Adding incremental id value to features with certain attribute value

Question

I am using QGIS 3.12. I have a layer of trees and a virtual field 'tree number' that counts all trees that I add to the layer ($id). I want to set that if a height of a tree is more than 3 (in field 'height') it will not be counted in the 'tree number' field. Can it be done?

MrXsquared · Accepted Answer

Use if("height" > 3,NULL,$id) as expression for your virtual field.

Taras · Answer

Despite a suggestion from @MrXsquared, I think this task has to be tackled a slightly different way.
To achieve a continuous id-chain, try the following expression in the Field Calculator
if("height">3, array_find(array_agg($id, filter:="height">3), $id), NULL)

Resulting in

If the "id"s have to start with 1, then use the following expression
if("height">3, array_find(array_agg($id, filter:="height">3), $id) + 1, NULL)

Adding incremental id value to features with certain attribute value

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