You might have copied `a**2 + b**2 + c**2` and called them `p`? You can
check by typing `type(p)`. If it is an Add then that is what happened
somehow. It's always best to show the actual self-verifying code snippet.
Here is what I used and everything work for me:
```
>>> from sympy.vector import dot
>>> from sympy.vector.coordsysrect import CoordSys3D
>>> O = CoordSys3D('O')
>>> r = O.x*O.i + O.y*O.j + O.z*O.k
>>> dot(r,r) # this works
O.x**2 + O.y**2 + O.z**2
>>> from sympy import symbols
>>> a, b, c = symbols("a, b, c")
>>> p = a*O.i +b*O.j + c*O.k
>>> dot(p,p) # this does not work
a**2 + b**2 + c**2
>>> p.dot(p)
a**2 + b**2 + c**2
```
/c
On Monday, October 25, 2021 at 10:17:09 AM UTC-5 Andreas Schuldei wrote:
> p.dot(p)
> a**2 + b**2 + c**2
> dot(p,p)
> Traceback (most recent call last):
> File "<input>", line 1, in <module>
> File "C:\Users\Andreas
> Schuldei\PycharmProjects\lissajous-achse\venv\lib\site-packages\sympy\physics\vector\functions.py",
>
> line 31, in dot
> raise TypeError('Dot product is between two vectors')
> TypeError: Dot product is between two vectors
>
>
> i am confused. why does this happen?
>
> Oscar schrieb am Montag, 25. Oktober 2021 um 12:35:47 UTC+2:
>
>> I don't see the exception that you showed:
>>
>> In [9]: O = CoordSys3D('O')
>> ...: r = O.x*O.i + O.y*O.j + O.z*O.k
>> ...: dot(r,r) # this works
>> ...: O.x**2 + O.y**2 + O.z**2
>> ...: from sympy import symbols
>> ...: a, b, c = symbols("a, b, c")
>> ...: p = a*O.i +b*O.j + c*O.k
>> ...: p
>> ...: a*O.i + b*O.j + c*O.k
>> ...: dot(p,p) # this does not work
>> Out[9]:
>> 2 2 2
>> a + b + c
>>
>> --
>> Oscar
>>
>> On Mon, 25 Oct 2021 at 11:28, Andreas Schuldei <and...@schuldei.org>
>> wrote:
>> >
>> >
>> > I am trying to figure out the basics here.
>> > I understand O.x, O.y and O.z have special meaning, first because they
>> magically appear, just by instanciating O, and also because they seem to
>> distinguish themselves from other generic symbols/skalars:
>> >
>> > O = CoordSys3D('O')
>> > r = O.x*O.i + O.y*O.j + O.z*O.k
>> > dot(r,r) # this works
>> > O.x**2 + O.y**2 + O.z**2
>> > from sympy import symbols
>> > a, b, c = symbols("a, b, c")
>> > p = a*O.i +b*O.j + c*O.k
>> > p
>> > a*O.i + b*O.j + c*O.k
>> > dot(p,p) # this does not work
>> > Traceback (most recent call last):
>> > File "<input>", line 1, in <module>
>> > File "C:\Users\Andreas
>> Schuldei\PycharmProjects\lissajous-achse\venv\lib\site-packages\sympy\physics\vector\functions.py",
>>
>> line 31, in dot
>> > raise TypeError('Dot product is between two vectors')
>> > TypeError: Dot product is between two vectors
>> > p = a*O.x*O.i + b*O.y*O.j + c*O.z*O.k
>> > dot(p,p) # this does not work, either!
>> > Traceback (most recent call last):
>> > File "<input>", line 1, in <module>
>> > File "C:\Users\Andreas
>> Schuldei\PycharmProjects\lissajous-achse\venv\lib\site-packages\sympy\physics\vector\functions.py",
>>
>> line 31, in dot
>> > raise TypeError('Dot product is between two vectors')
>> > TypeError: Dot product is between two vectors
>> >
>> > Why is this? What factors are allowed with vector components and unit
>> vectors?
>> >
>> > I feel a little restricted if my only vector component variables
>> allowed are O.x, O.y, and O.z.
>> > Oscar schrieb am Samstag, 23. Oktober 2021 um 16:49:10 UTC+2:
>> >>
>> >> A SymPy Vector is constructed algebraically from the unit vectors i, j
>> >> and k of the coordinate system. For a vector field you also use the
>> >> coordinate system base scalars x, y and z.
>> >>
>> >> In [11]: from sympy.vector import CoordSys3D, dot
>> >>
>> >> In [12]: O = CoordSys3D('O')
>> >>
>> >> In [13]: r = O.x*O.i + O.y*O.j + O.z*O.k
>> >>
>> >> In [14]: r
>> >> Out[14]: (x_O) i_O + (y_O) j_O + (z_O) k_O
>> >>
>> >> In [15]: dot(r, r)
>> >> Out[15]:
>> >> 2 2 2
>> >> x_O + y_O + z_O
>> >>
>> >> I often see confusion about this so clearly this is not very intuitive
>> >> and maybe it is not very clearly documented:
>> >> https://docs.sympy.org/latest/modules/vector/index.html
>> >>
>> >> I think there should be an easier way where you can just use a
>> >> "standard" coordinate system and just do:
>> >>
>> >> from sympy.vector.stdcoords import x, y, z, i, j, k
>> >> r = x*i + y*j + z*k
>> >> print(dot(r, r))
>> >>
>> >> The vector docs should just begin by showing how to do that and then
>> >> how to do simple calculations in a single coordinate system.
>> >>
>> >> --
>> >> Oscar
>> >>
>> >> On Sat, 23 Oct 2021 at 07:11, Andreas Schuldei <and...@schuldei.org>
>> wrote:
>> >> >
>> >> > I am putting together the components of a vector field (a magnetic
>> field, caused by a current in several conductors) in cartesian coordinates.
>> The field is derived from calculating the rotation of its magnetic vector
>> potential, which can be expressed as
>> >> >
>> >> > A_z = -Const * dot(r,a)(dot(r,r)
>> >> > A =(0,0,A_z
>> >> > and then the rot(A):
>> >> > B = curl(A)
>> >> >
>> >> > gives me (after some simplifications)
>> >> > u = (2 * C * r2 * (a1 * r1 + a2 * r2)) / np.square(r1*r1 + r2*r2) -
>> (C * a2) / (r1*r1 + r2*r2)
>> >> > v = (C * a1) / (r1*r1 + r2*r2) - (2 * C * r1 * (a1 * r1 + a2 * r2))
>> / np.square(r1*r1 + r2*r2)
>> >> > w = 0
>> >> >
>> >> > with B(u,v,w) being the vector field I am interested in.
>> >> >
>> >> > My initial question ("How to create a vector") is mostly a sympy
>> syntactical one. I look for a function like
>> >> >
>> >> > Sys = CoordSys3D("Sys")
>> >> > O = Sys.origin
>> >> > u = (2 * C * r2 * (a1 * r1 + a2 * r2)) / np.square(r1*r1 + r2*r2) -
>> (C * a2) / (r1*r1 + r2*r2)
>> >> > v = (C * a1) / (r1*r1 + r2*r2) - (2 * C * r1 * (a1 * r1 + a2 * r2))
>> / np.square(r1*r1 + r2*r2)
>> >> > w = 0
>> >> > V = O.vector(u,v,w)
>> >> > ^^^^^^^^^^^^^^^^^^^
>> >> > where I can specify a vector, relative to a coordinate system, by
>> its components. The O.vector() function would return a vector that can then
>> safely be transformed into other (resting) coordinate systems.
>> >> >
>> >> > brombo schrieb am Freitag, 22. Oktober 2021 um 19:06:03 UTC+2:
>> >> >>
>> >> >> You might want to look at this link -
>> >> >>
>> >> >> https://galgebra.readthedocs.io/en/latest/
>> >> >>
>> >> >> Also if you could show me symbolically (not code) what you are
>> doing perhaps I could give you an example of how to do it in galgebra.
>> >> >>
>> >> >> On 10/22/21 3:15 AM, Andreas Schuldei wrote:
>> >> >>
>> >> >> I saw this
>> https://stackoverflow.com/questions/46993819/how-to-create-a-vector-function-in-sympy
>>
>> which uses Matrix() as a workaround to create a vector. The author says,
>> that it can not be transformed between coordinate systems, like real
>> vectors, though.
>> >> >>
>> >> >> I need to transform my input and output vector from one coordinate
>> system to another (and back). How are vector functions done in that case?
>> My function is simple:
>> >> >>
>> >> >> def B_el(r_vec, I):
>> >> >> mu_0 = 4 * np.pi * 1e-7
>> >> >> a1 = -0.05
>> >> >> a2 = 0.0
>> >> >> C = mu_0 * I / np.pi
>> >> >> r1 = r_vec.i
>> >> >> r2 = r_vec.j
>> >> >> u = (2 * C * r2 * (a1 * r1 + a2 * r2)) / np.square(r1*r1 + r2*r2) -
>> (C * a2) / (r1*r1 + r2*r2)
>> >> >> v = (C * a1) / (r1*r1 + r2*r2) - (2 * C * r1 * (a1 * r1 + a2 * r2))
>> / np.square(r1*r1 + r2*r2)
>> >> >> return Matrix([u, v, 0])
>> >> >>
>> >> >> --
>> >> >> You received this message because you are subscribed to the Google
>> Groups "sympy" group.
>> >> >> To unsubscribe from this group and stop receiving emails from it,
>> send an email to sympy+un...@googlegroups.com.
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>> https://groups.google.com/d/msgid/sympy/8db840a8-20f9-47ac-a1c3-11b6658f00bfn%40googlegroups.com
>> .
>> >> >
>> >> > --
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>> .
>> >
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