My layer contains the following fields:

  • "B" is the identifier of a geometry
  • "B_N" are the identifiers of the neighboring geometries of "B".
  • "H" characterizes "B"
  • "B_N_H" characterizes the "B_N"(s)

How could I do the subtraction of "H" and "B_N_H" for each "B_N"?

For example: for b = 56, I would like to get in one column "0, 1, 3, 0" and in another column the maximum value 3 and the corresponding "b_n" identifier 57 for b = 57, I would like to get "0, -3, 0, -1" and the maximum value 3 (absolute value) and the corresponding b_n identifier 56.

enter image description here

1 Answer 1


(QGIS 3.22) You can use an expression like this on a Text field in the Field Calculator. It makes an array from the list of neighbours, iterates through that array and subtracts each value from the h field. Finally, it converts the resulting array to a string to be entered in the calculated field.

array_to_string(                   -- convert the final array to a string
    array_foreach(                 -- iterate through array
        string_to_array("b_n_h"),  -- make an array from the contents of "b_n_h" field
        "h" - @element             -- for each element of the b_n_h array, subtract it from the value in field "h"

enter image description here

For the other column:

"b" || ',' ||                             -- concatenate the value from "b" field with a , and the maximum absolute value as follows

array_max(                                -- get the maximum of the absolute values
        string_to_array("subtraction"),  -- convert the string field value to an array
        abs(@element)                     -- get the absolute value of each array element

enter image description here

  • thanks @Matt, the 1st part works perfectly. The 2nd part is more complicated: the array_max function is only available from the Qgis 3.18 version. As I am on my personal PC, I just installed version 3.22. The ok button activates, the preview of the result appears at the launch of the command, I have an error message : "An error occurred while evaluating the calculation chain: Unable to convert '' to duplicate". what version of Qgis do you have?
    – fcka
    Feb 5, 2022 at 21:11
  • 1
    I added the my version (3.22) to my answer. Do you have NULL values in one of your fields? I don't know where the error would come from otherwise
    – Matt
    Feb 5, 2022 at 21:16
  • As much for me, I removed all the lists with zeros and it worked perfectly! Congratulations and thank you
    – fcka
    Feb 5, 2022 at 22:05

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