# AbstractArray and Vector of AbstractArray as an input
**URL:** https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972
**Category:** New to Julia
**Tags:** question
**Created:** [May 7, 2024, 8:56pm UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972 "2024-05-07T20:56:27Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Kadii](https://discourse.julialang.org/u/Kadii)
#### Post date: [May 7, 2024, 8:56pm UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/1 "2024-05-07T20:56:27Z")
Hi,
I’m testing how the `AbstractArray` works as an input type declaration. When I defined two functions as follows, `test1` worked just fine but `test2`.
Is `test2` type declaration invalid? Or, the way to call the function is wrong?
```julia
julia> T = AbstractArray{T, 2} where T <: Union{Float32, Float64}
julia> test1(::T) = pritnln("I'm not a vector.")
julia> test2(::Vector{T}) = println("I'm a vector.")
julia> v = [1.0 2.0; 3.0 4.0]
julia> test1(v)
I'm not a vector.
julia> test2([v])
ERROR: MethodError: no method matching test2(::Vector{Matrix{Float64}})
Closest candidates are:
test2(::Vector{AbstractMatrix{T} where T<:Union{Float32, Float64}})
```
---
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [May 7, 2024, 9:06pm UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/2 "2024-05-07T21:06:20Z")
You need
```julia
test2(::Vector{<:T}) = println("I'm a vector.")
```
because the element type of `[v]` is not exactly `T`. Then
```julia
julia> test2([v])
I'm a vector.
```
---
### Author:  [@Kadii](https://discourse.julialang.org/u/Kadii)
#### Post date: [May 7, 2024, 9:27pm UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/3 "2024-05-07T21:27:04Z")
Thank you very much! It’s clear now. I confirmed that `test3` and `test5` work since `T1` is nontrivial but `T2` isn’t.
```julia
julia> T1 = Array{Float64, 2}
julia> T2 = Array{T, 2} where T <: Union(Float32, Float64}
julia> test3(::Vector{T1}) = println("Works!")
julia> test4(::Vector{T2}) = println("Not work")
julia> test5(::Vector{<:T2}) = println("Works!")
julia> test3([v])
Works!
julia> test4([v])
ERROR...
julia> test5([v])
Works!
```
---
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [May 7, 2024, 9:43pm UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/4 "2024-05-07T21:43:47Z")
You can also explicitly specify the element type of a vector. For example, `T2[v]` has element type `T2`, and `test4(T2[v])` works. (However, a non-concrete element type is usually not a good idea.)
BTW, I had to change the definition of `T2` to
```julia
T2 = Array{T, 2} where T <: Union{Float32, Float64}
```
---
### Author:  [@Kadii](https://discourse.julialang.org/u/Kadii)
#### Post date: [May 8, 2024, 3:07am UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/5 "2024-05-08T03:07:59Z")
Thanks. I correct my code. I’ve also tried this.
```julia
julia> T2 = Array{T, 2} where T <: Union{Float32, Float64}
julia> test4(::Vector{T2}) = println("Not work")
julia> v = [1.0 2.0; 3.0 4.0]
julia> vv = Vector{T2}(undef, 0)
julia> for n in 1:3
push!(vv, v)
end
julia> test4(vv)
Not work
```
---
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [May 8, 2024, 11:26am UTC](https://discourse.julialang.org/t/abstractarray-and-vector-of-abstractarray-as-an-input/113972/6 "2024-05-08T11:26:47Z")
The easiest way to create an empty vector of type `T2` is to say `T2[]`. In any case, if one creates a vector by adding elements to an empty vector, then it’s important to specify the element type, as you did. The empty list `[]` has element type `Any`, and that would lead to very slow code.