# Eachindex in map (and broadcast?)
**URL:** https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371
**Category:** Internals & Design
**Created:** [October 10, 2017, 9:36am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371 "2017-10-10T09:36:35Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@juthohaegeman](https://discourse.julialang.org/u/juthohaegeman)
#### Post date: [October 10, 2017, 9:36am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371/1 "2017-10-10T09:36:35Z")
I have a question about the julia implementation of `map` and `broadcast` and its relation to `eachindex`.
In `map!`, as well as in a few other places, I find the pattern `for (i,j) in zip(eachindex(dest),eachindex(A))`, i.e.
```julia
for (i,j) in zip(eachindex(dest),eachindex(A))
dest[i] = f(A[j])
end
```
Isn’t it the point of `eachindex` to return an iterator that is suitable even for multiple arrays, i.e.
```julia
for I in eachindex(dest,A)
dest[I] = f(A[I])
end
```
With the current implementation, if both `dest` and `A` have `IndexStyle( . ) == IndexCartesian`, two `CartesianRange` iterators will have to run, and this necessarily brings along some overhead. In fact, `ccopy!` uses the pattern `RB, RA = eachindex(B), eachindex(A)` and then checks excplicitly wether `RA==RB` in order to run the iterator only once.
Even if one of the two has `IndexLinear`, I assume there is little benefit of using linear indices for one if the other is still indexed using cartesian indices. However, if there is some benefit to having linear indices for only one of the two, I would actually like even more if `eachindex(dest,A)` returns an iterator that produces two separate indices, which could be the same or could be different, so that the above `map!` implementation would become:
```julia
for (i,j) in eachindex(dest,A)
dest[i] = f(A[j])
end
```
I think this has several benefits. If both are sparse, the indices could visit all locations that are zero in either one of the two (or more) arrays.
My use case is to have two arrays which have a strided memory layout, but with strides that are not necessarily increasing (not column major), e.g. resulting from a lazy `permutedims`. I have written a plain iterator that generates a cache efficient memory accessing pattern (i.e. by running first over some outer blocks and than over the indices within each block). Ideally, wrapping these strided arrays in a new `Array` type, and extending `eachindex` in this way, `map` etc would work out of the box, but with the current `map!` code this is not true.
And my final questions is, how does `broadcast` fit in. `broadcast` does not seem to be using `eachindex` at all, though I am still studying the implementation. Some blog post about the implementation of the current broadcast mechanism might be nice/interesting.
---
### Author:  [@mkborregaard](https://discourse.julialang.org/u/mkborregaard)
#### Post date: [October 10, 2017, 11:07am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371/2 "2017-10-10T11:07:43Z")
> [@juthohaegeman](#):
>
> for I in eachindex(dest,A)
> dest[I] = f(A[I])
> end
This doesn’t work if one of the arrays is an `OffsetArray`, e.g. with zero-based indexing.
---
### Author:  [@juthohaegeman](https://discourse.julialang.org/u/juthohaegeman)
#### Post date: [October 10, 2017, 11:38am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371/3 "2017-10-10T11:38:24Z")
Hence the proposal to change the semantics of `eachindex` with multiple arguments, which is more powerful in any respect.
(Though I guess with `OffsetArray`, one could discuss when arrays can be considered to have the same “domain”, such that they can be copied into each other etc.)
---
### Author:  [@nalimilan](https://discourse.julialang.org/u/nalimilan)
#### Post date: [October 11, 2017, 9:02am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371/4 "2017-10-11T09:02:16Z")
See these issues:
[https://github.com/JuliaLang/julia/issues/15648](https://github.com/JuliaLang/julia/issues/15648)
[https://github.com/JuliaLang/julia/pull/15356](https://github.com/JuliaLang/julia/pull/15356)
---
### Author:  [@juthohaegeman](https://discourse.julialang.org/u/juthohaegeman)
#### Post date: [October 11, 2017, 9:13am UTC](https://discourse.julialang.org/t/eachindex-in-map-and-broadcast/6371/5 "2017-10-11T09:13:00Z")
That’s great; thanks. I should have looked some further.