In QGIS Atlas how to get the index and the number of features grouped/filtered by an Attribute with expression?

Question

Using a QGIS Expression in the Layout Editor, how can one return the count of features matching a particular attribute value for an atlas feature, for example, "AREAS_Area".

In this particular example, the valid range of values for "Areas_Area" is 1-25. I would like to return the count of features matching the "Areas_Area" attributes for 1, 2, 3, etc.

Here is what I have currently, which provides me with an index (current Area number out of total Area numbers), as well as the page count (current feature number out of total number of features).

concat('Area: ',"Areas_AREA",' / ',maximum("Areas_AREA"),' | ','Segment: ',@atlas_featurenumber,' / ',@atlas_totalfeatures)

Image preview:

So given the above information and the result I have in the preview, my expectation would be that it would instead read something more like:
Area: 1 / 25 | Segment: 1 / 13

Answer 1

You want to get two information dynamically in your atlas.

Get the index of the current segment within the group of segment with the same Areas_AREA

You have to use an id that is unique for this operation. I would use RIN for this but you can replace it with any id you have for your segment as long as it is in the same order that your atlas is ordered. You can use the formula array_agg() to get an array (kind of list) of all the RIN of segment that have Areas_AREA equal to Areas_AREA of your current segment. Then you can use array_find() to get the index of your current segment inside this array. You have to add +1 because the first element of an array have index 0.

array_find(array_agg("RIN", filter:="Areas_AREA"=attribute (@atlas_feature,'Areas_AREA'),"RIN") +1

OR a calculation with more explicit values :

array_find ( aggregate( @atlas_layername, aggregate:='array_agg', expression:="RIN", filter:="Areas_AREA" =attribute (@atlas_feature ,'Areas_AREA')), "RIN" )+1

The result should be 1 for the first segment then 2 then 3, then when your segment is in another area it should start from 1 again.

The number of segment that have the same Areas_AREA

To get the number of segment that have the same area id you can use the formula count() number of feature with Areas_AREA equal with the attribute Areas_AREA of your current atlas feature :

count(1, filter:="Areas_AREA" =attribute (@atlas_feature ,'Areas_AREA'))

The result should be a number equal to the number of feature for each area

Or you can also use array like before and use formula

array_length(array_agg("RIN", filter:="Areas_AREA"=attribute (@atlas_feature,'Areas_AREA')) 

The final formula would be something like :

concat('Area: ',"Areas_AREA",' / ',maximum("Areas_AREA"),' | ','Segment: ', 
array_find (aggregate( @atlas_layername, aggregate:='array_agg', expression:="RIN", filter:="Areas_AREA" =attribute (@atlas_feature ,'Areas_AREA')), "RIN" )+1, 
'/',
count(1, filter:="Areas_AREA" =attribute (@atlas_feature ,'Areas_AREA')))

Note : To make atlas of atlas the report function could be useful and could add title page for each group of segment with specific maps.