Cardinal sine doesn't work.

[jdelamot...@gmail.com]

Hello everyone. I want to define the cardinal sine function, sinc. I do it as follows:

def Sinc(x):

    if x==0:

        sinc = 1.0

    else:

        sinc = sin(x)/(x)

    return sinc

I used this code in the past. However, I had to swich between computers and now I am getting the following error: 

PyomoException: Cannot convert non-constant expression to bool. This error is usually caused by using an expression in a boolean context such as an if statement. For example, 

    m.x = Var()

    if m.x <= 0:

        ...

would cause this exception.

The line in which I am using the expression is 

def Wind_lammda_definition3(model, i):

    return m.Wind_lammda[i,3]==32.07*Sinc(1.79*(-10. +m.lammda[i,3]*180/np.pi))*Sinc(2.09*(-40. + m.phi[i,3]*180/np.pi)) 

m.Wind_lammda_const3 = Constraint(m.N, rule = Wind_lammda_definition3)

I have no idea why this is happening now. Can it be a issue of version?

Please, any help is appreciated. 

Best regards.

Jaime.

[David Woodruff]

Dear Jaime,

   Using a Pyomo Var in an *if* statement will not do what you want. *If* statements are not passed to solvers and, generally, neither are user-defined functions. 

There may be various ways to get what you want, but they might cause some trouble:

- Maybe there is a solver that knows about sinc (but I don't think so),

- You could add binary Vars to indicate that the argument to sinc is not zero, but unless you already have other binaries, this would dramatically complicate solving the problem.

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[Jaime de la Mota Sanchis]

Hi David. As I said in my original message, this used to work on my old computer. I am writing this email from it. As you can see in the attached figures, the sinc works here. I need to know why it doesn't work on my new computer.

Can someone please assist me?

Best regards.

Jaime. 

To view this discussion on the web visit https://groups.google.com/d/msgid/pyomo-forum/CAERj1nVEdTP-eLK0gcKDwPjO_-tv5xW8nNQEBKvhY_GjmOqfsg%40mail.gmail.com.

[David Woodruff]

My recollection is that in older versions of Pyomo, there was no error message when a Var was used in a Python logical expression, so your code would execute, but not work the way you thought it was working in all cases (I recall that such a logical expression would always be False). 

To view this discussion on the web visit https://groups.google.com/d/msgid/pyomo-forum/CAApskacySOMHf-3Mx-mLVZHh15JYnM8JA3OkxZFpSEf1NAGfdg%40mail.gmail.com.