# BoundsError when trying to index certain dimensions of an array with a vector containing indices

**URL:** <https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983>\
**Category:** General Usage\
**Tags:** question, indexing\
**Created:** [August 7, 2021, 8:25am UTC](https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983 "2021-08-07T08:25:26Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Valery](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/valery/32/21803_2.png) [@Valery](https://discourse.julialang.org/u/Valery)\
**Post date:** [August 7, 2021, 8:25am UTC](https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983/1 "2021-08-07T08:25:27Z")

</div>

```julia
A = rand(3,3)
b = findall(vec(A).>0.5) # contains indices
c = rand(2,length(b)) # some values that we want to have at the corresponding indexes stored in b, in array B.
B = rand(2,3,3)
B[:,b] = c # this line returns an error

ERROR: BoundsError: attempt to access 2×3×3 Array{Float64, 3} at index [1:2, [1, 4, 5, 7, 8, 9]]
Stacktrace:
 [1] throw_boundserror(A::Array{Float64, 3}, I::Tuple{Base.Slice{Base.OneTo{Int64}}, Vector{Int64}})
   @ Base ./abstractarray.jl:651
 [2] checkbounds
   @ ./abstractarray.jl:616 [inlined]
 [3] _setindex!
   @ ./multidimensional.jl:886 [inlined]
 [4] setindex!(::Array{Float64, 3}, ::Matrix{Float64}, ::Function, ::Vector{Int64})
   @ Base ./abstractarray.jl:1267
 [5] top-level scope
   @ REPL[15]:1

```

In the last line, I would like that the value in `c` fill `B` in its 3x3 part (second and third dimension of `B`) at the locations contained in `b` for a 3x3 matrix (locations obtained from `A`). How should I write the last line ?

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [August 7, 2021, 8:56am UTC](https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983/2 "2021-08-07T08:56:20Z")

</div>

FYI, it would be much better to make your `B 3x3x2`.

* * *

Your `A` is a 3x3 mask (based on \> 0.5 or not), your `B` has 2 “layers” of 3x3 matrix.

My understanding is that you want to assign random values to each of those two layers at location according to mask `A`.

```julia
julia> for i = CartesianIndices(A)
           if A[i] > 0.5
               @. B[:, i] = rand()
           end
       end

```

this should do?

---

<div class="post-metadata">

**Author:** ![Valery](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/valery/32/21803_2.png) [@Valery](https://discourse.julialang.org/u/Valery)\
**Post date:** [August 7, 2021, 3:42pm UTC](https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983/3 "2021-08-07T15:42:27Z")

</div>

The solution I was looking for is

```julia
A = rand(3,3)
b = findall(vec(A).>0.5) # contains indices
c = rand(2,length(b)) # some values that we want to have at the corresponding indexes stored in b, in array B.
B = rand(2,3,3)
b = CartesianIndices(A)[b] # ADDING THIS LINE SOLVES THE PROBLEM
B[:,b] = c # b as CartesianIndices is legal here

```

Vector `b` as `CartesianIndices` is a legal input in this scenario (instead of ‘LinearIndices’).

@jling Thank you for your suggestion with the loop, but I will avoid using a loop if there is an alternative.

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [August 7, 2021, 4:32pm UTC](https://discourse.julialang.org/t/boundserror-when-trying-to-index-certain-dimensions-of-an-array-with-a-vector-containing-indices/65983/4 "2021-08-07T16:32:52Z")

</div>

> [@Valery](#):
>
> I will avoid using a loop if there is an alternative.

why, Julia is not MATLAB/Python/R, loops are fast in Julia and allocate less than your “vectorzied” code.

```julia
julia> A = rand(3,3);

julia> function f(A,B)
           b = findall(vec(A).>0.5) 
           c = rand(2,length(b))
           b = CartesianIndices(A)[b]
           B[:,b] = c
           B
       end

julia> @benchmark f($A, B) setup=(B=rand(2,3,3))
BenchmarkTools.Trial: 10000 samples with 376 evaluations.
 Range (min … max): 256.104 ns … 9.701 μs ┊ GC (min … max): 0.00% … 93.84%
 Time (median): 263.614 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 302.790 ns ± 381.676 ns ┊ GC (mean ± σ): 8.49% ± 6.50%

  ▅█▅▂▁ ▁
  ██████▆▆▇██▇▇▇▇▆▅▆▇▇▇▇▆▇▆▇█▇▇▆▅▅▄▄▄▄▆▄▅▅▃▅▄▄▄▄▅▅▅▅▄▅▆▆▆▆▅▅▄▃▄ █
  256 ns Histogram: log(frequency) by time 554 ns <

 Memory estimate: 608 bytes, allocs estimate: 7.

julia> function g(A,B)
           for i = CartesianIndices(A)
                  if A[i] > 0.5
                      @. B[:, i] = rand()
                  end
              end
        B
       end
g (generic function with 1 method)

julia> @benchmark g($A, B) setup=(B=rand(2,3,3))
BenchmarkTools.Trial: 10000 samples with 988 evaluations.
 Range (min … max): 51.816 ns … 381.994 ns ┊ GC (min … max): 0.00% … 0.00%
 Time (median): 52.579 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 53.107 ns ± 4.857 ns ┊ GC (mean ± σ): 0.00% ± 0.00%

  ▅▃█▁ ▁ ▁
  ████▇▇▆█▇▆▅▃▅▅▇▆▅▃▃▁▃▄▁▄▄▃▁▁▁▃▁▄▄▁▁▃▃▅▅▄▃▄▅▅▅▄▆▆▄▅▆▅▄▄▆▅▅▃▆▄ █
  51.8 ns Histogram: log(frequency) by time 72.2 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

```

So my version is 4x faster from micro benchmark and has 0 allocation?