Raising Variables to Substituted Variables

[codykarcher]

It would be nice if this worked:

from gpkit import Variable, Model, units
import gpkit

mwx = Variable("mwx", 1, "-", "Maximum Takeoff Weight Weighting Exponent")
MTOW = Variable("W_{MTO}", "-", "Maximum Takeoff Weight")

constraints = [MTOW >= 10]
objective = MTOW**mwx

m = Model(objective, constraints)
sol = m.solve('cvxopt', verbosity=1)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-6a678a48de69> in <module>()
      6 
      7 constraints = [MTOW >= 10]
----> 8 objective = MTOW**mwx
      9 
     10 m = Model(objective, constraints)

TypeError: unsupported operand type(s) for ** or pow(): 'Variable' and 'Variable'

So that I could sweep over the exponent in controlpanel...

[bqpd]

Ooh, interesting example. I don't think that's best done with a Variable,
though? But this is definitely a use case that comes up. @whoburg?
On Mar 24, 2016 13:06, "cjk12" [email protected] wrote:

It would be nice if this worked:

from gpkit import VectorVariable, Variable, Model, unitsimport gpkit

mwx = Variable("mwx", 1, "-", "Maximum Takeoff Weight Weighting Exponent")MTOW = Variable("W_{MTO}", "-", "Maximum Takeoff Weight")

constraints = [MTOW >= 10]
objective = MTOW**mwx

m = Model(objective, constraints)
sol = m.solve('cvxopt', verbosity=1)

---

TypeError Traceback (most recent call last)
in ()
8 constraints = [MTOW >= 10]
9
---> 10 objective = MTOW**mwx
11
12 m = Model(objective, constraints)

TypeError: unsupported operand type(s) for ** or pow(): 'Variable' and 'Variable'

So that I could sweep over the exponent in controlpanel...

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#597

[whoburg]

I'm noticing a lot of cases where users want to play with the objective after model creation (this is one example; another is flipping back and forth between MTOW and 1/endurance).

I don't think we should allow a gpkit.Variable to be used as an exponent. One problem is that it's not clear what to do with units. (note that the units of the objective change every time mwx is changed in the example above -- this is excusable for an objective, but probably not so in constraints).

So, my reaction is that we could handle this in controlpanel by allowing for sliders that aren't Variables in the model. @bqpd, do we already have a related issue with linkedsweeps? Another related issue might be feasibility models with varying exponents on the slack variables.

[bqpd]

Yeah, hooks for custom controlpanel sliders is probably good. I don't think we have a related issue with linkedsweeps?

In general, changing the cost function is a great thing for a user to be doing and we should definitely encourage it more than we do at present.

[whoburg]

agree with that.

[bqpd]

superseded by #825