Knot Iteration

Given an AbstractInterpolation itp get an iterator over it's knots using knots(itp)

using Interpolations
itp = interpolate(rand(4), options...)
kiter = knots(itp) # Iterator over knots
collect(kiter) # Array of knots [1, 2, 3, 4]

For multiple dimensions, the iterator will return tuples of positions (ie. (x, y, ...)), with the first coordinate changing the fastest.

julia> itp = interpolate(ones(3,3), BSpline(Linear()));

julia> kiter = knots(itp);

julia> collect(kiter)
3×3 Matrix{Tuple{Int64, Int64}}:
(1, 1)  (1, 2)  (1, 3)
(2, 1)  (2, 2)  (2, 3)
(3, 1)  (3, 2)  (3, 3)

The number of elements and size of the iterator can be found as shown:

julia> length(kiter)
9

julia> size(kiter)
(3, 3)

Extrapolated Knots

Given an AbstractExtrapolation etp, knots(etp) will also iterate over the the knots with the following behavior.

• For Throw, Flat, Line iterate the knots once
• For Periodic and Reflect generate an infinite sequence of knots starting at the first knot.

As Periodic and Reflect generate infinite sequences of knots, length and size are undefined. For Throw, Flat, Line, length and size behave as expected.

Periodic

With Periodic boundary condition, knots repeat indefinitely with the first and last knot being co-located. (ie. in the example below etp(2.0) = 1.0 not 4.0).

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation(x, x.^2, extrapolation_bc=Periodic());

julia> kiter = knots(etp);

julia> k = Iterators.take(kiter, 6) |> collect
6-element Vector{Float64}:
1.0
1.5
1.75
2.0
2.5
2.75

Extrapolating to the generated knots etp.(k), confirms that the extrapolated knots do map back to the correct inbound knots (ie. etp(k) == etp(k)).

julia> etp.(k)
6-element Vector{Float64}:
1.0
2.25
3.0625
1.0
2.25
3.0625

Reflect

With the Reflect boundary condition knots repeat indefinitely, following the pattern shown below (Offset terms are not shown for brevity).

k, k, ..., k[end-1], k[end], k[end+1], ... k, k, k, ...
julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation(x, x.^2, extrapolation_bc=Reflect());

julia> kiter = knots(etp);

julia> k = Iterators.take(kiter, 6) |> collect
6-element Vector{Float64}:
1.0
1.5
1.75
2.0
2.25
2.5

Evaluating the extrapolation at etp.(k) confirms that the extrapolated knots correspond to the correct inbound knots.

julia> etp.(k)
6-element Vector{Float64}:
1.0
2.25
3.0625
4.0
3.0625
2.25

Multiple Dimensions

As with an AbstractInterpolation, iterating over knots for a multi-dimensional extrapolation also supported.

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation((x, x), x.*x');

julia> knots(etp) |> collect
4×4 Matrix{Tuple{Float64, Float64}}:
(1.0, 1.0)   (1.0, 1.5)   (1.0, 1.75)   (1.0, 2.0)
(1.5, 1.0)   (1.5, 1.5)   (1.5, 1.75)   (1.5, 2.0)
(1.75, 1.0)  (1.75, 1.5)  (1.75, 1.75)  (1.75, 2.0)
(2.0, 1.0)   (2.0, 1.5)   (2.0, 1.75)   (2.0, 2.0)

Because some boundary conditions generate an infinite sequences of knots, iteration over knots can end up "stuck" iterating along a single axis:

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation((x, x), x.*x', extrapolation_bc=(Periodic(), Throw()));

julia> knots(etp) |> k -> Iterators.take(k, 6) |> collect
6-element Vector{Tuple{Float64, Float64}}:
(1.0, 1.0)
(1.5, 1.0)
(1.75, 1.0)
(2.0, 1.0)
(2.5, 1.0)
(2.75, 1.0)

Rearranging the axes so non-repeating knots are first can address this issue:

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation((x, x), x.*x', extrapolation_bc=(Throw(), Periodic()));

julia> knots(etp) |> k -> Iterators.take(k, 6) |> collect
6-element Vector{Tuple{Float64, Float64}}:
(1.0, 1.0)
(1.5, 1.0)
(1.75, 1.0)
(2.0, 1.0)
(1.0, 1.5)
(1.5, 1.5)

Directional Boundary Conditions

If the boundary conditions are directional, the forward boundary condition is used to determine if the iterator will generate an infinite sequence of knots.

For example the following extrapolation etp, will throw an error for values less than 1.0, but will use Periodic extrapolation for values above 2.0. As a result, the iterator will generate an infinite sequence of knots starting at 1.0.

julia> x = [1.0, 1.2, 1.3, 2.0];

julia> etp = LinearInterpolation(x, x.^2, extrapolation_bc=((Throw(), Periodic()),));

julia> kiter = knots(etp);

julia> kiter |> k -> Iterators.take(k, 5) |> collect
5-element Vector{Float64}:
1.0
1.2
1.3
2.0
2.2

We can also check if the iterator has a length using: Base.IteratorSize

julia> Base.IteratorSize(kiter)
Base.IsInfinite()

Swapping the boundary conditions, results in a finite sequence of knots from 1.0 to 2.0.

julia> x = [1.0, 1.2, 1.3, 2.0];

julia> etp = LinearInterpolation(x, x.^2, extrapolation_bc=((Periodic(), Throw()),));

julia> kiter = knots(etp);

julia> collect(kiter)
4-element Vector{Float64}:
1.0
1.2
1.3
2.0

As expected the iterator now has a defined length:

julia> Base.IteratorSize(kiter)
Base.HasLength()

julia> length(kiter)
4

julia> size(kiter)
(4,)

knotsbetween

Given an AbstractInterpolation itp or results of knots(itp) get an iterator over knots between start and stop.

using Interpolations
itp = interpolate(rand(4), options...)
krange = knotsbetween(itp; start=1.2, stop=3.0)
collect(kiter) # Array of knots between 1.2 and 3.0

We can iterate over all knots greater than start by omitting stop

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation(x, x.^2, extrapolation_bc=Periodic());

julia> krange = knotsbetween(etp; start=4.0);

julia> Iterators.take(krange, 5) |> collect
5-element Vector{Float64}:
4.5
4.75
5.0
5.5
5.75

If we omit start, iteration will range from the first knot to stop

julia> krange = knotsbetween(etp; stop=4.0);

julia> collect(krange)
9-element Vector{Float64}:
1.0
1.5
1.75
2.0
2.5
2.75
3.0
3.5
3.75

It is an error to not provided start and stop

julia> knotsbetween(etp)
ERROR: ArgumentError: At least one of start or stop must be specified
[...]

Multiple Dimensions

When used with a multi-dimensional interpolant, knotsbetween can be used to iterate overall knots such that: start[i] < k[i] stop[i] where i indexes dimensions.

julia> x = [1.0, 1.5, 1.75, 2.0];

julia> etp = LinearInterpolation((x, x), x.*x');

julia> knotsbetween(etp; start=(1.2, 1.5), stop=(1.8, 3.0)) |> collect
2×2 Matrix{Tuple{Float64, Float64}}:
(1.5, 1.75)   (1.5, 2.0)
(1.75, 1.75)  (1.75, 2.0)