added error message

Author: xtalax

src/derivative_operators/derivative_operator.jl

@@ -20,7 +20,7 @@ struct CenteredDifference{N} end
 function CenteredDifference{N}(derivative_order::Int,
                             approximation_order::Int, dx::T,
                             len::Int, coeff_func=nothing) where {T<:Real,N}
-
+    @assert approximation_order>1 "approximation_order must be greater than 1."
     stencil_length          = derivative_order + approximation_order - 1 + (derivative_order+approximation_order)%2
     boundary_stencil_length = derivative_order + approximation_order
     dummy_x                 = -div(stencil_length,2) : div(stencil_length,2)
@@ -30,8 +30,8 @@ function CenteredDifference{N}(derivative_order::Int,
     deriv_spots             = (-div(stencil_length,2)+1) : -1
     boundary_deriv_spots    = boundary_x[2:div(stencil_length,2)]
 
-    stencil_coefs           = convert(SVector{stencil_length, T}, calculate_weights(derivative_order, zero(T), dummy_x))
-    _low_boundary_coefs     = SVector{boundary_stencil_length, T}[convert(SVector{boundary_stencil_length, T}, calculate_weights(derivative_order, oneunit(T)*x0, boundary_x)) for x0 in boundary_deriv_spots]
+    stencil_coefs           = convert(SVector{stencil_length, T}, (1/dx^derivative_order) * calculate_weights(derivative_order, zero(T), dummy_x))
+    _low_boundary_coefs     = SVector{boundary_stencil_length, T}[convert(SVector{boundary_stencil_length, T}, (1/dx^derivative_order) * calculate_weights(derivative_order, oneunit(T)*x0, boundary_x)) for x0 in boundary_deriv_spots]
     low_boundary_coefs      = convert(SVector{boundary_point_count},_low_boundary_coefs)
     high_boundary_coefs     = convert(SVector{boundary_point_count},reverse(SVector{boundary_stencil_length, T}[reverse(low_boundary_coefs[i]) for i in 1:boundary_point_count]))