Updating a field with something like "if feature_1 is in field_1 then write category_1 to field_2" in QGIS Field Calculator?

Question

I'm grouping different geological time scales into more general time scales.

An example of what I would want is

"if Oligocene-miocene is in descript_title, write Oligocene-Miocene in descript_category".

How would I write this as an expression?

As suggested by @ahmadhanb, I could extract a specific part of the feature to the target field.

However, how should I write this code properly? The goal is to do what @ahmadhanb but there is a specific line for each feature.

IF 'Basement Complex (pre-jurassic)' IN descript_title
substr("descript_title",0,17)

Answer 1

If only you need to extract Oligocene-miocene and put it in the category column, you can use the substr() function from Field Calculator to update the descript_category column as follows:

substr("descript_title",0,17)

Which will extract only Oligocene-miocene from the descript_title and add it to the descript_category.

---

Another way is to create a custom function, name it splitText, to split the text based on first whitespace as follows:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def splitText(labeltext, feature, parent):
    split_text = labeltext.split()
    if '-' in split_text[0]:
        out_text = '{0}'.format(split_text[0])
    else:
        out_text = '{0} {1}'.format(split_text[0],split_text[1])
    return out_text

Then from Custom Expression menu you can use the defined function as follows:

 splitText("descript_title")

Here is the output:

Answer 2

ahmadhanb's answer works, but it seems to me that the part you want to remove always is in paranthesis, so if you split on '(', you can simplify as the first part split is returning is always what you are looking for:

from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def splitText(labeltext, feature, parent):
    split_text = labeltext.split('(')
    return(split_text[0].strip())

(strip() to remove trailing whitespace)