error setting scalar component X.set_coef(0, 0, value)

[chriswebers]

The scalar component of a multi-vector can not be set with
X.set_coef(igrade=0, ibase, value)
for any value of ibase .
Reason: The first element of self.Ga.blades or self.Ga.bases is the empty list [].
Any index ibase into this empty list will throw an error.

Example code:

x = 1 + a1 + a2 + a1*a2
x.set_coef(0, 0, 42)

/opt/local/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages/galgebra/mv.py in set_coef(self, igrade, ibase, value)
   1174     def set_coef(self, igrade, ibase, value):
   1175         if self.blade_rep:
-> 1176             base = self.Ga.blades[igrade][ibase]
   1177         else:
   1178             base = self.Ga.bases[igrade][ibase]

IndexError: list index out of range

[eric-wieser]

What version are you using? I think I already fixed this.

[chriswebers]

I am using version ‘0.4.5’ from the repository.

…

On 15 Jan 2020, at 8:53 am, Eric Wieser ***@***.***> wrote: What version are you using? I think I already fixed this. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or unsubscribe.

[eric-wieser]

Darn, well that was worth a try. I'm not really a fan of that function at all, it's the only one that mutates state.

I'll see if I can fix this in the next few days, but I think the API is just poorly designed here

[chriswebers]

I agree. But if you remove the set_coefs function, what is then the canonical way to create a multivector from an iterable (list, numpy.array etc.) of Sympy expression components? As an example, assume vector space dimension 2, and given Sympy variables r, s, t, u = symbols(‘r, s, t, u’), I want to define my own coefficients = [r, s, t, u] (or numpy.array([r, s, t, u]) etc.) and then create_base_multivector(coefficients) resulting in a multivector: r + s*a1 + t*a2 + u*a1*a2 or create_blade_multivector(coefficients) resulting in a multivector: r + s*a1 + t*a2 + u*a1^a2 (note the wedge in the last term) For full metric, those two multivectors are not the same (as you are surely aware) because a1*a2 = u*Q_12 + a1^a2 (and Q_12 is the inner product of a1 and a2).

[utensil]

what is then the canonical way to create a multivector from an iterable (list, numpy.array etc.) of Sympy expression components?

I've been doing something like:

def vector(ga, components):
    basis = ga.mv()
    return sum([components[i] * e for i, e in enumerate(basis)])

The above is just for vectors, but the idea applies to multivectors.