# What is the dx used in finite difference approximation in Calculus.jl?

#1

I tried to find what the value of \Delta x that is used in Calculus.jl for finite difference approximation (i.e. for a forward scheme \frac{f(x+\Delta x)-f(x)}{\Delta x }), without luck. Is there a way to retreive it myself, so that I can write my own customized FD approximation, or is it varied based on application?

#2

It depends on the rule:

For example, the forward rule use sqrt(eps(eltype(x))) * max(one(eltype(x)), abs(x)).

You may also want to check out:

Complex step finite differencing is really cool!

#3

Thanks! Is there a way to specify what rule one would like to use in Calculus.jl? My experience is that if you want to get the gradient, then the central scheme is employed anyhow.

#4

It takes a optional parameter dtype.

Here’s a basic example.

julia> using Calculus

julia> g(x) =  sin(x[1]) + cos(x[2])
g (generic function with 1 method)

julia> Calculus.finite_difference(g,[0,0])
2-element Array{Float64,1}:
1.0
0.0

julia> Calculus.finite_difference(g,[0,0],:central)
2-element Array{Float64,1}:
1.0
0.0

julia> Calculus.finite_difference(g,[0,0],:forward)
2-element Array{Float64,1}:
1.0
-7.45058e-9


You are right - the default method is central. I’d encourage you to look at the src file posted, it’s pretty easy to understand and modify for your needs.

Also the command methods(your_function_here) is often one of the first analysis tools I use when trying to understand a method.