Idiomatic way to specify exclusive range?

General Usage
1

The idiomatic way to specify a range from a to b inclusive is a:b. Is there an standard operator or idiom to express the range a to b in which b itself is excluded (something like range() in Python)? I’m planning to make some updates to the token system for sorted containers in DataStructures.jl that will introduce notation for exclusive ranges. I prefer to follow an idiom if one exists.

1 Like
2

a:b-1 ? Seems easy enough to parse and reads nicely as from a to b minus one.

4 Likes
3

This would be a great solution if I had time to implement and test performant operations +(::Token,::Int) and -(::Token,::Int), but unfortunately, that is outside the scope of work I could hope to accomplish in January. If there is an idiomatic notation that addresses just this limited usage, that would be great. If not, I will make something up.

4

You could also go crazy and do something like:

julia> a↦b = a:b-1
↦ (generic function with 1 method)

julia> collect(1↦4)
3-element Array{Int64,1}:
 1
 2
 3

However, array comprehension fails for some reason:

julia> [i for i in 1↦4]
syntax: expected "]"

Stacktrace:
 [1] top-level scope at In[12]:1
 [2] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091
1 Like
5

@tamasgal, what about:

a↦b = (a:b-1)

This does not work, as explained by @mbauman. But the similar function does not look too bad:

julia> ↦(a,b) = a:b-1
↦ (generic function with 1 method)

julia> [i for i in ↦(1,4)]
3-element Array{Int64,1}:
 1
 2
 3
1 Like
6

The trouble with that comprehension is that ↦ has a very low precedence. The fix is [i for i in (1↦4)]. You’ll generally need to add parens in places where you wouldn’t need to with :. There aren’t many other operators with the same precedence as : — IntervalSets uses .., but it’s also inclusive by default.

That said, intervals are perhaps the right mental model for the sort of operation I’m guessing you’re doing — and IntervalSets does have support for open intervals (it’s just the function OpenInterval or explicit Interval{:closed,:open}).

You could also add shorthands with one of these unicode operators which have the same precedence as : and .. — … ⁝ ⋮ ⋱ ⋰ ⋯ — but unfortunately none of them are very evocative of a half-open-on-one-side sort of thing.

3 Likes