# Sub2ind on julia

**URL:** <https://discourse.julialang.org/t/sub2ind-on-julia/109717>\
**Category:** New to Julia\
**Tags:** image-processing\
**Created:** [February 4, 2024, 11:49pm UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717 "2024-02-04T23:49:36Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![Afonso\_Britto](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/afonso_britto/32/205972_2.png) [@Afonso\_Britto](https://discourse.julialang.org/u/Afonso_Britto)\
**Post date:** [February 4, 2024, 11:49pm UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/1 "2024-02-04T23:49:36Z")

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I have this matlab code I want to convert it to julia, but there is no sub2ind in julia. Does anyone have any ideia how i can translate it?

‘’’  
for c = 1:num\_channels

```
    l1_c = sub2ind(size(SAT), row_indices, l_idx, repmat(c, h, w));
    u0_c = sub2ind(size(SAT), row_indices, u_idx, repmat(c, h, w));
    
    l0_c = sub2ind(size(SAT), row_indices, l_idx - 1, repmat(c, h, w));
    u1_c = sub2ind(size(SAT), row_indices, u_idx + 1, repmat(c, h, w));
    
    l1 = sub2ind(size(SAT), row_indices, l_idx);
    u0 = sub2ind(size(SAT), row_indices, u_idx);
    
    l0 = sub2ind(size(SAT), row_indices, l_idx - 1);
    u1 = sub2ind(size(SAT), row_indices, u_idx + 1);

    % Full (center) areas.
    C = SAT(u0_c) - SAT(l1_c);
    
    % Left fractional areas.
    alpha = (l_pos - xform_domain_position(l0)) ./ (xform_domain_position(l1) - xform_domain_position(l0));
    yi = I(l0_c) + alpha .* ( I(l1_c) - I(l0_c) );
    L = 0.5 .* (yi + I(l1_c)) .* (1-alpha) .* (xform_domain_position(l1) - xform_domain_position(l0));
    
    % Right fractional areas.
    alpha = (u_pos - xform_domain_position(u0)) ./ (xform_domain_position(u1) - xform_domain_position(u0));
    yi = I(u0_c) + alpha .* ( I(u1_c) - I(u0_c) );
    R = 0.5 .* (yi + I(u0_c)) .* (alpha) .* (xform_domain_position(u1) - xform_domain_position(u0));
    
    F(:,:,c) = (L + C + R) / (2 * box_radius);

end

```

‘’’

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<div class="post-metadata">

**Author:** ![Mattriks](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mattriks/32/351_2.png) [@Mattriks](https://discourse.julialang.org/u/Mattriks)\
**Post date:** [February 4, 2024, 11:56pm UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/2 "2024-02-04T23:56:57Z")

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> [@PSA: replacement of ind2sub/sub2ind in Julia 0.7+](https://discourse.julialang.org/t/psa-replacement-of-ind2sub-sub2ind-in-julia-0-7/14666):
>
> I’ve now seen a couple of cases where folks seem unhappy about the removal of the functions ind2sub and sub2ind. Some of the concerns may stem from the suggested replacement in the deprecation warning, as the replacement, while “safe,” seems considerably more awkward. This is a short post explaining their replacements and the motivation for the change; once you learn to think in the new way, I’d be surprised if you don’t become a fan of the new approach. In older versions of Julia, the way to c…

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**Author:** ![Afonso\_Britto](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/afonso_britto/32/205972_2.png) [@Afonso\_Britto](https://discourse.julialang.org/u/Afonso_Britto)\
**Post date:** [February 5, 2024, 1:00am UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/3 "2024-02-05T01:00:01Z")

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Could you give an example of one line of my code using this solution? I really could not implemented it, really new to Julia.

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**Author:** ![mkitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mkitti/32/12459_2.png) [@mkitti](https://discourse.julialang.org/u/mkitti)\
**Post date:** [February 5, 2024, 1:05am UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/4 "2024-02-05T01:05:35Z")

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We could give you a better answer if you gave us an executable example. I have no idea what most of the variables above look like.

```julia
julia> SAT_SIZE = (2,3,5)
(2, 3, 5)

julia> SAT = reshape(1:prod(SAT_SIZE), SAT_SIZE);

julia> linidx = LinearIndices(size(SAT));

julia> linidx[2,3,5]
30

julia> linidx[1,2,4]
21

julia> linidx[1:2, 2:3, 4:5]
2×2×2 Array{Int64, 3}:
[:, :, 1] =
 21 23
 22 24

[:, :, 2] =
 27 29
 28 30

```

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**Author:** ![oscarvdvelde](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscarvdvelde/32/202157_2.png) [@oscarvdvelde](https://discourse.julialang.org/u/oscarvdvelde)\
**Post date:** [February 5, 2024, 1:15am UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/5 "2024-02-05T01:15:41Z")

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I came from Scilab where also ind2sub/sub2ind was used. For my uses, I converted that as follows:

```julia
dims = (10,10,10) # example
lindex = LinearIndices((dims[1],dims[2],dims[3]))
grid = CartesianIndices((1:dims[1],1:dims[2],1:dims[3]))[lindex] 
gridx = getindex.(grid,1)
gridy = getindex.(grid,2)
gridz = getindex.(grid,3)

```

Here lindex can be any linear index you already have into a matrix. To get linear indices from x,y,z locations,

```julia
julia> grid[50]
CartesianIndex(10, 5, 1)

julia> lindex[grid[50]]
50

julia> lindex[grid[1:10,5,1]]
10-element Vector{Int64}:
 41
 42
 43
  ⋮
 49
 50

```

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<div class="post-metadata">

**Author:** ![mkitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mkitti/32/12459_2.png) [@mkitti](https://discourse.julialang.org/u/mkitti)\
**Post date:** [February 5, 2024, 1:42am UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/6 "2024-02-05T01:42:32Z")

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If you are in a pinch, you can just use the following.

```julia-repl
julia> function sub2ind(sz, args...)
           linidx = LinearIndices(sz)
           getindex.([linidx], args...)
       end
sub2ind (generic function with 1 method)

julia> row = [1 2 3 1];

julia> col = [2 2 2 3];

julia> sub2ind((3,3), row, col)
1×4 Matrix{Int64}:
 4 5 6 7

julia> I1 = [1 2 1 2];

julia> I2 = [2 2 1 1];

julia> I3 = [1 1 2 2];

julia> sub2ind((2,2,2), I1, I2, I3)
1×4 Matrix{Int64}:
 3 4 5 6

julia> sub2ind((3,4,2), 2,1,2)
1-element Vector{Int64}:
 14

```

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**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [February 5, 2024, 3:39pm UTC](https://discourse.julialang.org/t/sub2ind-on-julia/109717/7 "2024-02-05T15:39:38Z")

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Looking at your example, it appears that you don’t need a linear index _explicitly_, but rather the ability to index an array at arbitrary combinations of coordinates. In Julia, `CartesianIndex` is a great tool for this.

For example, I would use `l0 = @. CartesianIndex(row_indices, l_idx - 1)` ([note the `@.`](https://docs.julialang.org/en/v1/base/arrays/#Base.Broadcast.@ __dot__ ), which makes this equivalent to `l0 = CartesianIndex.(row_indices, l_idx .- 1)`). Then using these indices is as simple as `xform_domain_position[l0]`.