression.
An up-to-date list of all the Field Calculator functions can be read in the official docs.
@ArMoraer has developped an interesting plugin to be able to control the way this automatic numbering is made, it's named "Sort and Number". You can use it to fill a column with unique numbers by taking into account 3 other fields to sort your data like you want before numbering.
See the source topic : Is there a way to assign row number for sorted table by column in QGIS?
If you are using QGIS 3 you must used @row_number
For me the best and easiest way is just $id, this will return a id number to each row indexing it from 0.
$id, after deleting rows should corroborate @row_number. If I correctly understood your affirmation.