# Symbolic Calculations and Building Fast Parallel Functions

ModelingToolkit.jl is first and foremost a symbolic modeling language. The way to define symbolic variables is via the `@variables`

macro:

`@variables x y`

After defining variables as symbolic, symbolic expressions, which we call an `Operation`

, can be generated by utilizing Julia expressions. For example:

`z = x^2 + y`

Here, `z`

is the `Operation`

for "square `x`

and add `y`

". To make an array of symbolic expressions, simply make an array of symbolic expressions:

```
A = [x^2+y 0 2x
0 0 2y
y^2+x 0 0]
3×3 Array{Expression,2}:
x ^ 2 + y Constant(0) 2x
Constant(0) Constant(0) 2y
y ^ 2 + x Constant(0) Constant(0)
```

To better view the results, we can use Latexify.jl. ModelingToolkit.jl comes with Latexify recipes so it works automatically:

```
using Latexify
latexify(A)
```

Normal Julia functions work on ModelingToolkit expressions, so if we want to create the sparse version of `A`

we would just call `sparse`

:

```
using SparseArrays
spA = sparse(A)
3×3 SparseMatrixCSC{Expression,Int64} with 4 stored entries:
[1, 1] = x ^ 2 + y
[3, 1] = y ^ 2 + x
[1, 3] = 2x
[2, 3] = 2y
```

We can thus use normal Julia functions as generators for sparse expressions. For example, here we will define

```
function f(u)
[u[1]-u[3],u[1]^2-u[2],u[3]+u[2]]
end
f([x,y,z]) # Recall that z = x^2 + y
3-element Array{Operation,1}:
x - (x ^ 2 + y)
x ^ 2 - y
(x ^ 2 + y) + y
```

Or we can build array variables and use these to trace:

```
@variables u[1:3]
f(u)
3-element Array{Operation,1}:
u₁ - u₃
u₁ ^ 2 - u₂
u₃ + u₂
```

## Building Functions

The function for building functions is the aptly-named `build_function`

. The first argument is the symbolic expression or the array of symbolic expressions to compile, and the trailing arguments are the arguments for the function. For example:

```
to_compute = [x^2 + y, y^2 + x]
f_expr = build_function(to_compute,[x,y])
```

gives back two codes. The first is a function `f([x,y])`

that computes and builds an output vector `[x^2 + y, y^2 + x]`

. Because this tool was made to be used by all the cool kids writing fast Julia codes, it is specialized to Julia and supports features like StaticArrays. For example:

```
myf = eval(f_expr[1])
myf(SA[2.0,3.0])
2-element SArray{Tuple{2},Float64,1,2} with indices SOneTo(2):
7.0
11.0
```

The second function is an in-place non-allocating mutating function which mutates its first value:

```
f_expr[2]
:((var"##MTIIPVar#292", var"##MTKArg#290")->begin
@inbounds begin
let (x, y) = (var"##MTKArg#290"[1], var"##MTKArg#290"[2])
var"##MTIIPVar#292"[1] = (getproperty(Base, :+))(x ^ 2, y)
var"##MTIIPVar#292"[2] = (getproperty(Base, :+))(y ^ 2, x)
end
end
nothing
end)
```

Thus we'd use it like the following:

```
myf! = eval(f_expr[2])
out = zeros(2)
myf!(out,[2.0,3.0])
out
2-element Array{Float64,1}:
7.0
11.0
```

To save the symbolic calculations for later, we can take this expression and save it out to a file:

`write("function.jl", string(f_expr[2]))`

Note that if we need to avoid `eval`

, for example to avoid world-age issues, one could do `expression = Val{false}`

:

`build_function(to_compute,[x,y],expression=Val{false})`

which will use GeneralizedGenerated.jl to build Julia functions which avoid world-age issues.

## Building Non-Allocating Parallel Functions for Sparse Matrices

Now let's show off a little bit. `build_function`

is kind of like if `lambdify`

ate its spinach. To show this, **let's build a non-allocating function that fills sparse matrices in a multithreaded manner**. To do this, we just have to represent the operation that we're turning into a function via a sparse matrix. For example:

```
N = 8
A = sparse(Tridiagonal([x^i for i in 1:N-1],[x^i * y^(8-i) for i in 1:N], [y^i for i in 1:N-1]))
8×8 SparseMatrixCSC{Operation,Int64} with 22 stored entries:
[1, 1] = x ^ 1 * y ^ 7
[2, 1] = x ^ 1
[1, 2] = y ^ 1
[2, 2] = x ^ 2 * y ^ 6
[3, 2] = x ^ 2
[2, 3] = y ^ 2
[3, 3] = x ^ 3 * y ^ 5
[4, 3] = x ^ 3
[3, 4] = y ^ 3
⋮
[5, 5] = x ^ 5 * y ^ 3
[6, 5] = x ^ 5
[5, 6] = y ^ 5
[6, 6] = x ^ 6 * y ^ 2
[7, 6] = x ^ 6
[6, 7] = y ^ 6
[7, 7] = x ^ 7 * y ^ 1
[8, 7] = x ^ 7
[7, 8] = y ^ 7
[8, 8] = x ^ 8 * y ^ 0
```

Now we call `build_function`

:

`build_function(A,[x,y],parallel=ModelingToolkit.MultithreadedForm())[2]`

which generates the code:

```
(var"##MTIIPVar#317", var"##MTKArg#315")->begin
@inbounds begin
@sync begin
let (x, y) = (var"##MTKArg#315"[1], var"##MTKArg#315"[2])
begin
Threads.@spawn begin
(var"##MTIIPVar#317").nzval[1] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 1), (getproperty(Base, :^))(y, 7))
(var"##MTIIPVar#317").nzval[2] = (getproperty(Base, :^))(x, 1)
(var"##MTIIPVar#317").nzval[3] = (getproperty(Base, :^))(y, 1)
(var"##MTIIPVar#317").nzval[4] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 2), (getproperty(Base, :^))(y, 6))
(var"##MTIIPVar#317").nzval[5] = (getproperty(Base, :^))(x, 2)
(var"##MTIIPVar#317").nzval[6] = (getproperty(Base, :^))(y, 2)
end
end
begin
Threads.@spawn begin
(var"##MTIIPVar#317").nzval[7] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 3), (getproperty(Base, :^))(y, 5))
(var"##MTIIPVar#317").nzval[8] = (getproperty(Base, :^))(x, 3)
(var"##MTIIPVar#317").nzval[9] = (getproperty(Base, :^))(y, 3)
(var"##MTIIPVar#317").nzval[10] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 4), (getproperty(Base, :^))(y, 4))
(var"##MTIIPVar#317").nzval[11] = (getproperty(Base, :^))(x, 4)
(var"##MTIIPVar#317").nzval[12] = (getproperty(Base, :^))(y, 4)
end
end
begin
Threads.@spawn begin
(var"##MTIIPVar#317").nzval[13] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 5), (getproperty(Base, :^))(y, 3))
(var"##MTIIPVar#317").nzval[14] = (getproperty(Base, :^))(x, 5)
(var"##MTIIPVar#317").nzval[15] = (getproperty(Base, :^))(y, 5)
(var"##MTIIPVar#317").nzval[16] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 6), (getproperty(Base, :^))(y, 2))
(var"##MTIIPVar#317").nzval[17] = (getproperty(Base, :^))(x, 6)
(var"##MTIIPVar#317").nzval[18] = (getproperty(Base, :^))(y, 6)
end
end
begin
Threads.@spawn begin
(var"##MTIIPVar#317").nzval[19] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 7), (getproperty(Base, :^))(y, 1))
(var"##MTIIPVar#317").nzval[20] = (getproperty(Base, :^))(x, 7)
(var"##MTIIPVar#317").nzval[21] = (getproperty(Base, :^))(y, 7)
(var"##MTIIPVar#317").nzval[22] = (getproperty(Base, :*))((getproperty(Base, :^))(x, 8), (getproperty(Base, :^))(y, 0))
end
end
end
end
end
nothing
end
```

Let's unpack what that's doing. It's using `A.nzval`

in order to linearly index through the sparse matrix, avoiding the `A[i,j]`

form because that is a more expensive way to index a sparse matrix if you know exactly the order that the data is stored. Then, it's splitting up the calculation into Julia threads so they can be operated on in parallel. It synchronizes after spawning all of the tasks so the computation is ensured to be complete before moving on. And it does this with all in-place operations to the original matrix instead of generating arrays. This is the fanciest way you could fill a sparse matrix, and ModelingToolkit makes this dead simple.

(Note: this example may be slower with multithreading due to the thread spawning overhead, but the full version was not included in the documentation for brevity. It will be the faster version if `N`

is sufficiently large!)

## Derivatives

One common thing to compute in a symbolic system is derivatives. In ModelingToolkit.jl, derivatives are represented lazily via operations, just like any other function. To build a differential operator, use `@derivatives`

like:

```
@variables t
@derivatives D'~t
```

This is the differential operator $D = \frac{\partial}{\partial t}$: the number of `'`

is the order of the derivative and the second variable is the variable we're differentiating by. Now let's write down the derivative of some expression:

```
z = t + t^2
D(z) # derivative(t + t ^ 2, t)
```

Notice that this hasn't computed anything yet: `D`

is a lazy operator because it lets us symbolically represent "The derivative of z with respect to t", which is useful for example when representing our favorite thing in the world, differential equations. However, if we want to expand the derivative operators, we'd use `expand_derivatives`

:

`expand_derivatives(D(z)) # 1 + 2t`

We can also have simplified functions for multivariable calculus. For example, we can compute the Jacobian of an array of expressions like:

```
ModelingToolkit.jacobian([x+x*y,x^2+y],[x,y])
2×2 Array{Expression,2}:
1 + y x
2x Constant(1)
```

and similarly we can do Hessians, gradients, and define whatever other derivatives you want.

## Simplification and Substitution

ModelingToolkit interfaces with SymbolicUtils.jl to allow for simplifying symbolic expressions. This is done simply through the `simplify`

command:

`simplify(t+t) # 2t`

This can be applied to arrays by using Julia's broadcast mechanism:

```
B = simplify.([t^2+t+t^2 2t+4t
x+y+y+2t x^2 - x^2 + y^2])
2×2 Array{Operation,2}:
2 * t ^ 2 + t 6t
x + 2 * (t + y) y ^ 2
```

We can then use `substitute`

to change values of an expression around:

```
simplify.(substitute.(B,[x=>y^2]))
2×2 Array{Operation,2}:
2 * t ^ 2 + t 6t
y ^ 2 + 2 * (t + y) y ^ 2
```

and we can use this to interactively evaluate expressions without generating and compiling Julia functions:

```
substitute.(B,([x=>2.0,y=>3.0,t=>4.0],))
2×2 Array{ModelingToolkit.Constant,2}:
Constant(36.0) Constant(24.0)
Constant(16.0) Constant(9.0)
```

Where we can reference the values via:

`ModelingToolkit.Constant(2.0).value # 2.0`

## Non-Interactive Development (No Macro Version)

Note that the macros are for the high-level case where you're doing symbolic computation on your own code. If you want to do symbolic computation on someone else's code, like in a macro, you may not want to do `@variables x`

because you might want the name "x" to come from the user's code. For these cases, ModelingToolkit.jl allows for fully macro-free usage. For example:

```
x = Variable{Float64}(:x)()
y = Variable{Float64}(:y)()
x+y^2.0
```

Does what you'd expect. The reference documentation shows how to define any of the quantities in such a way that the names can come from runtime values.

If we need to use this to generate new Julia code, we can simply convert the output to an `Expr`

:

`Expr(x+y^2)`

## Variables as Operations

`Operation`

is the type name the ModelingToolkit.jl gives to symbolic expressions. In ModelingToolkit.jl, essentially everything is an `Operation`

. Notice that when we defined our variables above, they were represented as an `Operation`

as well, which means that variables alone are an operation that can then be composed to make bigger operations.

But since variables are functions, we can represent their dependencies as well. For example:

`@variables t x(t) y(t)`

defines `t`

as a dependent variable while `x(t)`

and `y(t)`

are independent variables. This is accounted for in differentiation:

```
z = x + y*t
expand_derivatives(D(z)) # derivative(x(t), t) + y(t) + derivative(y(t), t) * t
```

Since `x`

and `y`

are time-dependent, they are not automatically eliminated from the expression and thus the `D(x)`

and `D(y)`

operations still exist in the expanded derivatives for correctness.

We can also define unrestricted functions:

`@variables g(..)`

Here `g`

is a variable that is a function of other variables. Any time that we reference `g`

we have to utilize it as a function:

`z = g(x) + g(y)`

## Registering Functions

One of the benefits of a one-language Julia symbolic stack is that the primitives are all written in Julia, and therefore it's trivially extendable from Julia itself. By default, new functions are traced to the primitives and the symbolic expressions are written on the primitives. However, we can expand the allowed primitives by registering new functions. For example, let's register a new function `h`

:

```
h(x,y) = x^2 + y
@register h(x,y)
```

Now when we use `h(x,y)`

, it is a symbolic expression and doesn't expand:

`h(x,y) + y^2`

In order to use it with the differentiation system, we need to register its derivatives. We would do it like this:

```
# Derivative w.r.t. the first argument
ModelingToolkit.derivative(::typeof(h), args::NTuple{2,Any}, ::Val{1}) = 2args[1]
# Derivative w.r.t. the second argument
ModelingToolkit.derivative(::typeof(h), args::NTuple{2,Any}, ::Val{2}) = 1
```

and now it works with the rest of the system:

```
@derivatives Dx'~x Dy'~y
expand_derivatives(Dx(h(x,y) + y^2)) # 2x
expand_derivatives(Dy(h(x,y) + y^2)) # 1 + 2y
```