I'm using QGIS 3.10.0 A Coruna.

I'm using the field Calculator for an equation which requires a variable (which I Type in manually) and a (fixed) value within the same attribute table. This variable is different for each feature, meaning I have to write the equation (for my example) 55 times.

I dont have a problem with that so far; however, if for any reason my fixed value should change, I would have to repeat the work with the field calculator over and over again. Is there a way where

1) I can see which equation I used in the first place and

2) where I can automatically update my results in the attributetable in case the fixed value changes?

1 Answer 1


If adding new fields to your layer is an option for you, you can do it the following:

  1. Save your two values into the fields "fixvalue" and "varvalue"

  2. Save your expression into a field ("expression") using the string indicator ' (single quote) like this: '("fixvalue" * "varvalue")'

enter image description here

  1. You can now calculate your result using the expression eval( "expression"). The expression eval() turns a string into an expression.

  2. The result

enter image description here

When you now change your fixed value by using field calculator -> update fields, you can easily recalculate your result by using the expression in #3 like this:

Update the fixvalue:

enter image description here

update the result:

enter image description here

the new result:

enter image description here

  • 1
    Instead of saving the fixed value in a field, you can save it as a layer level variable. That way you only have to change it in one place when you change the fixed value.
    – csk
    Jan 25, 2020 at 19:54
  • Thanks csk! I never used the layer level variables, it opens up many new possibilities.
    – rtaani
    Jan 27, 2020 at 7:10
  • MrXsquared, thank you for the elaborate answer! I didnt know the "eval" funktion - I might just be able to combine your and csk answers, so I dont have that many new fields!
    – rtaani
    Jan 27, 2020 at 7:13

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