Working with expressions inside variables in QGIS

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This is a follow-up based on the question asked and answered here: (Using attributes tables as variables on QGIS),

I named all my layouts according to the "map_code" column + a short for the theme:

A1_HIDRO
A2_POPUL
A3_TOPOG

Since the trigger for the exhibition is the 'maptype' variable, I thought about extracting that value from somewhere else. So, I tried inputting the substr() function to extract the needed piece of the string from the layout name, as follows:

[%substr(@layout_name,2,1)%]

It happens that I got the expected results on regular textboxes (1, 2, 3, etc), but the variables can't seem to compute that expression and set the appropriate output based on the result of that operation. I wonder if they just can't do that. Am I missing something?

Or there's a workaround to link the value being used on the 'maptype' variable with the name of the current layout?

This way, I'd just need to name a new layout and make it match the value of 'maptype' column.

Best Answer

Interesting question, as far as I know these variables can only be static string values. But you can pass these string values to eval() to evaluate as an expression.

First store your @maptype variable as just the expression, without the [% on either side.

Now when you refer to the variable in a text box or data defined override as @maptype you will get a string output with the expression text - i.e., substr(@layout_name,2,1) - but it won't treat it like an expression. It's like the equivalent of seeing cells in Excel that say =A2*100 instead of showing the result.

To get around this use eval(@maptype) instead, any time you would otherwise use a direct reference to the variable. Now you should get the actual output of that expression and it will change dynamically based on the input fields/variables.

Note you can't get around this by saving the variable as eval(substr(... - the output is still a string result, not the result of an expression.

Example table set up per linked question in OP:

Example output using eval(@maptype) in text boxes and map item themes:

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Grouping features by attribute with sum function in QGIS

Try Aggregate:

This algorithm take a vector or table layer and aggregate features based on a group by expression. Features for which group by expression return the same value are grouped together. It is possible to group all source features together using constant value in group by parameter, example: NULL. It is also possible to group features using multiple fields using Array function, example: Array("Field1", "Field2"). Geometries (if present) are combined into one multipart geometry for each group.

QGIS User defined variables

Yes, you can define your own custom variables in QGIS in different levels like as in global Level, as project or layer variables and even as ad hoc variables in QGIS expressions using the function with_variable().

See functions help for with_variable() expression as well as an expample where three different variable are declared: vertex (=20), azimuth1 and azimuth2 (the last two have an expression assigned as values).

Go to the Settings / properties of the desired Level (e.g. right Click layer > properties > tab variables) and select variables to add your own variable by typing a name and the value of the variable. You can then call the variable by its Name preceded with @ - e.g. @my_variable.

See here for more information about custom variables: http://nyalldawson.net/2015/12/exploring-variables-in-qgis-pt-3-layer-level-variables/

The content (value) of the varible can be set to an attribute or a complex expression e.g. to generate all kind of sophisticated data defined styles. So this example: define a project variable called set_color with this value (string):

case 
when value = 1 then 'red' 
when value = 2 then 'blue' 
when value = 3 then 'green' 
else 'yellow' 
end

Now use the variable @set_color to create a data defined color of a layer, based on its attribute field value using the function eval(): eval(@set_color):

Variables you defined at global, project or layer level will appear in expression string builder dialog under variables:

Best Answer

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