# \`iterate\` returning iter, next-state and iter's original length
**URL:** https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656
**Category:** General Usage
**Created:** [September 7, 2018, 8:49am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656 "2018-09-07T08:49:07Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [September 7, 2018, 8:49am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656/1 "2018-09-07T08:49:07Z")
I am using a custom iterator that delegates into a struct to obtain the iterative grist, say a vector. I think it is appropriate to check if the current state (like an index) exceeds the length of the delegatee.
I do not want to recalculate the vector’s length at each iteration, and keeping the length in the struct does not work for external reasons.
What is an appropriate way to define `iterate` so that the next state returned is a tuple `(total_length, next_index)` and have it all work? I would prefer that the state stay simple (a two tuple) rather than embed both in a struct.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [September 7, 2018, 9:14am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656/2 "2018-09-07T09:14:35Z")
```julia
struct Foo{T <: AbstractVector} # type for MWE
v::T
end
function Base.length(c::Foo)
@info "length was called, and it was darn expensive"
length(c.v)
end
function Base.iterate(c::Foo)
len = length(c)
len > 0 || return nothing
(c.v[1], (2, len))
end
Base.iterate(c::Foo, (i, len)) = i > len ? nothing : (c.v[i], (i+1, len))
```
then
```julia
julia> foreach(println, Foo(1:10))
[ Info: length was called, and it was darn expensive
1
2
3
4
5
6
7
8
9
10
```
But that said, I would just define a wrapper type that caches the length.
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### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [September 7, 2018, 9:30am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656/3 "2018-09-07T09:30:54Z")
Doesn’t a vector know its length?
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### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [September 7, 2018, 9:32am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656/4 "2018-09-07T09:32:23Z")
strings do not know their length
although I am not using strings here
You may have resolved this as Julia does, often – problem? no, its no problem.
The length I need is several layers deep into substructure while iterating on `x` length(`x.partsofx.vecparts.bluevec`). I don’t believe the levels of dot indirection are very costly – I do believe they are not ameanable to compile time prelocation.
---
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [September 7, 2018, 9:37am UTC](https://discourse.julialang.org/t/iterate-returning-iter-next-state-and-iters-original-length/14656/5 "2018-09-07T09:37:47Z")
thanks for that … I was looking for `(c, (i, len))`
I’ll benchmark both approaches.