1

In QGIS 3, are aggregate functions over a set of array elemnts possible?

As simple as in the form of:

aggregate(array:=array(1,2,3), aggregate:='mean')

I'd use it for the task to get an average for an array of raster values, which I extract at the nodes of a linestring input feature layer. The raster values at the linestring nodes I can retrieve beautifully with the below expression, but obviuosly, there is no way to aggregate those values into on single aggregate for each feature.

with_variable('nodes',
    nodes_to_points( $geometry),
    array_foreach(  generate_series(1, num_points(@nodes)), raster_value(DGM_Slope', 1, point_n(@nodes, @element)))
)

2 Answers 2

2

array_mean() is not yet available in the latest release, but I found a neat workaround with eval() and array_to_String() :)

with_variable('nodes', 
    nodes_to_points( $geometry),
    eval(
        array_to_string(
            array_foreach(  generate_series(1, num_points(@nodes)), raster_value('Slope', 1, point_n(@nodes, @element)))
        , '+')
        ) * 1/num_points(@nodes)
)
1

You can simply use the function array_mean(array) (available since QGIS 3.18) - see the examples in the explanation on the right side of the expression string builder:

array_mean(array(0,1,7,66.6,135.4)) → 42

array_mean(array(0,84,'a','b','c')) → 42

3
  • In my 3.12.3 i don't see an array_mean function...
    – Kay
    Apr 14, 2021 at 16:56
  • 1
    ..I googled the function: It's not available in the latest stable release 3.16...
    – Kay
    Apr 14, 2021 at 16:58
  • 1
    Sorry, did not realize that it was introduced in 3.18 only - updated my answer accordingly
    – Babel
    Apr 14, 2021 at 17:04

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