# Map! using row iteration for output

**URL:** <https://discourse.julialang.org/t/map-using-row-iteration-for-output/92768>\
**Category:** General Usage\
**Tags:** question\
**Created:** [January 10, 2023, 7:13pm UTC](https://discourse.julialang.org/t/map-using-row-iteration-for-output/92768 "2023-01-10T19:13:18Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![prittjam](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/prittjam/32/21267_2.png) [@prittjam](https://discourse.julialang.org/u/prittjam)\
**Post date:** [January 10, 2023, 7:13pm UTC](https://discourse.julialang.org/t/map-using-row-iteration-for-output/92768/1 "2023-01-10T19:13:18Z")

</div>

why is this not possible?

Given

```julia
typeof((u₁,u₂))
Tuple{Vector{SVector{3, Float64}}, Vector{SVector{3, Float64}}}

```

and

```julia
typeof(M)
MMatrix{8, 9, Float64, 72} (alias for MArray{Tuple{8, 9}, Float64, 2, 72})

```

I want to fill M,

```julia
zs = @SVector zeros(eltype(M))
odd_rows(u₁,u₂) = SVector{9}(u₁.data...,zs...,(u₂[1]*u₁).data...)
map!(odd_rows,eachrow(M),u₁,u₂)

```

Extending this, can a row iterator server as an lvalue to mutate the underlying matrix?