# Understanding map/mapslices/eachslice/stack
**URL:** https://discourse.julialang.org/t/understanding-map-mapslices-eachslice-stack/132744
**Category:** New to Julia
**Tags:** stack, mapslices
**Created:** [September 29, 2025, 10:39am UTC](https://discourse.julialang.org/t/understanding-map-mapslices-eachslice-stack/132744 "2025-09-29T10:39:34Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Boris](https://discourse.julialang.org/u/Boris)
#### Post date: [September 29, 2025, 10:39am UTC](https://discourse.julialang.org/t/understanding-map-mapslices-eachslice-stack/132744/1 "2025-09-29T10:39:34Z")
Hi, I’ve been trying to learn to use `mapslices` and don’t understand the synthax/errors for my use case.
Suppose I have
```julia
# MWE
sb = [true false false; false false false; true true true] # dimensions (k \times j)
yh = rand(3,3,3) # dimensions (j \times k \times s), first two dimensions are transpose of sb
Mz = [0 1/3 1/3 1/3] # dimensions (n \times m), where m is # of true in sb
```
I want to calculate the following product on each slice in the third dimension:
```julia-auto
x1 = Mz*yh[:,:,1]'[sb] # yh slice is transposed
x2 = Mz*yh[:,:,2]'[sb]
x3 = Mz*yh[:,:,3]'[sb]
```
where `yh[:,:,1]'[sb]` is a essentially a vector of with `m` elements and I would like to stack the output in an array where the answers are along the third dimension as they are in `yh`, such that
```julia
out[:,:,i] = xi; # dimensions n \times 1 \times s
```
I never learned `mapslices`, `eachslice`, or `stack`, so I thought now is a good time and reading the documentation for `mapslices` I thought it is exactly what I need but I am definitely doing something wrong:
```julia
julia> mapslices(x->Mz*x'[sb],yh,dims=3)
ERROR: BoundsError: attempt to access 1×3 adjoint(::Vector{Float64}) with eltype Float64 at index [3×3 Matrix{Bool}]
Stacktrace:
[1] throw_boundserror(A::LinearAlgebra.Adjoint{Float64, Vector{Float64}}, I::Tuple{Base.LogicalIndex{Int64, Matrix{Bool}}})
@ Base .\essentials.jl:14
[2] checkbounds
@ .\abstractarray.jl:699 [inlined]
[3] _getindex(l::IndexLinear, A::LinearAlgebra.Adjoint{Float64, Vector{Float64}}, I::Base.LogicalIndex{Int64, Matrix{Bool}})
@ Base .\multidimensional.jl:914
[4] getindex
@ .\abstractarray.jl:1312 [inlined]
[5] (::var"#29#30")(x::Vector{Float64})
@ Main c:\Users\boris\Dropbox\JuliaNew\BEAVARs\main_dev.jl:124
[6] mapslices(f::var"#29#30", A::Array{Float64, 3}; dims::Int64)
@ Base .\abstractarray.jl:3281
[7] top-level scope
@ c:\Users\boris\Dropbox\JuliaNew\BEAVARs\main_dev.jl:124
```
My logic here was that `mapslices` will take a slice of `yh` along the third dimension called `x`, i.e. `x = yh[:,:,1]` and then transpose it, use `sb` to take the corresponding elements and then pre-multiply the vector with `Mz`, doing that along all slices and return a 3D object with the answers stacked along the third dimension.
```julia
help?> mapslices
mapslices(f, A; dims)
Transform the given dimensions of array A by applying a function f on each slice of the form A[..., :, ..., :, ...], with a colon at each
d in dims. The results are concatenated along the remaining dimensions.
For example, if dims = [1,2] and A is 4-dimensional, then f is called on x = A[:,:,i,j] for all i and j, and f(x) becomes R[:,:,i,j] in
the result R.
```
Is the problem stemming from the fact that the output has a lower dimension than the input (size of the input is independent from `j and k`, dimensions of `yh` )? Is the problem due to me having a vector (which, in contrast to other languages is not an m \times 1 array)? Or am I using the function wrong? I don’t understand the error (apart from trying to access elements that don’t exist).
---
### Author:  [@wheeheee](https://discourse.julialang.org/u/wheeheee)
#### Post date: [September 29, 2025, 1:45pm UTC](https://discourse.julialang.org/t/understanding-map-mapslices-eachslice-stack/132744/2 "2025-09-29T13:45:07Z")
The `dims` argument to `mapslices` refer to the dimension of each slice you want to apply `f` on, and I take it you probably meant to specify `dims=1:2` instead. To collapse dimensions, do `map(x -> Mz * x'[sb], eachslice(yh; dims=3))` with `Mz = [0, 1/3, 1/3, 1/3]'` instead.
I think the confusion is that the `dims` kwarg for `eachslice` and `mapslices` are basically the complement of each other…
---
### Author:  [@Boris](https://discourse.julialang.org/u/Boris)
#### Post date: [September 29, 2025, 2:22pm UTC](https://discourse.julialang.org/t/understanding-map-mapslices-eachslice-stack/132744/3 "2025-09-29T14:22:49Z")
Ooooooh. Now I get it. `dims 1:2` are the dimensions I want **to keep**.
Yes, this works exactly as I expected.
```julia
julia> mapslices(x->Mz*x'[sb],yh,dims=1:2)
1×1×3 Array{Float64, 3}:
[:, :, 1] =
0.453213930170162
[:, :, 2] =
0.44354265382116154
[:, :, 3] =
0.6598193145457338
```
I see, this type of syntax allows me to do more “powerful” slicing if I wanted, e.g. save arbitrary dimensions (in the above example 2:3 would also work), to do the slicing.
Thanks, this is nice!