# Document / export \`ht\_keyindex()\` from dict.jl?
**URL:** https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738
**Category:** Internals & Design
**Tags:** performance, dictionary
**Created:** [July 4, 2022, 11:16am UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738 "2022-07-04T11:16:07Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [July 4, 2022, 11:16am UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/1 "2022-07-04T11:16:07Z")
I’d like to avoid repeated lookup of the memory address of some key in a dictionary. Consider the following code:
```julia
function delete_if_even!(d::Dict, k)
if haskey(d, k) && iseven(d[k])
delete!(d, k)
else
d
end
end
```
Since a dictionary is implemented as a hash table, the hash computation happens 3 times in the above code, in `haskey`, `getindex`, and `delete!`. The repeated computation can be avoid by rewriting the above function as follows, directly using `ht_keyindex()` from `dict.jl`,
```julia
function delete_if_even_fast!(d::Dict, k)
index = Base.ht_keyindex(d, k)
@inbounds index >= 0 && iseven(d.vals[index]) ? Base._delete!(d, index) : d # return d if nothing is done
end
```
In fact, a similar functionality is exported by the C++ stdlib `unordered_map`. An iterator for the memory address of a key can be located by calling `find()`, then you can directly delete a key using the iterator by calling `erase()`. In Julia, the iterator of a dictionary is usually only produced by `iterate()` unless you dig into unexported parts of `dict.jl`. Or maybe there’s a more elegant solution to the problem?
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [July 4, 2022, 11:27am UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/2 "2022-07-04T11:27:08Z")
Have a look at `Dictionaries.jl`: [GitHub repo](https://github.com/andyferris/Dictionaries.jl), [Announcement post with some discussion](https://discourse.julialang.org/t/ann-dictionaries-jl-improved-productivity-and-performance-of-dictionaries-in-julia/32074).
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### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [July 4, 2022, 2:29pm UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/3 "2022-07-04T14:29:30Z")
Thanks, it looks neat. But for my use case (heavy insertion / deletion element by element rather than broadcast), the base Dict still seems faster according to my very limited testing. It would be nice if a few more methods are exported by dict.jl, so I can improve my existing code with minimal changes, without relying on undocumented internal details.
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### Author:  [@Stephen\_Vavasis](https://discourse.julialang.org/u/Stephen_Vavasis)
#### Post date: [July 5, 2022, 2:01am UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/4 "2022-07-05T02:01:52Z")
This topic has been discussed several times in Julia language issues, and there is an open PR to address it here: [Add modify! function for lookup/update/insert/delete in one go by tkf · Pull Request #33758 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/33758)
---
### Author:  [@andyferris](https://discourse.julialang.org/u/andyferris)
#### Post date: [August 2, 2022, 3:09am UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/5 "2022-08-02T03:09:15Z")
> [@greatpet](#):
>
> for my use case (heavy insertion / deletion element by element rather than broadcast), the base Dict still seems faster according to my very limited testing
Yeah this is correct.
For the die-hards, in Dictionaries.jl there is a module under contrib/HashDictionaries.jl with a dictionary like the one in `Base`. It is slightly faster than `Dict` because I removed the extra code used to support `WeakKeyDict`, and you can use the Dictionaries.jl token interface (and helper functions) with it. The plan is to turn this into `UnorderedDictionary`, export it, and document it as superior for this use case.
---
### Author:  [@andyferris](https://discourse.julialang.org/u/andyferris)
#### Post date: [August 8, 2022, 12:35pm UTC](https://discourse.julialang.org/t/document-export-ht-keyindex-from-dict-jl/83738/6 "2022-08-08T12:35:13Z")
To follow up on that, the new Dictionaries.jl 0.3.23 has an `UnorderedDictionary` dictionary type (and `UnorderedIndices` set type) for insertion/deletion/lookup heavy workloads. It would be good to know how it performs for people in the wild.