# Strange behavior of intersect() when applied to ranges

**URL:** <https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313>\
**Category:** General Usage\
**Created:** [March 25, 2019, 5:24pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313 "2019-03-25T17:24:07Z")\
**Posts on this page:** 11\
**Page:** 1

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**Author:** ![adriano.vilela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/adriano.vilela/32/7596_2.png) [@adriano.vilela](https://discourse.julialang.org/u/adriano.vilela)\
**Post date:** [March 25, 2019, 5:24pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/1 "2019-03-25T17:24:07Z")

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When trying to intersect two ranges I stumbled upon the following behavior:

```julia
julia> intersect(2:8, 4:10)
4:8

julia> intersect(2:7, 5:10)
5:7

julia> intersect(2:6, 6:10)
6:6

julia> intersect(2:5, 7:10)
7:6

julia> intersect(2:4, 8:10)
8:7

julia> intersect(2:3, 9:10)
9:8

```

Is this expected? It works fine for arrays:

```julia
julia> intersect(collect(2:8), collect(4:10))
5-element Array{Int64,1}:
 4
 5
 6
 7
 8

julia> intersect(collect(2:7), collect(5:10))
3-element Array{Int64,1}:
 5
 6
 7

julia> intersect(collect(2:6), collect(6:10))
1-element Array{Int64,1}:
 6

julia> intersect(collect(2:5), collect(7:10))
0-element Array{Int64,1}

julia> intersect(collect(2:4), collect(8:10))
0-element Array{Int64,1}

julia> intersect(collect(2:3), collect(9:10))
0-element Array{Int64,1}

```

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<div class="post-metadata">

**Author:** ![under-Peter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/under-peter/32/3626_2.png) [@under-Peter](https://discourse.julialang.org/u/under-Peter)\
**Post date:** [March 25, 2019, 5:33pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/2 "2019-03-25T17:33:05Z")

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Note that if you do e.g. `collect(intersect(1:2,4:5))` it returns a `0-element Array{...}` too, so if you compare the collected results you get the same, i.e. `collect(intersect(r1,r2)) == intersect(collect(r1),collect(r2))` for unitranges `r1` and `r2`.  
Effectively, something like `4:2` is an empty range since the step is `+1` (not to be confused with a non-empty range like `4:-1:2`). The advantage is that intersect of unitranges seems to always return unitranges which is important for type stability. Quite nice if you ask me! (And you might wanna take a look at the implementation - very simple and understandable)

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<div class="post-metadata">

**Author:** ![adriano.vilela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/adriano.vilela/32/7596_2.png) [@adriano.vilela](https://discourse.julialang.org/u/adriano.vilela)\
**Post date:** [March 25, 2019, 5:44pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/3 "2019-03-25T17:44:20Z")

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Ok, that makes sense. I don’t know why, but for a moment I was thinking of `4:2` as `4:-1:2`. The argument about type stability makes total sense. Thanks a lot.

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<div class="post-metadata">

**Author:** ![stillyslalom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stillyslalom/32/45687_2.png) [@stillyslalom](https://discourse.julialang.org/u/stillyslalom)\
**Post date:** [March 25, 2019, 6:01pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/4 "2019-03-25T18:01:02Z")

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The automatic modification of range bounds still seems somewhat strange:

```julia
julia> 4:2
4:3

```

\*edit: this is definitely intentional behavior, but I don’t understand the rationale.

```julia
unitrange_last(start::T, stop::T) where {T<:Integer} =
    ifelse(stop >= start, stop, convert(T,start-oneunit(stop-start)))

```

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<div class="post-metadata">

**Author:** ![adriano.vilela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/adriano.vilela/32/7596_2.png) [@adriano.vilela](https://discourse.julialang.org/u/adriano.vilela)\
**Post date:** [March 25, 2019, 6:01pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/5 "2019-03-25T18:01:39Z")

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I also noticed that intersect!() throws an error when applied to unit ranges:

```julia
julia> s = 1:5
1:5

julia> s = intersect(s, 4:5)
4:5

julia> s = 1:5
1:5

julia> intersect!(s, 4:5)
ERROR: setindex! not defined for UnitRange{Int64}
Stacktrace:
 [1] error(::String, ::Type) at ./error.jl:42
 [2] error_if_canonical_setindex(::IndexLinear, ::UnitRange{Int64}, ::Int64) at ./abstractarray.jl:1028
 [3] setindex! at ./abstractarray.jl:1019 [inlined]
 [4] filter!(::getfield(Base, Symbol("##83#84")){typeof(∉),typeof(push!),Set{Int64}}, ::UnitRange{Int64}) at ./array.jl:2339
 [5] _shrink!(::Function, ::UnitRange{Int64}, ::Tuple{UnitRange{Int64}}) at ./array.jl:2384
 [6] intersect!(::UnitRange{Int64}, ::UnitRange{Int64}) at ./array.jl:2389
 [7] top-level scope at none:0

```

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<div class="post-metadata">

**Author:** ![stillyslalom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stillyslalom/32/45687_2.png) [@stillyslalom](https://discourse.julialang.org/u/stillyslalom)\
**Post date:** [March 25, 2019, 6:05pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/6 "2019-03-25T18:05:06Z")

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That’s not surprising: unit ranges aren’t mutable objects, so a mutating method shouldn’t exist.

```julia
struct UnitRange{T<:Real} <: AbstractUnitRange{T}
    start::T
    stop::T
    UnitRange{T}(start, stop) where {T<:Real} = new(start, unitrange_last(start,stop))
end

```

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<div class="post-metadata">

**Author:** ![adriano.vilela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/adriano.vilela/32/7596_2.png) [@adriano.vilela](https://discourse.julialang.org/u/adriano.vilela)\
**Post date:** [March 25, 2019, 7:35pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/7 "2019-03-25T19:35:26Z")

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Right. I was wondering why the method below is defined then:

`[6] intersect!(::UnitRange{Int64}, ::UnitRange{Int64}) at ./array.jl:2389`

But then I noticed it comes from here:

`intersect!(v::AbstractVector, itrs...) = _shrink!(intersect!, v, itrs)`

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<div class="post-metadata">

**Author:** ![stillyslalom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stillyslalom/32/45687_2.png) [@stillyslalom](https://discourse.julialang.org/u/stillyslalom)\
**Post date:** [March 25, 2019, 9:00pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/8 "2019-03-25T21:00:19Z")

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Yeah, that method may be too broadly typed; there are a lot of immutable concrete types `<:` AbstractVector. This is a case where trait-based dispatch would be useful.

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [March 25, 2019, 10:01pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/9 "2019-03-25T22:01:22Z")

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I suppose that empty ranges could retain independent start and stop values but what would the advantage be? The current behavior allows the length of a range to always be computed as `stop-start+1`. It also normalizes empty ranges starting at a given index.

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<div class="post-metadata">

**Author:** ![jameson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jameson/32/23_2.png) [@jameson](https://discourse.julialang.org/u/jameson)\
**Post date:** [November 18, 2021, 8:58pm UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/10 "2021-11-18T20:58:50Z")

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We don’t do the clever trick anymore for computing the length of the range, so we could change this now ([https://github.com/JuliaLang/julia/pull/43058#issuecomment-973218652](https://github.com/JuliaLang/julia/pull/43058#issuecomment-973218652)), if there is support for that.

And there is also now this issue [https://github.com/JuliaLang/julia/issues/40331](https://github.com/JuliaLang/julia/issues/40331) for printing these empty ranges more clearly.

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [November 19, 2021, 10:32am UTC](https://discourse.julialang.org/t/strange-behavior-of-intersect-when-applied-to-ranges/22313/11 "2021-11-19T10:32:50Z")

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Sorry if this doesn’t make sense, but wouldn’t it be clear to output the existing form: `UnitRange[]`