Get more than one elements from array in QGIS expressions using square brackets

Question

Using QGIS expressions, if you have an array, you can get the n-th element by simply adding a number (n) in square brackets as index operator, starting with 0 for the first element. Like this, array (1,2,3,4,5,6,7,8)[2] returns 3 (index no. 2/third element).

Is there a way to retrieve more than one element in this way with QGIS expressions?

What I tried, based on similar options in R, but doesn't work here:

array (1,2,3,4,5,6,7,8)[2,3] -- to retrieve elements 2 and 3
array (1,2,3,4,5,6,7,8)[array(2,5)] -- to retrieve elements 2, 3 and 5

Answer 1

I guess no. At least in Python they are called index brackets and are not meant to contain several indizes. Don't know about other languages like R or C. But you can build a custom function:

from qgis.core import *
from qgis.gui import *
@qgsfunction(args='auto', group='Custom', referenced_columns=[])
# arr is an array-input you want to return several values from by their index
# idxarr is an array-input containing the indizes you want to return from arr
def array_get_several(arr, idxarr, feature, parent):
 elements = [arr[i] for i in idxarr]
 return elements

and use it as array_get_several(array(1,2,3,4,5,6),array(1,2)) which for example returns [2,3]. Note that it returns the values ordered, so e.g. array_get_several(array(1,2,3,4,5,6),array(2,1)) does return [3,2].

• geozelot: docs.qgis.org/3.22/en/docs/user_manual/expressions/…
• MrXsquared: @geozelot good point, but will only work for a range. Not e.g. to return only index 1 and 4, but not 2 and 3. However, worth posting as answer ;)

Answer 2

Addendum to @MrXsquared's answer. You can also define a range of indexes.

from qgis.core import *
from qgis.gui import *
@qgsfunction(args='auto', group='Custom', referenced_columns=[])
def array_get_range(arr, start, end, feature, parent):
 return arr[start:end] # exclusive end

array_get_range( array(0,1,2,3,4,5,6,7), 2, 6 ) returns [2, 3, 4, 5].

• geozelot: docs.qgis.org/3.22/en/docs/user_manual/expressions/…
• Kadir Şahbaz: @geozelot QGIS already has a function for the second approach. Trying to solve directly in PyQGIS instead of searching if there is a solution in Expression is my bad habit. :DD
• geozelot: Oh that's not the worst habit ,) No need to reinvent the wheel, though.

Answer 3

The answer by @MrXsquared is indeed an elegant way to do that, lacking the simple way with just square brackets.

With just native functions, a workaround would be:

array_foreach (
 array(0,3),
 array_get (array(1,2,3,4,5,6), @element)
)

returns: [ 1, 4 ] (position 0 and 3, defined in line 2).

• geozelot: ...and the proper way would be to use array_slice. In lower level languages you are referring to the extraction of consecutive array element as slicing the (underlying) array, usually implemented additionally for convenience using bracket slice notation [<start>:<end>].