# Reducing sum of OffsetArray

**URL:** <https://discourse.julialang.org/t/reducing-sum-of-offsetarray/36571>\
**Category:** General Usage\
**Created:** [March 26, 2020, 10:29pm UTC](https://discourse.julialang.org/t/reducing-sum-of-offsetarray/36571 "2020-03-26T22:29:39Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [March 26, 2020, 10:29pm UTC](https://discourse.julialang.org/t/reducing-sum-of-offsetarray/36571/1 "2020-03-26T22:29:39Z")

</div>

I’m getting an error with `sum(B, dims=1)`, where `B` is an `OffsetArray`.

```julia
using OffsetArrays
A = rand(3,3)
B = OffsetArray(A, -1:1, 5:7)
sum(A, dims=1) # OK
sum(B, dims=1) # errors

```

Here’s the output:

```julia
Julia-1.4.0> sum(A, dims=1)
1×3 Array{Float64,2}:
 1.15857 1.72228 1.86123

```

```julia
Julia-1.4.0> sum(B, dims=1)
ERROR: ArgumentError: No method is implemented for reducing index range of type typeof(i). Please implement
reduced_index for this index type or report this as an issue.

Stacktrace:
 [1] reduced_index(::OffsetArrays.IdOffsetRange{Int64,Base.OneTo{Int64}}) at .\reducedim.jl:8
 [2] reduced_indices(::Tuple{OffsetArrays.IdOffsetRange{Int64,Base.OneTo{Int64}},OffsetArrays.IdOffsetRange{Int64,Base.OneTo{Int64}}}, ::Int64) at .\reducedim.jl:23
 [3] reduced_indices at .\reducedim.jl:15 [inlined]
 [4] reducedim_initarray(::OffsetArray{Float64,2,Array{Float64,2}}, ::Int64, ::Float64, ::Type{Float64}) at .\reducedim.jl:92
 [5] reducedim_initarray at .\reducedim.jl:93 [inlined]
 [6] reducedim_init at .\reducedim.jl:172 [inlined]
 [7] _mapreduce_dim at .\reducedim.jl:317 [inlined]
 [8] #mapreduce#580 at .\reducedim.jl:307 [inlined]
 [9] _sum at .\reducedim.jl:679 [inlined]
 [10] _sum at .\reducedim.jl:678 [inlined]
 [11] #sum#583 at .\reducedim.jl:652 [inlined]
 [12] top-level scope at REPL[5]:1

```

I’m not really sure what I expected, but perhaps the same as this:

```julia
Julia-1.4.0> sum(B[i:i, :] for i in axes(B,1))
1×3 OffsetArray(::Array{Float64,2}, 1:1, 5:7) with eltype Float64 with indices 1:1×5:7:
 1.15857 1.72228 1.86123

```

---

<div class="post-metadata">

**Author:** ![AndiMD](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/andimd/32/6531_2.png) [@AndiMD](https://discourse.julialang.org/u/AndiMD)\
**Post date:** [March 26, 2020, 11:30pm UTC](https://discourse.julialang.org/t/reducing-sum-of-offsetarray/36571/2 "2020-03-26T23:30:06Z")

</div>

This looks like a bug/missing implementation, maybe open an issue? Possible workaround (not tested):

```julia
import Base: reduced_index
Base.reduced_index(i::OffsetArrays.IdOffsetRange) = oftype(i, first(i):first(i))

```

---

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [March 27, 2020, 3:57am UTC](https://discourse.julialang.org/t/reducing-sum-of-offsetarray/36571/3 "2020-03-27T03:57:42Z")

</div>

Issue logged here: [Reduction over OffsetArray error · Issue #107 · JuliaArrays/OffsetArrays.jl · GitHub](https://github.com/JuliaArrays/OffsetArrays.jl/issues/107)  
Workaround seems to work for me.  
Thanks!