# Linking two values in either direction
**URL:** https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407
**Category:** General Usage
**Created:** [March 1, 2018, 5:28am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407 "2018-03-01T05:28:56Z")
**Posts on this page:** 14
**Page:** 1
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 1, 2018, 5:28am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/1 "2018-03-01T05:28:56Z")
Hi all,
Is anyone aware of a type that can be used to link two values, in either direction? A trivial example would be if I wanted to link `"a"` to `:z`. I’d like to have some object `obj` such that `obj("a")` returns `:z`, and `obj(:z)` returns `"a"`.
I guess I could build this using a type that nests two dictionaries (one for each direction), or else using a `Vector{Tuple{T1, T2}}`, although both these options become a bit tricky if `T1` and `T2` are of the same type (i.e. linking `"a"`, and `"z"`). I thought it worth asking if there is already something pre-existing that solves this problem.
Cheers all,
Colin
---
### Author:  [@Nosferican](https://discourse.julialang.org/u/Nosferican)
#### Post date: [March 1, 2018, 5:45am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/2 "2018-03-01T05:45:37Z")
One possible way to efficiently store multi key-values would be with `Set`.
For example,
```julia
d = Tuple([:a,:b])
setdiff(d, [:a])
```
Each collection holds the key and all possible values. One can obtain the values by taking the key out. You can adapt this process in various ways such as a `Dict` which uses the key to find the `Set` and returns the `setdiff`.
---
### Author:  [@y4lu](https://discourse.julialang.org/u/y4lu)
#### Post date: [March 1, 2018, 5:57am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/3 "2018-03-01T05:57:11Z")
The simplest way would just be the flat file approach, you have an array of type Any(/etc) with two columns, pretty much like the `Vector{Tuple{T1, T2}}`, then find the one you want in one column (`find-x or .==`) and check the other column of that row.
It can get quite complicated/involved depending on what kind of link properties are needed
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [March 1, 2018, 8:03am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/4 "2018-03-01T08:03:51Z")
Sounds like a `Dict` would work:
```julia
julia> pairs = ["a" => :z, "foo" => :bar]
2-element Array{Pair{String,Symbol},1}:
"a"=>:z
"foo"=>:bar
julia> d = Dict(p for p in [pairs; reverse.(pairs)])
Dict{Any,Any} with 4 entries:
:bar => "foo"
:z => "a"
"a" => :z
"foo" => :bar
julia> d["a"]
:z
julia> d[:z]
"a"
```
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 1, 2018, 10:29am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/5 "2018-03-01T10:29:45Z")
Thanks for responding. Yes, that’s a neat way of doing things. After reading it, I was thinking of implementing something like this… until I saw DNF’s solution 🙂
Cheers,
Colin
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 1, 2018, 10:30am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/6 "2018-03-01T10:30:21Z")
Thanks for responding. Yes, my first instinct was to do something like this. But now that I’ve seen DNF’s solution, I think I’ll go with that.
Cheers,
Colin
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 1, 2018, 10:32am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/7 "2018-03-01T10:32:06Z")
I had no idea you could mix types in a Dict like that! That is awesome. I guess the downside is that every link is stored twice, but I think this is more than made up for by the simplicity of the solution.
Cheers and thanks, I’ll definitely be using this.
Colin
---
### Author:  [@tk3369](https://discourse.julialang.org/u/tk3369)
#### Post date: [March 1, 2018, 2:58pm UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/8 "2018-03-01T14:58:39Z")
It wouldn’t work in all cases, however, depending on what you want. If both `"a"` and `"b`" maps to `:z` then the later one wins.
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 1, 2018, 10:21pm UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/9 "2018-03-01T22:21:35Z")
Yes agreed - to be totally robust, you’d need to check all existing pairs before adding a new one. But for my purposes it is more than sufficient.
Cheers,
Colin
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [March 2, 2018, 8:06am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/10 "2018-03-02T08:06:22Z")
> [@tk3369](#):
>
> It wouldn’t work in all cases, however, depending on what you want. If both “a” and “b” maps to :z then the later one wins.
not only - there could resist single links too. And who will wins depends on internal Dict order not on order in vector of pairs:
```julia
julia> ppairs = ["a" => :z, "foo" => :bar, :z=>"c"] # Julia 0.7 don't allow using pairs! :/
3-element Array{Pair{Any,Any},1}:
Pair{Any,Any}("a", :z)
Pair{Any,Any}("foo", :bar)
Pair{Any,Any}(:z, "c")
julia> d = Dict(p for p in [ppairs; reverse.(ppairs)])
Dict{Any,Any} with 5 entries:
"c" => :z # ("c", :z) is represented only with this direction
:bar => "foo"
:z => "a" # first pair won
"a" => :z # first pair won
"foo" => :bar
```
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 2, 2018, 9:21am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/11 "2018-03-02T09:21:38Z")
Interesting point.
FYI, pairs work just fine for me on v0.7. I’m on `Version 0.7.0-DEV.4379 (2018-02-24 23:50 UTC)`
Cheers,
Colin
---
### Author:  [@Liso](https://discourse.julialang.org/u/Liso)
#### Post date: [March 2, 2018, 9:43am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/12 "2018-03-02T09:43:35Z")
Maybe it changed after your version?
```julia
julia> versioninfo()
Julia Version 0.7.0-DEV.4439
Commit cde00cf* (2018-03-01 17:22 UTC)
[...]
julia> pairs = ["a" => :z, "foo" => :bar, :z=>"c"]
ERROR: cannot assign variable Base.pairs from module Main
```
---
### Author:  [@colintbowers](https://discourse.julialang.org/u/colintbowers)
#### Post date: [March 2, 2018, 9:54am UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/13 "2018-03-02T09:54:30Z")
Maybe… although various uses of pairs is mentioned quite a bit in the v0.7 release notes… it certainly doesn’t sound like the intention is to remove them.
I guess we’ll find out 🙂
Cheers,
Colin
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [March 2, 2018, 2:03pm UTC](https://discourse.julialang.org/t/linking-two-values-in-either-direction/9407/14 "2018-03-02T14:03:58Z")
Cf [this discussion about invertable dicts](https://discourse.julialang.org/t/invertable-dict-structure/7225). TL;DR: this is a useful datastructure, and it would be great if someone made a PR to [DataStructures.jl](https://github.com/JuliaCollections/DataStructures.jl) implementing it.