# Extracting Array element type for subsequent use
**URL:** https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840
**Category:** General Usage
**Tags:** question
**Created:** [November 2, 2017, 6:17am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840 "2017-11-02T06:17:39Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@purplishrock](https://discourse.julialang.org/u/purplishrock)
#### Post date: [November 2, 2017, 6:17am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/1 "2017-11-02T06:17:39Z")
I want to do something like:
```julia
function foo(x::Array{T,1}, y::Array{T,1})
n1 = length(x)
n2 = length(y)
z = zeros(T, n1+n2)
...
end
```
First problem is that I actually need to write
```julia
function foo(x::Array{T,1} where T, y::Array{T,1} where T)
...
```
But then the subsequent use of T gives me an UndefVarError.
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [November 2, 2017, 6:38am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/2 "2017-11-02T06:38:58Z")
Use:
```julia
function foo(x::Array{T,1}, y::Array{T,1}) where T
...
```
---
### Author:  [@xiaodai](https://discourse.julialang.org/u/xiaodai)
#### Post date: [November 2, 2017, 6:39am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/3 "2017-11-02T06:39:00Z")
```julia
function foo{T}(x::Array{T,1}, y::Array{T,1})
```
i think you meant the above .
what you wrote will give you a different T for every value in the arrays I think.
---
### Author:  [@purplishrock](https://discourse.julialang.org/u/purplishrock)
#### Post date: [November 2, 2017, 4:12pm UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/4 "2017-11-02T16:12:03Z")
That did it, Thank you !
```julia
function foo{T}(x::Array{T,1}, y::Array{T,1}) zeros(T,length(x)+length(y)) end
foo (generic function with 1 method)
```
julia\> foo(rand(10), rand(5))
15-element Array{Float64,1}:
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
julia\>
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [November 2, 2017, 4:25pm UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/5 "2017-11-02T16:25:48Z")
> [@xiaodai](#):
>
> `function foo{T}(x::Array{T,1}, y::Array{T,1})`
This syntax is going to be removed. Use the `where` version instead.
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [November 2, 2017, 9:04pm UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/6 "2017-11-02T21:04:02Z")
That’s the old syntax, @fredrikekre was correct for v0.6 and later.
That doesn’t not mean that the elements of the Vectors can have a different type, that would be:
```
function foo(x::Vector{Any}, y::Vector{Any})
```
The following means that both x and y have to have the same element type (which can be `Any`)
```
function foo(x::Vector{T}, y::Vector{T}) where T
```
---
### Author:  [@Stephen\_Vavasis](https://discourse.julialang.org/u/Stephen_Vavasis)
#### Post date: [November 2, 2017, 11:58pm UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/7 "2017-11-02T23:58:16Z")
I’ll also point out: if you declare a function like this: `function foo(x::Vector{T} where T)`, you can still obtain the element type of `x` in the function body using the `eltype()` function even though you can’t get it from `T`.
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [November 3, 2017, 12:54am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/8 "2017-11-03T00:54:24Z")
Yes, and there’s even a nice short form:
`function foo(x::Vector{<:Any})`
---
### Author:  [@NaOH](https://discourse.julialang.org/u/NaOH)
#### Post date: [November 3, 2017, 2:27am UTC](https://discourse.julialang.org/t/extracting-array-element-type-for-subsequent-use/6840/9 "2017-11-03T02:27:27Z")
The signature should be defined as: `foo(x::Vector{T}) where {T}` instead.
Notice you are indeed able to get the type of the `Vector` from `T` as well as from `eltype(x)`, ie.:
```julia
julia> function foo(x::Vector{T}) where {T}
@show(T, T == eltype(x))
return nothing
end
foo (generic function with 1 method)
julia> a = rand(5);
julia> b = rand(Int, 5);
julia> foo(a)
T = Float64
T == eltype(x) = true
julia> foo(b)
T = Int64
T == eltype(x) = true
```