How to get the first state of a iterator?

General Usage
1

I need to create a vector with the states of an iterator for which the element satisfies a certain condition.
However, to do that, I would need to know the first state of an iterator (i.e. the default argument of iterator), in case the first element satisfies the condition. Is there a way to get it?

2

Perhaps I misunderstand the question, but it sounds like you are looking for

Required methods Brief description
iterate(iter) Returns either a tuple of the first item and initial state or nothing if empty

from the docs.

Perhaps you could provide more detail on what you are trying to achieve?

3

Thanks but that’s not what I am looking for. I would like to create a function g(f::Function, iter) that returns for an arbitrary iterator iter the vector of states for which the corresponding element satisfies f(x) = 0

The returned vector by g must satisfy following:

for state in g(f, iter)
   f(iterate(iter,  state)) == 0
end

For instance,

g(x -> x == 0, [0, 1, 0]) = [1, 3]

Note that, because the first element satisfies the condition, the function should return the first state of the iterator.

4

There is (semantically) no state for the first iterate call though? Some iterators might implement their iterate with a default value as the state but that is not required and not every iterator does this.

1 Like
6

As @kristoffer.carlsson says, unfortunately your question has no answer. It’s true that many iterators are implemented through

iterate(::Iterator, state=....)

but many others are not.

What you could do is represent state by (state,) and the empty state by (). Example:

function g(f, iter)
    zerostates = []
    curstate = ()
    i = iterate(iter, curstate...)
    while !isnothing(i)
        val, state = i
        curstate = (state,)
        iszero(f(val)) && push!(zerostates, curstate)
        i = iterate(iter, curstate...)
    end
    return zerostates
end

But it has the downside that you now have a Vector{Any}. It’s probably as close as you’ll get, though.

1 Like
8

Thanks everyone!