# Length(skipmissing( )) not implemented
**URL:** https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943
**Category:** Performance
**Created:** [August 6, 2021, 1:40pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943 "2021-08-06T13:40:35Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@tomtom](https://discourse.julialang.org/u/tomtom)
#### Post date: [August 6, 2021, 1:40pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/1 "2021-08-06T13:40:35Z")
```julia
julia> x = [2.0, missing, 3.0];
julia> length(x)
3
julia> length(skipmissing(x))
ERROR: MethodError: no method matching length(::Base.SkipMissing{Vector{Union{Missing, Float64}}})
julia> count(a -> true, skipmissing(x))
2
```
it would be nice if `length` is implemented for `SkipMissing`. Using `count` in this case is less readable and counter intuitive.
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### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [August 6, 2021, 1:42pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/2 "2021-08-06T13:42:49Z")
There is an issue for this [here](https://github.com/JuliaLang/julia/issues/37014).
The problem is that `length` will always be `O(n)` for `SkipMissing`. It would be very slow, which is against the design of `skipmissing` in the first place.
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### Author:  [@tomtom](https://discourse.julialang.org/u/tomtom)
#### Post date: [August 6, 2021, 1:46pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/3 "2021-08-06T13:46:04Z")
thanks.
but the situation is quite strange: if O(n) is the best we can possibly get, why hesitate to do so? After all, O(n) should be considered as “very good” for most algorithms…
now, using `count` is still O(n) (and with some overhead in the anonymous function). We just cannot avoid it…
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### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [August 6, 2021, 1:49pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/4 "2021-08-06T13:49:23Z")
I don’t think O(n) is very good for something like `length`, which is `O(1)` for vectors. The consequence of supporting, out of the box, `length` is that a lot of people will complain that `length(skipmissing(x))` is slow, and they would be right, when thinking about other `length` calls.
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### Author:  [@tomtom](https://discourse.julialang.org/u/tomtom)
#### Post date: [August 6, 2021, 1:51pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/5 "2021-08-06T13:51:12Z")
… but the point is there is **no way** to achieve O(1) for `length(skipmissing())`, in my understanding at least.
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### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [August 6, 2021, 1:55pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/6 "2021-08-06T13:55:21Z")
you want users to be aware they are doing this, where other wise they may be calling `length()` over and over like they would normally do.
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### Author:  [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)
#### Post date: [August 6, 2021, 1:59pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/7 "2021-08-06T13:59:20Z")
`skipmissing` is designed to be a very lightweight and fast iterator which skips missing values. It comes with a guarantee “every function in Base which supports `SkipMissing` will be fast”. That unfortunately means it won’t support all common functions.
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### Author:  [@cjdoris](https://discourse.julialang.org/u/cjdoris)
#### Post date: [August 6, 2021, 4:24pm UTC](https://discourse.julialang.org/t/length-skipmissing-not-implemented/65943/8 "2021-08-06T16:24:48Z")
Even more readable:
```julia
count(!ismissing, x)
```