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adds the nth function for iterables #56580

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adds the nth function for iterables #56580

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6a0b4bd
adds `nth` function to iterators plus tests
ghyatzo Nov 16, 2024
1de4a8f
removed useless module annotations
ghyatzo Nov 16, 2024
11ee27a
Merge branch 'master' into nth-api
ghyatzo Nov 16, 2024
3870e3e
fix one based indexing, more generic
ghyatzo Nov 16, 2024
8bdd4a0
fix: simpler generic-based indexing
ghyatzo Nov 17, 2024
7326e87
feat: adds the fix2 wrapper.
ghyatzo Jan 5, 2025
58b60e8
fix: better wording for the fix2 version
ghyatzo Jan 10, 2025
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70 changes: 69 additions & 1 deletion base/iterators.jl
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Expand Up @@ -30,7 +30,7 @@ import .Base:
getindex, setindex!, get, iterate,
popfirst!, isdone, peek, intersect

export enumerate, zip, rest, countfrom, take, drop, takewhile, dropwhile, cycle, repeated, product, flatten, flatmap, partition
export enumerate, zip, rest, countfrom, take, drop, takewhile, dropwhile, cycle, repeated, product, flatten, flatmap, partition, nth
public accumulate, filter, map, peel, reverse, Stateful

"""
Expand Down Expand Up @@ -1595,4 +1595,72 @@ end
# be the same as the keys, so this is a valid optimization (see #51631)
pairs(s::AbstractString) = IterableStatePairs(s)

"""
nth(itr, n::Integer)

Get the `n`th element of an iterable collection. Return `nothing` if not existing.
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Returning nothing makes it impossible to distinguish between "the nth element was nothing", and "there was no nth element". Perhaps return Union{Nothing, Some}?

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@ghyatzo ghyatzo Nov 16, 2024 •
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Fair point.
Should it be Union{nothing, Some} even in those cases where we know there can't be a nothing value in the iterator (for sake of uniform api)? I.e. Count Iterator or Repeated (with its element different than nothing) or AbstractRanges

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I think it should, otherwise it would be too confusing.

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I would just throw an error if there is no nth element. There could also be a default argument as in get, where a user can pass a value that should be returned if no nth element exists.

I don't really follow the logic that the spirit of iterators is to return nothing in such cases?

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@mcabbott mcabbott Nov 17, 2024 •
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Agree nothing is weird, your iterator can produce that. Some seems a bit technical & unfriendly? An error seems fine. Matches what first([]) does.

I suppose it can't literally be a method of get since it goes by enumeration not keys:

julia> first(Dict('a':'z' .=> 'A':'Z'), 3)
3-element Vector{Pair{Char, Char}}:
 'n' => 'N'
 'f' => 'F'
 'w' => 'W'

julia> nth(Dict('a':'z' .=> 'A':'Z'), 3)
'w' => 'W'


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See also: [`first`](@ref), [`last`](@ref)

# Examples
```jldoctest
julia> nth(2:2:10, 4)
8

julia> nth(reshape(1:30, (5,6)), 6)
6
```
"""
nth(itr, n::Integer) = _nth(IteratorSize(itr), itr, n)
nth(itr::AbstractArray, n::Integer) = n > length(itr) ? nothing : itr[n]
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This assumes one-based indexing. Perhaps do itr[begin + n - 1].

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you are absolutely correct.
would something like getindex(itr, nth(eachindex(IndexLinear(), itr), n)) be too overkill?
and adding a specialization with nth(itr::AbstractRange, n::Integer) = getindex(itr, n)

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I went with the probably overkill approach, if it's too much i'll revert back to your suggestion.

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AbstractRanges are not always one-based either, so that approach runs into the same issue

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@ghyatzo ghyatzo Nov 16, 2024 •
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From what I could gather that is included in the getindex already, since it ends up calling

unsafe_getindex(v::AbstractRange{T}, i::Integer) where T = convert(T, first(v) + (i - oneunit(i))*step_hp(v))

which should pretty much be the same sa [begin + n -1]
unless I'm missing the point completely?

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@mcabbott mcabbott Nov 17, 2024 •
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The line nth(itr::AbstractRange, n) = getindex(itr, n) will for sure fail on the axes of an OffsetArray. (In fact, it will first be ambiguous, as n::Any is less specific.)

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I was overthinking it, I'll just stick with [begin + n - 1]. Sorry.


_nth(::SizeUnknown, itr, n) = _fallback_nth(itr, n)
_nth(::Union{HasShape,HasLength}, itr, n) = _withlength_nth(itr, n, length(itr))
_nth(::IsInfinite, itr, n) = _inbounds_nth(itr, n)

_inbounds_nth(itr, n) = iterate(drop(itr, n - 1))[1]
_inbounds_nth(itr::AbstractArray, n) = itr[n]
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_withlength_nth(itr, n, N) = n > N ? nothing : _inbounds_nth(itr, n)

function _fallback_nth(itr, n)
y = iterate(drop(itr, n - 1))
y === nothing && return nothing
y[1]
end

# specialized versions to better interact with existing iterators
# Count
nth(itr::Count, n::Integer) = n > 0 ? itr.start + itr.step * (n - 1) : nothing

# Repeated
nth(itr::Repeated, n::Integer) = itr.x

# Take(Repeated)
nth(itr::Take{Repeated{O}}, n::Integer) where {O} = n > itr.n ? nothing : itr.xs.x

# infinite cycle
nth(itr::Cycle{I}, n::Integer) where {I} = _nth_inf_cycle(IteratorSize(I), itr, n)
_nth_inf_cycle(::IsInfinite, itr, n) = _inbounds_nth(itr.xs, n)
_nth_inf_cycle(::SizeUnknown, itr, n) = _fallback_nth(itr.xs, n)
_nth_inf_cycle(::Union{HasShape,HasLength}, itr, n) = _repeating_cycle_nth(itr.xs, n, length(itr.xs))

# finite cycle
# a finite cycle iterator is in reality a Flatten{Take{Repeated{O}}} iterator
nth(itr::Flatten{Take{Repeated{O}}}, n::Integer) where {O} = _nth_finite_cycle(IteratorSize(O), itr, n)
_nth_finite_cycle(::IsInfinite, itr, n) = _inbounds_nth(itr, n)
_nth_finite_cycle(::SizeUnknown, itr, n) = _fallback_nth(itr, n)
_nth_finite_cycle(::Union{HasShape,HasLength}, itr, n) = begin
N = itr.it.n # `Take` iterator n
torepeat = itr.it.xs.x # repeated object
K = length(torepeat)
n > K * N && return nothing
_repeating_cycle_nth(torepeat, n, K)
end


_repeating_cycle_nth(inner_itr, n, inner_N) = _inbounds_nth(inner_itr, 1 + ((n - 1) % inner_N))



end
24 changes: 24 additions & 0 deletions test/iterators.jl
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Expand Up @@ -1074,6 +1074,30 @@ end
end
end

@testset "nth" begin
Z = Array{Int,0}(undef)
Z[] = 17
itrs = (collect(1:1000), 10:6:1000, "∀ϵ>0", (1, 3, 5, 10, 78), reshape(1:30, (5, 6)),
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Z, 3, true, 'x', 4 => 5, view(Z), view(reshape(1:30, (5, 6)), 2:4, 2:6),
(x^2 for x in 1:10), Iterators.Filter(isodd, 1:10), Iterators.flatten((1:10, 50:60)),
pairs(50:60), zip(1:10, 21:30, 51:60), Iterators.product(1:3, 10:12),
Iterators.repeated(3.14159, 5), (a=2, b=3, c=5, d=7, e=11), Iterators.cycle(collect(1:100)),
Iterators.cycle([1, 2, 3, 4, 5], 5))
ns = (
234, 123, 3, 2, 21, 1, 1, 1, 1, 1, 1, 10, 9, 3, 15, 7, 6, 3, 4, 4, 99999, 25
)
expected = (
234, 742, '>', 3, 21, 17, 3, true, 'x', 4, 17, 22, 81, 5, 54, (7 => 56), (6, 26, 56), (3, 10), 3.14159, 7, 99, 5
)
@test length(itrs) == length(ns) == length(expected)
testset = zip(itrs, ns, expected)
@testset "iter: $IT" for (IT, n, exp) in testset
@test exp == nth(IT, n)
IT isa Cycle && continue # cycles are infinite so never OOB
@test nth(IT, 999999999) === nothing
end
end

@testset "Iterators docstrings" begin
@test isempty(Docs.undocumented_names(Iterators))
end