# How do I incorporate a boundary function for a vector in another function

**URL:** <https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005>\
**Category:** New to Julia\
**Created:** [April 3, 2023, 8:48am UTC](https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005 "2023-04-03T08:48:17Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![alekee](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alekee/32/48715_2.png) [@alekee](https://discourse.julialang.org/u/alekee)\
**Post date:** [April 3, 2023, 8:48am UTC](https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005/1 "2023-04-03T08:48:17Z")

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Y’all gonna see a lot from me cuz i dunno what the fuck i’m doing :). Working on problem set 2 of the MIT course and i’ve hit a wall. The question is:

Write a function `box_blur(v, l)` that blurs a vector `v` with a window of length `l` by averaging the elements within a window from `-l` to `l`. This is called a **box blur**. Use your function `extend` to handle the boundaries correctly.

Return a vector of the same size as `v`.

My extend function is:

```julia
function extend(v::AbstractVector, i)
	n = length(v)
	
if i in 1:n
		return v[i]
	elseif i < 1
		return v[1]
	else 
		return v[n]
	end
end

```

my mean function is

```julia
function mean(v)
	total = 0
	n = length(v)
	for s in 1:n
		total += v[s]
	end
	return total / n
end

```

my initial just straight up non-programmer logic for the box blur was:

```julia
function box_blur(v::AbstractArray, l)
	vn = copy(v)
	n = length(v)
	for i in vn
		window = i-l:i+l
		vn[i] = mean(extend(window, i))
	end
	return vn
end

```

I still end up with “ArgumentError: invalid index: 1.0 of type Float64” so i know the code must be nonsense but I don’t know how to incorporate the extend function to ensure the proper boundary.

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**Author:** ![fnin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fnin/32/23300_2.png) [@fnin](https://discourse.julialang.org/u/fnin)\
**Post date:** [April 3, 2023, 9:01am UTC](https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005/2 "2023-04-03T09:01:19Z")

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You are iterating over the vector elements of vn and in your loop you index with the entries of you vector. This doesn’t work as you cant index an array with floats.

Maybe you meant to loop over `1:n`?

```julia
function box_blur(v::AbstractArray, l)
	vn = copy(v)
	n = length(v)
	for i in 1:n
		window = i-l:i+l
		vn[i] = mean(extend(window, i))
	end
	return vn
end

```

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<div class="post-metadata">

**Author:** ![alekee](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alekee/32/48715_2.png) [@alekee](https://discourse.julialang.org/u/alekee)\
**Post date:** [April 3, 2023, 9:13am UTC](https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005/3 "2023-04-03T09:13:01Z")

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Aaaaah yes I did mean to do that, I even prepared in the line before to do that. Well my code is still clearly nonsense and idk what it’s even doing lmao. This is what it’s doing:

example\_vector = [0.8, 0.2, 0.1, 0.7, 0.6, 0.4]

After i input it into: box\_blur(example\_vector, 1), it turns into this

0.0, 3.0, 2.0, 3.0, 4.0, 5.0

computational logic makes my head hurt

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<div class="post-metadata">

**Author:** ![fnin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fnin/32/23300_2.png) [@fnin](https://discourse.julialang.org/u/fnin)\
**Post date:** [April 3, 2023, 10:33am UTC](https://discourse.julialang.org/t/how-do-i-incorporate-a-boundary-function-for-a-vector-in-another-function/97005/4 "2023-04-03T10:33:56Z")

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I do probably do it something like this:

```julia
extend(v::AbstractVector{T}, n) where T = [fill(v[1],n); v; fill(v[end], n)]

function box_blur(v,l)
   v_ex = extend(v,l)
   v_blurred = [sum(v_ex[i:(i+2*l)])/(2l+1) for i in 1:length(v)]
end

```

EDIT: I thought extend meant filling with 0, but apparently it is extending it with the same value.