expressionfields-attributesfilterqgis
I'm using QGIS and I want to create a rule to filter a dataset by size and by name (all areas over 80000 squarekilometers that begin with an A)
This is the expression I built:
"areakm2" > 80000 AND left("NAME", 1="A")
I always get an error message stating :
Field 'A' not found!
Why does QGIS not recognize that A is regarding the first letter of the selected string, how would I need to change my expression?
Use this expression to get an array with all fieldnames starting with per_:
array_filter(
map_akeys(attributes()),
left (@element,4)='per_'
)
Now for each of these fieldnames (elements of this array), run your expression separately, using array_foreach() .
To run your expression for each of these fieldnames, create the expression as a string and access the varying fieldnames with @element (this inserts the values of the elements from the array of step 2) by concatenating @element and the rest of the expression with pipes || to get everything in one string . Then convert this string to a working expression with eval():
array_foreach(
array_filter(
map_akeys(attributes()),
left (@element,4)='per_'
),
eval (
'array_sum(
aggregate(
layer:= ''your_layer_name'',
aggregate:=''array_agg'',
expression:=' || @element || ',
filter:=is_selected()
)
)'
)
)
Remark: inside the eval() function, you need to define the expression as a string, defined by single quotes '. As the expression that should be evaluated contains itself a string ('your_layer_name'), you must mask the single quotes here so that QGIS understands that the string used to be evaluated by eval is not over. Use two single quotes '' (not double quotes ") for masking.
Visualizing the sum for all selected points (yellow) of the values of all fields starting with per_; selected points are additionally labeled with their individual values:
You approach is possible, but very inefficient. I would heavily advice to create an atlas (see my other solution). However, just to answer your question and show a way to solve your problem with the approach you tried. There are in fact two problems with what you tried:
Use the same CRS for the project and the layer(s) you use to get the intersection. You had EPSG:32629 as project CRS (used also for your layout), but EPSG:3763 for the layer Cont_AAD_CAOP2021. Reproject the layer to the project CRS, then intersection will work.
Now use the following expression - see below for explanation:
array_max(
array_foreach (
aggregate( 'Cont_AAD_CAOP2021', 'array_agg', $id),
if(
intersects (
map_get (item_variables('Map 1'), 'map_extent_center'),
geometry (get_feature_by_id ('Cont_AAD_CAOP2021', @element))
),
attribute (
get_feature_by_id ('Cont_AAD_CAOP2021', @element),
'Concelho'
),
''
)
))
Explanation:
The correct syntax to get the center of the current layout page is: map_get (item_variables ('Map 1'), 'map_extent_center'), see item_variables() and map_get().
Instead of aggregate/concatenate the names of the attribute "Concelho" and filtering by intersection, rather use aggregate/array_agg to get an array of all id's of the layer, then use get_feature_by_id () and geometry() to get an array of the geometries from the layer. With array-foreach() an an if() clause then test if these geometries intersect with the center of the layout (step 2).
For the geometry that returns true (intersects with the center of the current layout), get the value of the field Concelho using attribute().
Use array_max() to get rid of the empty values and keep only the value of the field Concelho that indeed intersects the layout's center.
Best Answer
In QGIS text strings are enclosed in single quotes (') while double quotes (") are for field names, so your expression should be: