3

I'm trying to get the maximum value from an array, using the max() function. The syntax for the max() function is this:

max(value1,value2…)

I have an array: [ 1, 2, 3, 4, 5, 6, 7, 8 ] which I got by using this expression:

array_remove_all( array_agg( "fieldname"), maximum("fieldname"))

I need to obtain the maximum value from the array, but the max() function doesn't work. When I calculate the max() of the array, it gives this error:

Eval Error: Cannot convert '' to double

I can convert the array to a string using the array_to_string() function. When I calculate the max of the string version of the array, it gives this error:

Eval Error: Cannot convert '1,2,3,4,5,6,7,8' to double

So it seems the max() function can't accept an array or a list in string format as input. I also tried the maximum() function, with similar lack of results. I don't see any other function in the Field Calculator for finding the maximum value

How to extract the maximum value from an array in the Field Calculator?

3

To get the largest value from an array you could sort it and take the last value:

array_last(array_sort( an_array ))

If you want the second highest value in a field called 'fieldname' (I assume this because you remove the maximum in your array construction) you would need remove duplicates and slice:

array_slice(
  array_sort(
    array_distinct(
      array_agg("fieldname")
    )
  ), -2, -2
)

An alternative would be to use the function editor and write it in python.


Note: array_sort is a new function introduced in QGIS version 3.6:

enter image description here

  • It turns out array_sort is a new function, only available from QGIS 3.6 on. I'm still using the current LTR, 3.4. In fact I did look for a sort function, and I was surprised it didn't seem to exist. I guess I'll have to update my QGIS version. – csk Mar 29 at 18:48
  • Aha! I was looking for that sort function and couldn't find it. – Gabriel C. Mar 29 at 18:49

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