getting a float result from solve

asked 2019-09-02 17:24:12 +0200

Wayne
47 ●1 ●3 ●8

updated 2019-09-02 19:06:31 +0200

tmonteil
27333 ●31 ●203 ●514 http://wiki.sagemath.o...

I am trying to solve a system of 2eqs, and sage keeps giving me integer results. image description When I try to print a.n() I get an error cannot evaluate symbolic expression numerically.

Sorry, I tried to attach an image of it and I messed it up. Here is the code:

a,b=var('a b')
solve([7.0==a*sqrt(16)/(1+b/16.0),22==a*sqrt(4)/(1+b/4)],a,b)
print a.n()

The result the solve command gives is the ratios of integers.

(I typed an asterisk between the a and the sqrt, but it doesn't show up...)

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Comments

1

Post your code please.

rburing ( 2019-09-02 17:26:15 +0200 )edit

1

I added the complements you put in an answer into the question.

tmonteil ( 2019-09-02 19:06:49 +0200 )edit

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answered 2019-09-02 19:05:57 +0200

tmonteil
27333 ●31 ●203 ●514 http://wiki.sagemath.o...

updated 2019-09-03 07:25:58 +0200

When you run the solve command, the function returns a list of expressions, that are equalities. It does not touch the Python variables a and b which continue to point to the symbols a and b. Those are symbols, not numbers, so it is normal that they do not have any numerical value.

How to extract the values provided by the solve function ?

The simplest way, is first to return the solutions as dictionaries, not as symbolic equalities, by using the solution_dict=True option:

sage: s = solve([7.0==a*sqrt(16)/(1+b/16.0),22==a*sqrt(4)/(1+b/4)],a,b, solution_dict=True)
sage: s
[{b: -592/169, a: 231/169}]

Here, you see that there is only one solution:

sage: s[0]
{b: -592/169, a: 231/169}

You can extract the value for a as follows:

sage: s[0][a]
231/169

and take its numerical value:

sage: s[0][a].n()
1.36686390532544

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answered 2019-09-02 19:14:09 +0200

Emmanuel Charpentier
7057 ●6 ●46 ●137

updated 2019-09-02 19:17:55 +0200

1) When you type code, mark it as such by selecting it and using the "1 0 1 0 1 0" button of the damn' editor...

2) the following one-liner does what you want:

[[u.lhs()==u.rhs().n() for u in S] for S in solve([7.0==a*sqrt(16)/(1+b/16.0),22==a*sqrt(4)/(1+b/4)],a,b)]
[[a == 1.36686390532544, b == -3.50295857988166]]

3) I seriously question the need to get floats (i. e. inexact values). I'd try to keep exact values as far as possible (including in the equations):

sage: solve([7==a*sqrt(16)/(1+b/16),22==a*sqrt(4)/(1+b/4)],a,b)
[[a == (231/169), b == (-592/169)]]

EDIT: I didn't see tmonteil's excellent answer, which was more or less simultaneous. It differs fro mine, but is better on certain aspects(use of solution dictionaries...).