# Is there a function that extracts the index from an element of a CategoricalArray created by the function \`cut\`?

**URL:** <https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551>\
**Category:** General Usage\
**Tags:** question\
**Created:** [January 31, 2022, 10:49pm UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551 "2022-01-31T22:49:13Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![Soldalma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/soldalma/32/29388_2.png) [@Soldalma](https://discourse.julialang.org/u/Soldalma)\
**Post date:** [January 31, 2022, 10:49pm UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/1 "2022-01-31T22:49:13Z")

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The function `cut` in the `CategoricalArray` package returns a CategoricalArray object, as in this example:

```julia
x = rand(10)
ct = cut(x, 3)

"Q3: [0.735346, 0.972399]"
 "Q2: [0.335188, 0.735346)"
 "Q1: [0.00586079, 0.335188)"
 "Q3: [0.735346, 0.972399]"
 "Q3: [0.735346, 0.972399]"
 "Q1: [0.00586079, 0.335188)"
 "Q3: [0.735346, 0.972399]"
 "Q1: [0.00586079, 0.335188)"
 "Q2: [0.335188, 0.735346)"
 "Q2: [0.335188, 0.735346)"

```

One can convert the elements of the CategoricalArray object into strings:

```julia
string(ct[1])
"Q3: [0.735346, 0.972399]"

```

It is easy to write a function that would extract the index from such a string:

```julia
foo(ct[1])
3

```

or alternatively return the interval:

```julia
foo(ct[1])
(0.735346, 0.972399) # a tuple, not a string

```

Given the latter function another function that would find whether a value lies in the interval could be easily written:

```julia
bar(0.8, ct)
true

bar(2.0, ct)
false

```

I hope such functions exist and are faster than the functions I would write. However, I could not find references to either one. Do such functions exist?

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**Author:** ![Jeff\_Emanuel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeff_emanuel/32/15440_2.png) [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)\
**Post date:** [January 31, 2022, 11:25pm UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/2 "2022-01-31T23:25:07Z")

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I think you want `levelcode`

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**Author:** ![Soldalma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/soldalma/32/29388_2.png) [@Soldalma](https://discourse.julialang.org/u/Soldalma)\
**Post date:** [January 31, 2022, 11:57pm UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/3 "2022-01-31T23:57:59Z")

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Thanks. But as far as can gather `levelcode` only accepts a CategoricalValue object (or Missing) as argument. I need to find the interval where an arbitrary Float64 is.

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**Author:** ![Soldalma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/soldalma/32/29388_2.png) [@Soldalma](https://discourse.julialang.org/u/Soldalma)\
**Post date:** [February 1, 2022, 1:11am UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/4 "2022-02-01T01:11:53Z")

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I realized that my original question was not clear. My apologies. I have rewritten the question, hope that makes things clearer.

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<div class="post-metadata">

**Author:** ![Jeff\_Emanuel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeff_emanuel/32/15440_2.png) [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)\
**Post date:** [February 1, 2022, 1:52am UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/5 "2022-02-01T01:52:19Z")

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I think it’s still not clear, because you seem to want that information from the ct array elements. I think, instead, you mean to be passing x[1] to your proposed functions. Anyway, the bounds of the categories do not seem to preserved in the CategoricalArray. But this is the line where those are calculated [https://github.com/JuliaData/CategoricalArrays.jl/blob/fad8dc4d7fa5e6bf9ea30cd098e0e4cc99ff5eeb/src/extras.jl#L241](https://github.com/JuliaData/CategoricalArrays.jl/blob/fad8dc4d7fa5e6bf9ea30cd098e0e4cc99ff5eeb/src/extras.jl#L241). You could call quantile yourself and use its results.

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<div class="post-metadata">

**Author:** ![Soldalma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/soldalma/32/29388_2.png) [@Soldalma](https://discourse.julialang.org/u/Soldalma)\
**Post date:** [February 1, 2022, 9:30am UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/6 "2022-02-01T09:30:01Z")

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You are right that I could call `quantile` but I think that would mean it would be called twice. I also need to call `cut` and `cut` calls `quantile`. I wanted to avoid that for efficiency reasons. Calling `quantile` is expensive.

Since `cut` calls `quantile` there could be a version of `cut` that would also return the return value of `quantile`. Alternatively, there could exist a function that would do what I was asking for: extract the return value of `quantile` from the return value of `cut`.

Still another alternative would be to have a version of `cut` that would have a parameter `breaks` (replacing `ngroups`). Then one would first call `quantile` and its return value would be used as the argument for the `breaks` parameter when calling this version of `cut`. Maybe the easiest way would be to write such a version of `cut`. Something along these lines:

```julia

function mycut(v::Vector{Float64}, breaks::Vector{Float64})::Vector{Int64}
    @assert is_monotonic(breaks) "Argument `breaks` should be strictly monotonically increasing"

    @assert breaks[1] <= minimum(x) "The first element of `breaks` should be less than or equal to the smallest element of `x`."
    @assert breaks[end] >= maximum(x) "The last element of `breaks` should be greater than or equal to the greatest element of `x`."

    y::Vector{Int64} = fill(0, length(x))

    lenb = length(breaks)

    for i in eachindex(v)
        for b in 2:lenb
            if v[i] < breaks[b]
                y[i] = b - 1
                break
            end
        end
    end

    return y
end

```

It would be used in this way:

```julia
x = rand(20);
mycut(x, [minimum(x), 0.2, 0.4, 0.6, 0.8, maximum(x)])
20-element Vector{Int64}:
 3
 2
 1
 1
 2
 4
 4
 2
 4
 4
 0
 1
 2
 2
 2
 3
 3
 5
 3
 4

```

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<div class="post-metadata">

**Author:** ![Jeff\_Emanuel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeff_emanuel/32/15440_2.png) [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)\
**Post date:** [February 1, 2022, 3:40pm UTC](https://discourse.julialang.org/t/is-there-a-function-that-extracts-the-index-from-an-element-of-a-categoricalarray-created-by-the-function-cut/75551/7 "2022-02-01T15:40:38Z")

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There are already methods for `cut` in CategoricalArrays that accept `breaks`. If you need both range checking on the breaks and the categorical array, you could create a higher level type that knows both and provides the combined functionality.