Solve indexed symbol

[Will]

Dear all,

So I'm trying to use Sympy to solve a simple system with indexed symbols, something like this:

    Eq 1.            y[i] = sqrt(x[i])

    Eq 2.            m = Sum(z[i] * y[i], (i, 0, n)) / Sum(x[i], (i, 0, n))

where, x, y, z, i, n and m are symbols (x, y and z are IndexedBase and i is Idx, n and m are Symbol).

Now given each x[i], n and z[i], I'd like to lambdify a callable function to evaluate m, how can I achieve this?

Issue with my attempt: I tried using solve([Eq1, Eq2]) to generate a symbolic solution for m first, but it seems that y[i] is not resolved as sqrt(x[i]) but kept in its original form y[i].

Any help is appreciated!

[Aaron Meurer]

The code generation functionality needed to make this sort of thing automatic isn't implemented yet. Ideally, you'd be able to lambdify the two statements as a single block of code. Also, for solve(), elimination is not implemented yet (https://github.com/sympy/sympy/issues/2720). For now, your best bet is to simply substitute the one expression in the other (m.subs(y[i], sqrt(x[i]))). 

Aaron Meurer

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[Will]

Hi Aaron,

Thanks for helping!

In this case I'll probably implement a wrapper parser to roll out the indexes first and then feed the generated equations to sympy.

Any plan on automating this in future release?

Best,

Will

On Thursday, April 20, 2017 at 12:28:25 PM UTC-7, Aaron Meurer wrote:

The code generation functionality needed to make this sort of thing automatic isn't implemented yet. Ideally, you'd be able to lambdify the two statements as a single block of code. Also, for solve(), elimination is not implemented yet (https://github.com/sympy/sympy/issues/2720). For now, your best bet is to simply substitute the one expression in the other (m.subs(y[i], sqrt(x[i]))). 

Aaron Meurer

On Wed, Apr 19, 2017 at 7:08 PM, Will <cuiweil...@gmail.com> wrote:

Dear all,

So I'm trying to use Sympy to solve a simple system with indexed symbols, something like this:

    Eq 1.            y[i] = sqrt(x[i])

    Eq 2.            m = Sum(z[i] * y[i], (i, 0, n)) / Sum(x[i], (i, 0, n))

where, x, y, z, i, n and m are symbols (x, y and z are IndexedBase and i is Idx, n and m are Symbol).

Now given each x[i], n and z[i], I'd like to lambdify a callable function to evaluate m, how can I achieve this?

Issue with my attempt: I tried using solve([Eq1, Eq2]) to generate a symbolic solution for m first, but it seems that y[i] is not resolved as sqrt(x[i]) but kept in its original form y[i].

Any help is appreciated!

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[Aaron Meurer]

We'd like to make it so that you can pass a block of statements to
lambdify and it will generate one function that runs them all. Once we
have that, you will be able to pass in your expressions as you have
them directly. We already have a way to represent a block of
statements (CodeBlock in sympy.codegen.ast), but we need to add
support to lambdify.

Aaron Meurer

> https://groups.google.com/d/msgid/sympy/422ff8d1-83a7-472b-9556-e5da4c74168c%40googlegroups.com.