Solving inequations

Specific Domains Numerics
1

is there a package to solve inequations in Julia?
2x + 3 <= x + 7
answer: x<= 4

Thanks

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2

I’m not sure if there is a package for it. You can do it with Symbolics, though.

julia> using Symbolics

julia> function solve_inequality(g, x)
           f = ~(g.val.arguments...)
           xeq = Symbolics.solve_for(f, x)
           if substitute(g, x=>xeq+1).val
               return g.val.f(xeq, x)
           else
               return g.val.f(x, xeq)
           end
       end

julia> @variables x
(x,)

julia> g = 2x + 3 <= x + 7
3 + 2x <= 7 + x

julia> solve_inequality(g, x)
x <= 4.0

Note: You’ll probably want to put some kind of check to make sure it even is an inequality if you want to be serious about this. This is just a quick and dirty solution.

6 Likes
3

Thanks for the solution. I am a matlab user and its more easy to solve this kind of problem with matlab. I thought that would be some package to do it in a more clean way in julia, guess not. Thanks for solution tho.
xD

4

One of the other symbolics packages might have this already implemented, I’m just not familiar with most of them. SymPy.jl might be a good place to check.

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5

It looks like this is how you’d do it in SymPy:

julia> using SymPy

julia> @syms x
(x,)

julia> g = LessThan(2x + 3, x + 7)
2⋅x + 3 ≤ x + 7

julia> solve(g, x)
x ≤ 4 ∧ -∞ < x
7 Likes
6

wow. Now its much better. Thank for all you help Jonniedie. I am already reading SymPy documentation. Thanks mate, you made my day.

1 Like
7

I think you can also do “easy” inequalities with JuMP.jl:

using JuMP

model = Model()

@variable(model, x)
@constraint(model, 2x + 3 <= x + 7)
# x ≤ 4.0
3 Likes
8

this is also a good answer. Thanks man.

1 Like
9

@YingboMa
Is this something that can be done easily in Symbolics at this point, or is it on the roadmap?