# Testing for scalar indexes from to\_indices

**URL:** <https://discourse.julialang.org/t/testing-for-scalar-indexes-from-to-indices/22534>\
**Category:** Internals & Design\
**Tags:** question, proposal\
**Created:** [March 30, 2019, 10:52am UTC](https://discourse.julialang.org/t/testing-for-scalar-indexes-from-to-indices/22534 "2019-03-30T10:52:43Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [March 30, 2019, 10:52am UTC](https://discourse.julialang.org/t/testing-for-scalar-indexes-from-to-indices/22534/1 "2019-03-30T10:52:43Z")

</div>

I wonder if there is an API for testing if something returned by `to_indices` is a “scalar” index (ie one that would drop dimensions). I could not find anything, and I think it would be useful to have, and thought I would ask here before making a PR before the 1.2 feature freeze.

One can of course [rely on the documentation](https://docs.julialang.org/en/v1/manual/arrays/#man-supported-index-types-1) and note that currently “scalar” indexes are `Int` or `CartesianIndex`. But this may change in the future, just like `CartesianIndex` was introduced.

The proposal is something trivial like

```julia
is_scalar_index(::Integer) = true
is_scalar_index(::CartesianIndex) = true
is_scalar_index(_) = false

```

An MWE for some context is

```julia
"""
Wrapper for vectors, supports subsetting via [].
"""
struct Foo
    a # both are <: AbtractVector, but here omit
    b # type parameters to simplify example
    function Foo(a, b)
        @assert axes(a) == axes(b)
        new(a, b)
    end
end

Base.firstindex(f::Foo) = firstindex(f.a)

Base.lastindex(f::Foo) = lastindex(f.a)

function Base.getindex(f::Foo, i)
    ι = first(to_indices(f.a, (i, )))
    if ι isa Union{Integer, CartesianIndex} # this would be more robust
        error("subset using non-scalar indexes")
    else
        Foo(f.a[ι], f.b[ι])
    end
end

```

with

```julia
julia> f = Foo(1:5, ones(5))
Foo(1:5, [1.0, 1.0, 1.0, 1.0, 1.0])

julia> f[2:3]
Foo(2:3, [1.0, 1.0])

julia> f[2]
ERROR: subset using non-scalar indexes
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] getindex(::Foo, ::Int64) at ./REPL[19]:4
 [3] top-level scope at REPL[27]:1

```