# Why modyfing view do not lead to modification of source array

**URL:** <https://discourse.julialang.org/t/why-modyfing-view-do-not-lead-to-modification-of-source-array/66354>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [August 13, 2021, 7:00pm UTC](https://discourse.julialang.org/t/why-modyfing-view-do-not-lead-to-modification-of-source-array/66354 "2021-08-13T19:00:24Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Jakub\_Mitura](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jakub_mitura/32/19496_2.png) [@Jakub\_Mitura](https://discourse.julialang.org/u/Jakub_Mitura)\
**Post date:** [August 13, 2021, 7:00pm UTC](https://discourse.julialang.org/t/why-modyfing-view-do-not-lead-to-modification-of-source-array/66354/1 "2021-08-13T19:00:24Z")

</div>

Hello I have some 3 dimensional array (CT scan) - then for convinience I define view to given slice (slicing can be done in diffrent dimensions so that is why I have this meta.parse…). I want to be able to modify source 3 dimensional array by modifying the 2 dimensional view - what am I doing wrong?

below structs holding data

````julia
abstract type RawDataToDisp end

```@doc

2 dimensional ata for displaying single slice

struct is mutable becouse in case of the masks data can be changed multiple times and rapidly   

```

@with_kw mutable struct TwoDimRawDat{T} <: RawDataToDisp

   type::Type{T}= UInt8# easy access to type

   name::String=""#associated name

   dat::Array{T, 2}=ones(type,2,2)# raw pixel data

end#2DimRawDat

```@doc

3 dimensional data for displaying single slice

struct is mutable becouse in case of the masks data can be changed multiple times and rapidly   

```

@with_kw mutable struct ThreeDimRawDat{T} <: RawDataToDisp

   type::Type{T}= UInt8# easy access to type

   name::String=""#associated name

   dat::Array{T, 3}=ones(type,2,2,2)# raw voxel data

end#2DimRawDat

````

functions defined to work on this structs

````julia
```@doc

given two dim dat it sets points in given coordinates in given slice to given value

coords - coordinates in a plane of chosen slice to modify

value - value to set for given points

return reference to modified slice

```

function modSlice!(data::TwoDimRawDat{T}

                ,coords::Vector{CartesianIndex{2}}

                ,value::T ) where {T}

   data.dat[coords].=value

   data.dat

end#modSlice

```@doc

gives access to the slice of intrest - way of slicing is defined at the begining

typ - type of data 

slice - slice we want to access

sliceDim - on the basis of what dimension we are slicing

return 2 dimensional array wrapper -TwoDimRawDat object representing slice of given 3 dimensional array

!! important returned TwoDimRawDat holds view to the original 3 dimensional data  

```

function threeToTwoDimm(typ::Type{T}

                ,slice::Int

                ,sliceDim::Int

                ,threedimDat::ThreeDimRawDat{T})::TwoDimRawDat{T} where {T}

                arr=[":",":",":"]

                arr[sliceDim]="$slice"

               return TwoDimRawDat{T}(typ,threedimDat.name,view(threedimDat.dat,eval(Meta.parse(arr[1])),eval(Meta.parse(arr[2])),eval(Meta.parse(arr[3])) ) )

end#ThreeToTwoDimm

````

Then I test It on zero arrays of appropriate types and dimensionality

```julia
        zz=..# this is some TwoDimRawDat
        sa= zz.dat # 3 dim array
        maximum(sa) # 0 - as should be 
        tD= threeToTwoDimm(zz.type,2,2,zz ) # creating view in struct 
        modSlice!(tD, [CartesianIndex(1,1)], UInt8(3)) # modyfing view
        maximum(tD.dat)# is 3 as expected
maximum(sa)# still 0!!! - why?

```

thanks for help!

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [August 13, 2021, 7:38pm UTC](https://discourse.julialang.org/t/why-modyfing-view-do-not-lead-to-modification-of-source-array/66354/2 "2021-08-13T19:38:23Z")

</div>

I took the liberty of editing your post to change block (text) quotes to use code quoting instead — please use that in the future to help make your posts more readable.

On the actual question, I really want to dissuade you from using `Meta.parse` and `eval`. Instead, you can use `selectdim` to just do this operation for you (if I’m understanding it correctly).

Ok, that’s still not addressing the actual question… the root of the challenge you asked about is that `TwoDimRawDat` has its `dat` field defined as an `Array{T,3}`. This doesn’t support views and will convert (and thus copy) any non-Array as needed/able. If you want your `TwoDimRawDat` struct to support holding views, you need to change that field to also allow `SubArray`s (likely by allowing all `AbstractArray`s in general).