Help setting up an absolute value for expressions
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Hello!
I have an absolute value that I need to set to be an absolute value.
https://stackoverflow.com/questions/58612670/how-to-handle-absolute-value-in-gurobi-constraints
I followed the advice found here but when I run my model I find that the variables in my code below both are negative. The code sample and values can be seen below in screenshots below.
freq, price_down and price_up are all read from a pandas dataframe while tot_e_del_quart is a list containing different expressions.
I've been quite stumped at this issue as I don't see what I'm doing wrong compared to other examples. Like usual any help would be greatly appreciated!
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Hi Bill,
The solution values of z and z1 are numerically 0, i.e., with regards to floating-point tolerances. Please consult our guide on numerics for more details on this.
Cheers,
Matthias1 -
So what you're saying is that the optimizer has made the values so small that they effectivly become 0 and the absolute value of -0 is 0?
On a more hypothetical note. Would the way the code is currently structured work in your opinion or is there some flaw in it?
If we were to say that z = -2, would z1 then be equal to 2?0 -
Any value close enough to 0 is interpreted as 0. When taking the absolute value of a variable, more things happen behind the scenes of the solver than just taking the absolute value of the value in the solution.
You should maybe relax the z variable to allow negative values in the first place. By default, all variables are non-negative and you need to specify a specific negative lower bound to change that:
z = m.addVar(lb=-GRB.INFINITY)
See also the documentation about variables.
Cheers,
Matthias1 -
Thank you so so so much Matthias.
You've helped clear this up something immensly.
Have a wonderful day!
Cheers,
Bill0
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