# Casting \`Vector{Complex{T}}\` to \`Vector{T}\` of twice the size

**URL:** <https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689>\
**Category:** General Usage\
**Created:** [November 24, 2020, 3:24pm UTC](https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689 "2020-11-24T15:24:53Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jlbosse](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jlbosse/32/11274_2.png) [@jlbosse](https://discourse.julialang.org/u/jlbosse)\
**Post date:** [November 24, 2020, 3:24pm UTC](https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689/1 "2020-11-24T15:24:53Z")

</div>

I have an Vector of complex numbers and want to interpret it as a vector of real numbers with twice the length. Currently I allocate a new vector and then manually fill it, but I wonder if there is a faster way without reallocations since I suspect, that the data is already stored in exactly the desired way.

My current implementation:

```julia
function realize(v::AbstractVector{Complex{T}}) where {T}
    v_real = Array{T}(undef, 2 *length(v))
    @inbounds for i in eachindex(v)
        v_real[2i-1], v_real[2i] = real(v[i]), imag(v[i])
    end
    return v_real
end

```

Is there a way to get the same `v_real` by simple typecasting?

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<div class="post-metadata">

**Author:** ![fredrikekre](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fredrikekre/32/1688_2.png) [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)\
**Post date:** [November 24, 2020, 3:27pm UTC](https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689/2 "2020-11-24T15:27:08Z")

</div>

```julia
julia> x = rand(Complex{Float64}, 2)
2-element Array{Complex{Float64},1}:
 0.8199153266362134 + 0.6218323711392901im
 0.9962922034287771 + 0.8647751860682487im

julia> reinterpret(Float64, x)
4-element reinterpret(Float64, ::Array{Complex{Float64},1}):
 0.8199153266362134
 0.6218323711392901
 0.9962922034287771
 0.8647751860682487

```

---

<div class="post-metadata">

**Author:** ![jlbosse](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jlbosse/32/11274_2.png) [@jlbosse](https://discourse.julialang.org/u/jlbosse)\
**Post date:** [November 24, 2020, 3:36pm UTC](https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689/3 "2020-11-24T15:36:34Z")

</div>

Well, that is surprisingly straight forward 😃

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<div class="post-metadata">

**Author:** ![tim.holy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tim.holy/32/52_2.png) [@tim.holy](https://discourse.julialang.org/u/tim.holy)\
**Post date:** [November 24, 2020, 6:14pm UTC](https://discourse.julialang.org/t/casting-vector-complex-t-to-vector-t-of-twice-the-size/50689/4 "2020-11-24T18:14:49Z")

</div>

Starting with Julia 1.6, you’ll get considerably better performance on certain operations if you use `reinterpret(reshape, Float64, x)`. That’s available to earlier versions via `Compat.jl`, but it doesn’t help performance, just API compatibility.

```julia
julia> x = rand(Complex{Float64}, 3)
3-element Vector{ComplexF64}:
 0.3844083358603547 + 0.33079943531346534im
 0.6951482675152381 + 0.01509661529380657im
  0.517480198308429 + 0.21516809906601297im

julia> reinterpret(reshape, Float64, x)
2×3 reinterpret(reshape, Float64, ::Vector{ComplexF64}) with eltype Float64:
 0.384408 0.695148 0.51748
 0.330799 0.0150966 0.215168

```