# IndexStyle for pairs iterator?

**URL:** <https://discourse.julialang.org/t/indexstyle-for-pairs-iterator/96450>\
**Category:** General Usage\
**Tags:** indexing, iterators\
**Created:** [March 22, 2023, 3:06pm UTC](https://discourse.julialang.org/t/indexstyle-for-pairs-iterator/96450 "2023-03-22T15:06:40Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [March 22, 2023, 3:06pm UTC](https://discourse.julialang.org/t/indexstyle-for-pairs-iterator/96450/1 "2023-03-22T15:06:40Z")

</div>

I was a bit surprised to see that the `pairs` iterator for `Matrix` returns `CartesianIndices` by default. The docstring of `pairs` includes a section on specifying index style, showing these three signatures:

```julia
pairs(IndexLinear(), A)
pairs(IndexCartesian(), A)
pairs(IndexStyle(A), A)

```

I would have expected the default behaviour of `pairs` to be

```julia
pairs(A) = pairs(IndexStyle(A), A)

```

but in fact it is

```julia
pairs(A::AbstractVector) = pairs(IndexLinear(), A)
pairs(A::AbstractArray) = pairs(IndexCartesian(), A)

```

Note that `IndexStyle(Array)` is always `IndexLinear()`, for all dimensionalities.

The `eachindex` function is defined as I expected:

```julia
eachindex(A::AbstractArray) = (@inline(); eachindex(IndexStyle(A), A))

```

and I was treating `pairs` as simply a convenient version of `eachindex` that also spat out values, with surprising results.

So two questions:

1. Why isn’t the defult behaviour of `pairs` for an array consistent with the array’s `IndexStyle`? Or why isn’t `IndexStyle(Matrix)` equal to `IndexCartesian()`?
2. What is a convenient way to query the type of the indices of `A`? For now I have `typeof(first(first(pairs(A))))`, which seems a bit awkward. What if I only have the _type_ of `A`, and not `A` itself?

If I want to do the ‘safe and correct’ version of `enumerate(A)` (which many misuse, actually), I have to write `pairs(IndexLinear(), A)`, which a bit awkward.