Rewind iterators

General Usage
1

Hello,

I wonder if it’s possible to rewind iterator (ie without) recreating it.

I tried

julia> itr = Iterators.Stateful([10,11,12])
Base.Iterators.Stateful{Array{Int64,1},Union{Nothing, Tuple{Int64,Int64}}}([10, 11, 12], (10, 2), 0)

julia> collect(itr)
3-element Array{Int64,1}:
 10
 11
 12

julia> collect(itr)
0-element Array{Int64,1}

julia> rewind(itr)
ERROR: UndefVarError: rewind not defined
Stacktrace:
 [1] top-level scope at REPL[28]:1

Any idea?

2

It depends on what you will do next. the thing is, when you collect, you allocate anyways. If you need your itr to survive multiple action, you can use rest
https://docs.julialang.org/en/v1/base/iterators/#Base.Iterators.rest

1 Like
3

Well…

I really think that such a rewind function shouldn’t be difficult to implement.

julia> itr = Iterators.Stateful([10,11,12])
Base.Iterators.Stateful{Array{Int64,1},Union{Nothing, Tuple{Int64,Int64}}}([10, 11, 12], (10, 2), 0)

julia> collect(itr)
3-element Array{Int64,1}:
 10
 11
 12

julia> collect(itr)
0-element Array{Int64,1}

julia> rewind(itr::Iterators.Stateful) = Iterators.Stateful(itr.itr)
rewind (generic function with 1 method)

julia> itr = rewind(itr)
Base.Iterators.Stateful{Array{Int64,1},Union{Nothing, Tuple{Int64,Int64}}}([10, 11, 12], (10, 2), 0)

julia> collect(itr)
3-element Array{Int64,1}:
 10
 11
 12

I wonder if an inplace rewind function (named for example rewind!) couldn’t be implemented

Your opinion?

4

Rewinding iterators in general is impossible. For example, consider:

proc = open(`sh -c 'while true; do echo $RANDOM; done'`)
iter = eachline(proc)

For stateless iterators, you can use IterTools.peekiter. It should be possible to implement this for stateful iterators (or iterators that perform expensive some computation) by buffering peeked elements.

1 Like
5

you are basically recreating your itr, what (from an efficiency perspective) matters is the relation between the data (i.e itr and its elements), can you maybe construct a use case for this and see what’s the best options are?

1 Like