# Correct way to declare and use vector parameters in ModelingToolkit and PDE
**URL:** https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714
**Category:** New to Julia
**Tags:** modelingtoolkit
**Created:** [January 9, 2023, 1:40pm UTC](https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714 "2023-01-09T13:40:50Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Philippe\_Peeters](https://discourse.julialang.org/u/Philippe_Peeters)
#### Post date: [January 9, 2023, 1:40pm UTC](https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714/1 "2023-01-09T13:40:50Z")
Hi,
I’m experimenting with PDE and the MethodOfLines.jl. I’m trying to figuring out how to correctly declare and use a parameter which is a float64 vector of variable size.
Here is very basic (and useless) code sample where the system is parametrised by a function fc which takes a vector as argument.
```julia
using DifferentialEquations, ModelingToolkit
@parameters α
@variables t X(t)
Dt = Differential(t)
function fc(t,α::Vector{Float64})
y = 0.0
for var in α
y += var * sin(t)
end
return y
end
@register fc(t,α::Vector{Float64})
#α=[1.0,2.0]
eq = Dt(X) ~ fc(t,α) * X
@named sys= ODESystem(eq);
X0 = [X=>1.0]
p = [α => [1.0,2.0]];
tspan=(0.0,10.0)
prob=ODEProblem(sys,X0,tspan,p);
```
Howerver, it fails with the error
```julia
ERROR: MethodError: no method matching fc(::Num, ::Num)
Closest candidates are:
fc(::Num, ::SymbolicUtils.Symbolic{<:Vector{Float64}}) at ~/.julia/packages/Symbolics/UrqtQ/src/register.jl:51
fc(::Real, ::SymbolicUtils.Symbolic{<:Vector{Float64}}) at ~/.julia/packages/Symbolics/UrqtQ/src/register.jl:51
fc(::Num, ::Vector{Float64}) at ~/.julia/packages/Symbolics/UrqtQ/src/register.jl:51
Stacktrace:
[1] top-level scope
@ ~/Documents/SISTest.jl:18
```
when defining eq.
I’m new to ModelingToolkit and Symbolics, I guess I’m not using the parameter \alpha correctly.
Is there a correct and standard way to use vector parameters ?
BTW, the example doesn’t make use of MethodOfLines.jl but it’s the final goal to solve PDE involving functions taking vectors as argument.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [January 9, 2023, 2:07pm UTC](https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714/2 "2023-01-09T14:07:45Z")
you can’t right now.
---
### Author:  [@Qfl3x](https://discourse.julialang.org/u/Qfl3x)
#### Post date: [July 18, 2023, 11:00am UTC](https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714/3 "2023-07-18T11:00:26Z")
Sorry for pinging an old thread. Is there a still no way? Even if the Vector is of fixed length? Or do I need to cut into smaller parameters? I currently get the error:
```julia
ERROR: MethodError: no method matching hasmetadata(::Vector{Num}, ::Type{Symbolics.VariableDefaultValue})
```
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [July 21, 2023, 2:24pm UTC](https://discourse.julialang.org/t/correct-way-to-declare-and-use-vector-parameters-in-modelingtoolkit-and-pde/92714/4 "2023-07-21T14:24:52Z")
@xtalax this should be handled now right?