[Qgis-user] model builder - merge several inputs
Nicolas Cadieux
njacadieux.gitlab at gmail.com
Wed Jan 26 12:58:45 PST 2022
Hi,
Ok I looked at the Statistic by categories. I thought it operated on two
vector files (something like zonal statistics). I now see it's on a
single vector file and it's fields so my advice was not great. I'am not
quite sure I follow you on why you need to combine field names from 2
files but I few months ago, I had difficulty passing an input string as
a field with that name. I solved the problem using this
"(attribute(to_string( @NamelengthField ))" (Thanks Nyall). This was
Nyall's explanation:
"There's no bug here -- the variable value is just a pure text string,
and the expression engine can't automatically guess that you actually
want the value of the field with that name, and not just the name string
itself. Hence the need for the attribute function, which allows you to
retrieve a feature's fields via a text string containing the field name."
Using that idea (the attribute() function), I was able to combine 2 text
strings into a "field" recognized by the model. Hope this helps? If it
does not, send us a simplified model of the problem. Maybe we can look
at it.
Nicolas
On 2022-01-26 11:51 a.m., Simon Dietmann wrote:
>
> Hi Nicolas,
>
> thank you for your quick reply. I am not sure if I understand
> correctly what you are suggesting, because the ‘join by attribute’ and
> ‘aggregate’ functions both will change the attributes of the vector
> layer.
>
> What I´m trying to do instead is to give a list of field names as a
> parameter to the function ‘statistics by categories’, and my problem
> is that I don´t have this single list, but two separate lists that
> each contain a part of these field names. So I would like to merge the
> two lists of field names and pass it over to the algorithm, but I have
> no idea how to do this.
>
> Another solution could be to perform the ‘union’ first, and only
> afterwards ask the user to specify the field names for evaluation.
> Unfortunately, I have no idea either how to determine a chronological
> sequence where the ‘vector field’ input is queried after the ‘union’
> has already been performed.
>
> Thank you very much for your help,
>
>
>
> Simon
>
> *Von:*Nicolas Cadieux <njacadieux.gitlab at gmail.com>
> *Gesendet:* Mittwoch, 26. Januar 2022 17:21
> *An:* Simon Dietmann <simon.dietmann at pgnu.de>; qgis-user at lists.osgeo.org
> *Betreff:* Re: [Qgis-user] model builder - merge several inputs
>
> Hi,
>
> You could try a "join attribute by location" followed by a "Aggregate"...
>
> Nicolas
>
> On 2022-01-26 3:43 a.m., Simon Dietmann wrote:
>
> Dear list,
>
> I am trying to find a solution for the following problem with the
> model builder:
>
> My model asks for two vector polygon layers that will be combined
> by ‘union’. The resulting layer gets a new field (field
> calculator) and is evaluated by ‘Statistics by categories’
> algorithm. The fields for the categories are selected by the user
> when calling the model. Since those fields are present in the two
> separate input layers, I have to use two separate ‘Vector field’
> inputs.
>
> The problem arises when I try to combine those two input variables
> to use them together for the ‘Statistics by categories’ algorithm.
> AFAICS there is no function inside the model builder to achieve
> this, so I suggest I have to export the model as a python script.
> There I found that the call of the Statistics function gives the
> name of the variable as a parameter and not the variable itself:
>
> alg_params = {
>
> 'CATEGORIES_FIELD_NAME': parameters['fields'], ….
>
> How can I combine my two lists (~like
> “parameters[‘fields’+’fields2’], or is there a way to do this
> inside the model builder that I didn´t find?
>
> Thanks in advance for help, kind regards
>
> Simon Dietmann
>
>
>
> Mit freundlichen Grüßen
> im Auftrag
>
>
> Simon Dietmann
> M.Sc. Physische Geografie
>
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>
>
>
>
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> --
> Nicolas Cadieux
> https://gitlab.com/njacadieux
--
Nicolas Cadieux
https://gitlab.com/njacadieux
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