# Defining \`getindex\` for \`Tables.columntable\` objects

**URL:** <https://discourse.julialang.org/t/defining-getindex-for-tables-columntable-objects/31944>\
**Category:** General Usage\
**Created:** [December 6, 2019, 9:15pm UTC](https://discourse.julialang.org/t/defining-getindex-for-tables-columntable-objects/31944 "2019-12-06T21:15:02Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![pdeffebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdeffebach/32/10320_2.png) [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)\
**Post date:** [December 6, 2019, 9:15pm UTC](https://discourse.julialang.org/t/defining-getindex-for-tables-columntable-objects/31944/1 "2019-12-06T21:15:02Z")

</div>

Let’s say you are working with tabular objects and for convenience and want to use `t[i, j]` `getindex`ing (and not too many other functions) throughout your code.

On the other hand you want to support the Tables.jl interface. Therefore when you receive a function argument `t`, you wrap it in `Tables.columntable(t)`.

As far as I can tell, the Tables interface does not support indexing. But many implementations of `Tables.columntable` do support that, for instance `Tables.columntable(df::DataFrame) = df`.

Therefore it makes sense to do the following in your code

```julia
struct IndexableColumnTable{T}
    table::T
end

function Base.getindex(t::IndexableColumnTable)
	p = propertynames(t.table)[j]
	getproperty(t.table, p)[i]
end

```

Then in your function do

```julia
function foo(t)
    t = columntable(t)
    if hasmethod(getindex, (typeof(t), Int, Int))
    	return t
    else 
    	return IndexableColumnTable(t)
    end
end

```

Is this a reasonable approach?

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<div class="post-metadata">

**Author:** ![nalimilan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nalimilan/32/147_2.png) [@nalimilan](https://discourse.julialang.org/u/nalimilan)\
**Post date:** [December 11, 2019, 9:28am UTC](https://discourse.julialang.org/t/defining-getindex-for-tables-columntable-objects/31944/2 "2019-12-11T09:28:13Z")

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I’m afraid this isn’t right. First, I think you are confusing `Tables.columntable` and `Tables.columns`: the former always returns a named tuple of vectors, so if you do `t = columntable(t)` you never get an object which can be indexed with two integers (even if the input was e.g. a `DataFrame`).

Second, supposing you did `t = Tables.columns(t)`, the Tables.jl interface does not guaranty that `getproperty(t.table, p)` returns a vector, only an iterator. So I think you have to check whether the returned iterators are `AbstractVector`, and if not call `collect` on them. Then you can store the result as a named tuple of vectors (which is the most basic type of table) or use `Tables.materializer(t)(cols)` to create a table object of the same type as `t`.

Maybe Tables.jl could provide a function to do this more easily. Feel free to file an issue.

---

<div class="post-metadata">

**Author:** ![pdeffebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdeffebach/32/10320_2.png) [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)\
**Post date:** [December 11, 2019, 6:15pm UTC](https://discourse.julialang.org/t/defining-getindex-for-tables-columntable-objects/31944/3 "2019-12-11T18:15:49Z")

</div>

Thanks for the answer. It’s probably best just to use `Tables.matrix` in this context, even if it copies.

I get why Tables would want to be agnostic about the layout of columns, allowing for iterators or infinite streaming.