Adding new row by command in field calculator in QGIS

Question

I am actually trying to create a new field in a table with the Model Builder (Field calculator function).

Is there a command that creates a new field, so I can do calculations directly in that new field?

I know that there is a function, which creates a new field when using the field calculator directly but I need a command for it.

In Detail: I want to calculate the percent of an area with this command ($area / sum(area))*100 by using the Modeler, but I want it to be written in an existing layer, creating a new field for it.

• Babel: Sorry, your question is unclear. When you apply the fieldcalculator, by default every time you run the tool, a new field is created (you must check the box next to Update existing field to avoid that). Insert your expression, give a name to the new field and run the field calculator - you're done. So your question does not make sense to me: can you add more details about what the problem is?
• lesto: if u use the field caluclator with the Modellbuilder, you havent got the option "Update existing field" or " Add a new field" or something. You just have a command line. So I need a command to create that new field in the existing layer.
• lesto: To be clear, i am not using the field calculator by itself, i am using it in the Modellbuilder. Try it ;), you will see.
• Babel: It does work, even with Modelbuilder, see screenshot: i.sstatic.net/iCboM.png where you see where I introduced the name of the field that will be created. I'm not sure if we speak about the same thing? Can you add additional information or a screenshot?
• Babel: In your input-layer you need to have a field named area for your expression to work, by the way.

Answer 1

I found out what I did wrong. I forgot to set up the Output (Green mark) and only set up the input because I thought the Model Builder will create a second layer, what I didn't want it to, because it shall be written in the same layer (Red mark), if I would do it the way in the picture. I deleted the Red mark now and directly putted it from "Add geometry" to the "Field calculator" and set up the one output layer.

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