attribute-tableerrorqgistext;
I am attempting to re-code some text data in the attribute table using the QGIS Field Calculator tool, but I keep getting an error message:
An Error occured while evaluating the calculation string: "No Root
Node! Parsing Failed?
I am using the expressions bar, this is happening while in editing mode.
The equation for the field calculator should simply be something like:
(FIELDNAME_1 / FIELDNAME_2) * 100
Where FIELDNAME_1 = the field with the number of a particular crime and FIELDNAME_2 = the field with the total number of crimes.
Remember that the data should be stored as numbers. If its as a string bad things will probably happen (possibly the reason for your error).
This could be the solution to your issue.
My line dataset had lines dissolved by an attribute, so that two (or more) lines in separate locations were merged (the dissolve command has an option for this or the merge command may have been used) to a single record of the attribute table.
When editing the vertices of such a record you can see the multiparts.
When you define the starting vertex (xat(0), yat(0)) of the line there are actually two. I think that this is where the error originates from.
A way to avoid this is to use the 'Explode lines' tool from the processing toolbox (from the "Processing" plugin) or the explode tool from the edit toolbar.
This will create a new record in your attribute table for every line segment meaning there is only one start and end vertex for each record.
I used this on my line dataset and the x- or yat or (0 "or-1" ) expressions worked without error.
After your calculations you can dissolve your lines again using any of the dissolve functions from the processing toolbox.
Best Answer
It is necessary to enclose the string value you wish to calculate in single quotes like this:
'SAMPLE TEXT'