# BoundsError -\> NaN ? (in Interpolations)

**URL:** <https://discourse.julialang.org/t/boundserror-nan-in-interpolations/110372>\
**Category:** General Usage\
**Tags:** question, anonymous-function\
**Created:** [February 18, 2024, 2:23pm UTC](https://discourse.julialang.org/t/boundserror-nan-in-interpolations/110372 "2024-02-18T14:23:26Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![ryofurue](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ryofurue/32/24531_2.png) [@ryofurue](https://discourse.julialang.org/u/ryofurue)\
**Post date:** [February 18, 2024, 2:23pm UTC](https://discourse.julialang.org/t/boundserror-nan-in-interpolations/110372/1 "2024-02-18T14:23:26Z")

</div>

I need a NaN when the target isn’t in the “interior” in linear interpolation:

```julia
using Distributions
using Interpolations

const imax_in = 20
const imax_out = 20

xax_in, v_in =
  let dx = rand(Uniform(1.0, 10.0), imax_in)
    f(x) = x - x^3
    xax = [0; cumsum(dx)]
    xax, f.(xax)
  end

xax_out =
  let dx = rand(Uniform(0.5, 20.0), imax_out)
    [2.3; cumsum(dx)]
  end

ip = linear_interpolation(xax_in, v_in)

v_out = ip.(xax_out) #-> BoundsError as expected.

```

Currently, I get `BoundsError` of course, but I want NaN values there.

Is there an option to the interpolator to return NaN?

* * *

The following is an off-topic question. Are there arrays and dictionaries that return a prescribed values (such as `NaN` and `nothing`) instead of throwing BoundsError or KeyError?

---

<div class="post-metadata">

**Author:** ![bertschi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bertschi/32/33462_2.png) [@bertschi](https://discourse.julialang.org/u/bertschi)\
**Post date:** [February 18, 2024, 6:06pm UTC](https://discourse.julialang.org/t/boundserror-nan-in-interpolations/110372/2 "2024-02-18T18:06:34Z")

</div>

This is documented [here](http://juliamath.github.io/Interpolations.jl/stable/convenience-construction/), i.e.,

```julia
ip = linear_interpolation(xax_in, v_in; extrapolation_bc = NaN)

```

You may also want to check the section on [Extrapolation](http://juliamath.github.io/Interpolations.jl/stable/extrapolation/).

PS: For vectors, arrays or dictionaries you can just use `get` which takes a default argument for out-of-bound keys:

```julia-repl
julia> A = reshape(1:6, 2, 3);

julia> get(A, (1, 2), NaN)
3

julia> get(A, (4, 2), NaN)
NaN

```

---

<div class="post-metadata">

**Author:** ![ryofurue](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ryofurue/32/24531_2.png) [@ryofurue](https://discourse.julialang.org/u/ryofurue)\
**Post date:** [February 19, 2024, 5:40am UTC](https://discourse.julialang.org/t/boundserror-nan-in-interpolations/110372/3 "2024-02-19T05:40:03Z")

</div>

Thank you!!

I’ve been using Julia more than a year but still I’m not able to find right documentations . . . Perhaps because Julia is too young, Google search isn’t working for me. Of course I visited the official documentation of `Interpolations.jl` but somehow I failed to locate the information.

Not just `Interpolations` but all sorts of Julia packages. I end up asking simple questions whose answers are in the official documentations. . . .

Regarding `get()`: Thank you for that, too!

It’s strange that I’ve never heard of such a fundamental function as that. I’ve sometimes used `getindex( )` as a function form of `[]`, but `get()` seems to be more fundamental (generic) than `getindex()`. For one thing, `getindex(A, 4, 2)` could be realized as `get(A, (4,2); outofrange=Throw())` (mimicking `linear_interpolation()`). I mean that `getindex()` could be viewed as a special case of `get()`, but not the other way round.