Is it possible to inject a value into an algorithm text setting in QGIS Processing Model?

Question

I have a SAGA Raster Calculator algorithm inside a model that I'm building.

For the formula, I'm using the function (g1>20)*1 to give me a binary raster output. This categorises pixels to buildings (1) or ground (0) depending on whether the altitude is 20m or more. This works as expected!

I can link this string expression into a string parameter in my model, and use this when I run my model.

For ease-of-use, I'd rather provide the threshold value of 20 as a numerical input value. So rather than

a string parameter "Minimum Height Expression" of "(g1>20)*1"

It would be something like

a number parameter "Minimum Height" of 20

And the expression inside the algorithm would be something like

(g1>[Minimum Height])*1

Is token subsititution like this possible with Processing? Or is there another way to get around this?

Answer 1

Thanks to @Joseph, I've found a way to do this. It's inspired by the existing user script "Batch String Replace via regex dictionary".

It takes three numeric parameters (all optional) and three string parameters (again, all optional).

Finally, there's a format parameter, which is a string like this

(g1>#1)*1

• $N (where N is 1,2 or 3) will be replaced with string parameter number N.
• #N (where N is 1,2 or 3) will be replaced with numeric parameter number N.

Here's the code..

##Table=group
##Format string from multiple numeric and string parameters=name
##String_input_1=string None
##String_input_2=string None
##String_input_3=string None
##Number_input_1=number 0
##Number_input_2=number 0
##Number_input_3=number 0
##Format_string=string $0
##output=output string

import string
import re

# at least, return an empty string
if not Format_string: 
    Format_string = ''

# Set output string
output = Format_string

# replace tokens

dix = { "[$]1":String_input_1,
        "[$]2":String_input_2,
        "[$]3":String_input_3,
        "[#]1":Number_input_1,
        "[#]2":Number_input_2,
        "[#]3":Number_input_3 }
for key in dix:
    output = re.sub(key,str(dix[key]),output)

And the dialog...

And here's what it looks like inside a model.