# Plan FFTs both in julia 0.6.3 and 0.7

**URL:** <https://discourse.julialang.org/t/plan-ffts-both-in-julia-0-6-3-and-0-7/11728>\
**Category:** General Usage\
**Tags:** fftw\
**Created:** [June 17, 2018, 12:22am UTC](https://discourse.julialang.org/t/plan-ffts-both-in-julia-0-6-3-and-0-7/11728 "2018-06-17T00:22:30Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![navidcy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/navidcy/32/2750_2.png) [@navidcy](https://discourse.julialang.org/u/navidcy)\
**Post date:** [June 17, 2018, 12:22am UTC](https://discourse.julialang.org/t/plan-ffts-both-in-julia-0-6-3-and-0-7/11728/1 "2018-06-17T00:22:30Z")

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I’m trying to make my package (`FourierFlows.jl`) compatible with v0.7.0. I have some trouble planning the fft’s. I can plan the ffts and do in-place ffts in both 0.7.0 and 0.6.3. However, I don’t know how to code it up in a manner that would work in both versions. I looked up `Compat.jl` but I couldn’t find FFTW modules included in it.

I attach here two minimal examples of what I’m trying to do. Both examples work in the corresponding julia versions.

In julia 0.6.3

```julia
export AbstractGrid

abstract type AbstractGrid end

export OneDGrid

struct OneDGrid{T} <: AbstractGrid
  nx::Int
  nk::Int
  nkr::Int
  Lx::T
  dx::T
  x::Array{T,1}
  kr::Array{T,1}
  rfftplan::FFTW.rFFTWPlan{T,-1,false,1}
  irfftplan::Base.DFT.ScaledPlan{Complex{T},Base.DFT.FFTW.rFFTWPlan{Complex{T},1,false, 1},T}
end

function OneDGrid(nx, Lx; x0=-0.5*Lx)
  T = typeof(Lx)
  dx = Lx/nx
  x = Array{T}(linspace(x0, x0+Lx-dx, nx))

  nk = nx
  nkr = Int(nx/2+1)

  kr = Array{T}(2π/Lx*cat(1, 0:Int(nx/2)))

  rfftplan = plan_rfft(Array{T,1}(nx))
  irfftplan = plan_irfft(Array{Complex{T},1}(nkr), nx)

  OneDGrid(nx, nk, nkr, Lx, dx, x, kr, rfftplan, irfftplan)
end

n, L = 2^8, 2π
g = OneDGrid(n, L)

freal = rand(n)
fhr1 = rfft(freal)
fhr2 = deepcopy(0*fhr1)
A_mul_B!(fhr2, g.rfftplan, freal)
freal1 = irfft(fhr1, n)
freal2 = deepcopy(0*freal1)
A_mul_B!(freal2, g.irfftplan, fhr2)

println("tests for rfft")
println(vecnorm(fhr1-fhr2))
println(" ")
println("tests for irfft")
println(vecnorm(freal-freal1))
println(vecnorm(freal-freal2))

```

while in julia 0.7.0

```julia
using FFTW

if VERSION > v"0.6.3-"
    using LinearAlgebra
end

export AbstractGrid

abstract type AbstractGrid end

export OneDGrid

struct OneDGrid{T} <: AbstractGrid
  nx::Int
  nk::Int
  nkr::Int
  Lx::T
  dx::T
  x::Array{T,1}
  kr::Array{T,1}
  rfftplan::FFTW.rFFTWPlan{T,-1,false,1}
end

function OneDGrid(nx, Lx; x0=-0.5*Lx)
  T = typeof(Lx)
  dx = Lx/nx
  x = Array{T}(range(x0, stop=x0+Lx-dx, length=nx))

  nk = nx
  nkr = Int(nx/2+1)

  kr = Array{T}(2π/Lx*cat(0:Int(nx/2); dims = 1))

  rfftplan = plan_rfft(Array{Float64, 1}(undef, nx))

  OneDGrid(nx, nk, nkr, Lx, dx, x, kr, rfftplan)
end

n, L = 2^8, 2π
g = OneDGrid(n, L)

freal = rand(n)
fhr1 = rfft(freal)
fhr2 = deepcopy(0*fhr1)
mul!(fhr2, g.rfftplan, freal)
freal1 = irfft(fhr1, n)
freal2 = deepcopy(0*freal1)
ldiv!(freal2, g.rfftplan, fhr2)

println("tests for rfft")
println(vecnorm(fhr1-fhr2))
println(" ")
println("tests for irfft")
println(vecnorm(freal-freal1))
println(vecnorm(freal-freal2))

```

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<div class="post-metadata">

**Author:** ![favba](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/favba/32/2735_2.png) [@favba](https://discourse.julialang.org/u/favba)\
**Post date:** [June 17, 2018, 1:31am UTC](https://discourse.julialang.org/t/plan-ffts-both-in-julia-0-6-3-and-0-7/11728/2 "2018-06-17T01:31:02Z")

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You are having trouble with the inverse plan, right? This is how I did it, at line 17  
[https://github.com/favba/InplaceRealFFT.jl/blob/master/src/InplaceRealFFT.jl](https://github.com/favba/InplaceRealFFT.jl/blob/master/src/InplaceRealFFT.jl)

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<div class="post-metadata">

**Author:** ![navidcy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/navidcy/32/2750_2.png) [@navidcy](https://discourse.julialang.org/u/navidcy)\
**Post date:** [June 17, 2018, 4:04am UTC](https://discourse.julialang.org/t/plan-ffts-both-in-julia-0-6-3-and-0-7/11728/3 "2018-06-17T04:04:51Z")

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@favba

Actually no. I _did_ have some trouble with `irfft` plan in the beginning but then I figure out that the inverse plans are not needed! You can do forward fft using `mul!` (which is analogous to `A_mul_B!`) and inverse fft with `ldiv!`. (Have a look to the last part of my v0.7.0 code above.

My question is: How should I implement all these so that **_both_** julia v0.6.3 and v.0.7 work?