Automating Attribute Table with Field Calculator

Question

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?

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")'

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

4. The result

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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:

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update the result:

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the new result:

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• csk: 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.
• rtaani: Thanks csk! I never used the layer level variables, it opens up many new possibilities.
• rtaani: 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!