relate x[0] back to x

[Nico]

After defining a vector `x` and one of its components, say `x0`,

```

import sympy

x = sympy.DeferredVector('x')

x0 = x[0]

```

is there any way to relate `x0` back to `x`? I.e., I'm looking for a Boolean function that returns `True` for the question "Is x0 a component of x?"

[Aaron Meurer]

It doesn't look like there is. DeferredVector just works by creating Symbol('x[0]'). I think for the use-case it was designed for, this is fine. You probably want to be using Indexed and IndexedBase instead. 

Aaron Meurer

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[Jason Moore]

There is also the MatrixSymbol class that may be better here. The DeferredVector was created with a very narrow need for the lambdify functions, I wouldn't recommend using it for anything other.

Jason
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[Nico Schlömer]

> There is also the MatrixSymbol class that may be better here.

Quite interesting!

Is there such a thing as a VectorSymbol as well? (I saw the `is_Vector` property here but never saw it True, neither for MatrixSymbols nor for DeferredVectors.)

--Nico

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[Jason Moore]

We don't distinguish between vectors and column matrices. You can check the matrix's shape.

Jason
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