Thanks for the quick advice, Aaron. I'm making progress again as a result.
Jeremy
On Thursday, November 24, 2022 at 5:02:19 AM UTC asme...@gmail.com wrote:
> The problem is that eq1 is a list. solve() returns a list because in
> general an equation can have more than one solution. To get the
> solution, use eq1[0]. This probably should have produced an error, but
> for some reason it didn't.
>
> Aaron Meurer
>
> On Wed, Nov 23, 2022 at 3:11 PM Jeremy Swift wrote:
> >
> > Hi there,
> >
> > I have just discovered sympy and am making my first tentative
> experiments.
> >
> > Over the years I've probably forgotten more maths than I remember so my
> knowledge is somewhere between basic engineering maths and multiplication
> tables. Having retired I'm trying to dredge some of it back from the depths.
> >
> > I'm comfortable with programming and have been writing some python
> lately, primarily for use in Rhino3D.
> >
> > I have some simultaneous equations to solve for four variables, F1, F2,
> F3 and F4. and a lot of given values V1x, V2x etc. Given the number of
> parameters, making substitutions manually is error prone, so I figured I'd
> get sympy to do the grunt work. However, subs() doesn't seem to be working
> for me. Could somebody show me where I am going wrong?
> >
> > Here's a simplified interactive example:
> >
> > >>> import sympy as s
> > >>> s.init_printing()
> > >>> F1,F2,F3,F4=s.symbols('F1 F2 F3 F4')
> > >>> V1x,V2x,V3x,V4x=s.symbols('V1x V2x V3x V4x')
> > >>> V1y,V2y,V3y,V4y=s.symbols('V1y V2y V3y V4y')
> > >>>
> > >>> eqx=F1*V1x+F2*V2x+F3*V3x+F4*V4x
> > >>> eqy=F1*V1y+F2*V2y+F3*V3y+F4*V4y
> > >>> eqx
> > F₁⋅V1x + F₂⋅V2x + F₃⋅V3x + F₄⋅V4x
> > >>> eq1=s.solve(eqx, F1)
> > >>> eq1
> > ⎡-F₂⋅V2x - F₃⋅V3x - F₄⋅V4x⎤
> > ⎢─⎥
> > ⎣ V1x ⎦
> > >>> eq2=eqy.subs(F1, eq1)
> > >>> eq2
> > F₁⋅V1y + F₂⋅V2y + F₃⋅V3y + F₄⋅V4y
> > >>>
> >
> > I was expecting sympy to substitute the solution in eq1 into the formula
> in eqy, but that hasn't happened. Why not?
> >
> > TIA
> > Jeremy
> >
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