# SymEngine Not Calculating Function Correctly

See the code from the REPL below. What gives?

``````julia> using SymEngine

julia> @vars x1, x2, x3
(x1, x2, x3)

julia> f = sin(x1*x2) + exp(x2 + x3) - x3
-x3 + exp(x2 + x3) + sin(x2*x1)

julia> gf_x1 = diff(f,x1)
x2*cos(x2*x1)

julia> gf_x2 = diff(f,x2)
x1*cos(x2*x1) + exp(x2 + x3)

julia> gf_x3 = diff(f,x3)
-1 + exp(x2 + x3)

julia> gf_x1(1.,2.,3.)
-0.416146836547142

julia> 2.0*cos(2.0)
-0.8322936730942848

julia> gf_x2(1.,2.,3.)
53.76585636005

julia> cos(2.0)+exp(5.0)
147.99701226602946

julia> gf_x3(1.,2.,3)
19.0855369231877

julia> -1+exp(5.)
147.4131591025766
``````

Now I’m not sure how `SymEngine` determines the argument order, but

It seems that you are evaluating `x1 = 2; x2 = 1`, which would agree with the order these variables appears in the expression.

1 Like

Thank you for your reply. From that perspective, means that x2 and x1 have “swapped” their respective positions. Interesting. Explicitly:

From what I see:
gf_x1 => cos(2.0)
gf_x2 => 2.0*cos(2.0) + exp(4.0)
gf_x3 => -1 + exp(3.)

``````julia> cos(2.0)
-0.4161468365471424

julia> 2.0*cos(2.0) + exp(4.0)
53.76585636004995

julia> -1 + exp(3.)
19.085536923187668
``````

I wonder if there is a way to have SymEngine hold respect to the variables as they are originally defined in the parent function f? Must be some trick. Maybe someone out there knows.

As @yuyichao said, the heuristic SymEngine is using here to decide on the order of positional arguments is a bit mysterious. I’d say you shouldn’t use the positional arguments like that anyways and instead use the `Pair` methods:

``````julia> using SymEngine

julia> @vars x1, x2, x3
(x1, x2, x3)

julia> f = sin(x1*x2) + exp(x2 + x3) - x3; gf1 = diff(f, x1)
x2*cos(x2*x1)

julia> gf_x1(x1 => 1., x2 => 2., x3 => 3.)
-0.832293673094285

julia> 2 * cos(2)
-0.8322936730942848
``````

as this is much clearer what is going on.

Also, I should mention that your title is a little misleading. It’s the SymEngine.jl external package that’s causing the mysterious behaviour. This isn’t about julia itself doing something wrong.

3 Likes

Thank you very much for your response also. Sorry about the misleading title. At the time I posted, I wasn’t aware that this was a SymEngine related error until @yuyichao replied . Let me see if there is a way to change it. Sorry but I’m obviously brand new to Julia.

And thank you a second time for the Pair example of using this properly.

2 Likes

No need to apologize, and I’m happy I was able to help.

Welcome to the community!

4 Likes

@yuyichao is right. Just try @vars a,b,c followed by f(a,b,c)

1 Like

I’ve taken the liberty to change the thread title to be more descriptive of the actual problem 2 Likes

Thank you,@Sukera!