# First matching element in collection
**URL:** https://discourse.julialang.org/t/first-matching-element-in-collection/18324
**Category:** General Usage
**Tags:** question
**Created:** [December 5, 2018, 2:48pm UTC](https://discourse.julialang.org/t/first-matching-element-in-collection/18324 "2018-12-05T14:48:57Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [December 5, 2018, 2:48pm UTC](https://discourse.julialang.org/t/first-matching-element-in-collection/18324/1 "2018-12-05T14:48:57Z")
Sometimes I want to find the first element in a collection that satisfies some predicate `f`. The collection may not be (O(1)) indexable, so `findfirst` then lookup is not ideal.
I am looking for something like
```julia
function firstmatching(f, itr)
state = ()
while true
y = iterate(itr, state...)
y ≡ nothing && return nothing
elt = first(y)
f(elt) && return Some(elt)
state = Base.tail(y)
end
end
```
Eg
```julia
julia> firstmatching(isodd, (2, 4, 5))
Some(5)
julia> firstmatching(x -> last(x) ≡ :a, [1 => :b, 2 => :c, 3 => :a])
Some(3 => :a)
```
Does something similar exist in `Base`, or a commonly used package?
---
### Author:  [@bennedich](https://discourse.julialang.org/u/bennedich)
#### Post date: [December 5, 2018, 3:15pm UTC](https://discourse.julialang.org/t/first-matching-element-in-collection/18324/2 "2018-12-05T15:15:46Z")
How about `first(Iterators.filter(f, itr))`?
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [December 5, 2018, 4:37pm UTC](https://discourse.julialang.org/t/first-matching-element-in-collection/18324/3 "2018-12-05T16:37:55Z")
> [@bennedich](#):
>
> How about `first(Iterators.filter(f, itr))` ?
I thought about that too, but besides being kind of complicated,
```julia
julia> first(Iterators.filter(isodd, 2:2:10))
ERROR: ArgumentError: collection must be non-empty
Stacktrace:
[1] first(::Base.Iterators.Filter{typeof(isodd),StepRange{Int64,Int64}}) at ./abstractarray.jl:289
[2] top-level scope at none:0
```
---
### Author:  [@bennedich](https://discourse.julialang.org/u/bennedich)
#### Post date: [December 5, 2018, 7:43pm UTC](https://discourse.julialang.org/t/first-matching-element-in-collection/18324/4 "2018-12-05T19:43:16Z")
Complicated? I feel like this is precisely the kind of thing `Iterators.filter` was made for. To support `nothing`, you could do:
```julia
function firstmatching2(f, itr)
it = iterate(Iterators.filter(f, itr))
it ≡ nothing ? nothing : it[1]
end
```
The advantage of reusing existing code is that you get maintenance, testing and (usually) a decent quality implementation for free. For example:
```julia
julia> A = [0:2:1000000; 1];
julia> @btime firstmatching(isodd, A)
34.428 ms (1999497 allocations: 38.14 MiB)
Some(1)
julia> @btime firstmatching2(isodd, A)
334.707 μs (2 allocations: 48 bytes)
1
```
I’m sure you can fix this in your implementation (if it matters to you), but if you reuse existing code, you don’t have to worry about these things.