# When function shouldn't return anything

**URL:** <https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836>\
**Category:** General Usage\
**Tags:** question\
**Created:** [July 13, 2017, 12:12pm UTC](https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836 "2017-07-13T12:12:21Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Saran\_S](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/saran_s/32/1155_2.png) [@Saran\_S](https://discourse.julialang.org/u/Saran_S)\
**Post date:** [July 13, 2017, 12:12pm UTC](https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836/1 "2017-07-13T12:12:21Z")

</div>

Below is the code which I am working on. I have a Composite type called NearestNeighbor which has arrays to be passed on to processing.

```julia
type NearestNeighbor
  Xrows::Array{Float64,2}
  Yrows::Vector{Float64}

  function NearestNeighbor()
    new(Array{Float64,2}(0,0), Vector{Float64}(0))
  end
end

```

nn=NearestNeighbor()

```julia
function trainModel{Float64}(nn::NearestNeighbor,Xvals::SharedArray{Float64,2},Yvals::Vector{Float64})
  nn.Xrows=Xvals
  nn.Yrows=Yvals
end

```

The above trainModel just assigns training data, it does not have to return anything. And I have predictModel function which uses nn.Xrows and nn.Yrows for further processing.  
Should I return **`nothing or return or none`** in the trainModel function or  
Kindly let me know what is right way to go about this issue

Thank You.

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**Author:** ![bernhard](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bernhard/32/2619_2.png) [@bernhard](https://discourse.julialang.org/u/bernhard)\
**Post date:** [July 13, 2017, 12:33pm UTC](https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836/2 "2017-07-13T12:33:56Z")

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1. I am not sure if you need such a function, when you can define a constructor (see example below) which initializes your data.  
Does this achieve what you want to do?
2. I have no experience with SharedArrays, but Xval seems to be a different type than nn.Xrows. I do not know whether your function will work given the type of Xrows.
3. if it ‘does not have to return anyhting’ you can put ‘return nothing’ on the last line (see below)
4. It is customary to use ! (at the end of the function name) for Julia functions that modify arguments (like `push!([1,2,3],99)` (in case you want to keep your `trainModel` function I would rename it to `trainModel!`)

hope that helps

```julia
type NearestNeighbor
  Xrows::Array{Float64,2}
  Yrows::Vector{Float64}

  function NearestNeighbor()
    new(Array{Float64,2}(0,0), Vector{Float64}(0))
  end
  function NearestNeighbor(a,b)
    new(a,b)
  end
end

x=rand(2,9)
y=rand(9)

my_nn=NearestNeighbor(x,y) #no need to call trainModel function

function trainModel!{Float64}(nn::NearestNeighbor,Xvals::SharedArray{Float64,2},Yvals::Vector{Float64})
  nn.Xrows=Xvals
  nn.Yrows=Yvals
  return nothing
end

```

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<div class="post-metadata">

**Author:** ![MA\_Laforge](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ma_laforge/32/385_2.png) [@MA\_Laforge](https://discourse.julialang.org/u/MA_Laforge)\
**Post date:** [July 13, 2017, 12:50pm UTC](https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836/3 "2017-07-13T12:50:24Z")

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I am guessing you are asking this because languages like C/C++ with a void return type do not return a value when the return statement is omitted.

Julia seems to inherit from the Lisp programming languages when it comes to return values:  
 → It returns whatever value that results from the last statement in your function (in your case, that means it returns `Yvals`).

I personally find it cleaner to return `nothing` in this case - but I do tend to forget to do so coming from a C/C++ background. I find that not returning `nothing` causes potential confusion for the function’s user.

There are 3 (equally valid) ways I might use to do this. All 3 return `nothing`:

### Method 1

```julia-auto
function myfunc()
    #do something
    return
end

```

### Method 2

```julia-auto
function myfunc()
    #do something
    return nothing
end

```

### Method 3

```julia-auto
function myfunc()
    #do something
    nothing
end

```

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<div class="post-metadata">

**Author:** ![Saran\_S](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/saran_s/32/1155_2.png) [@Saran\_S](https://discourse.julialang.org/u/Saran_S)\
**Post date:** [July 13, 2017, 1:00pm UTC](https://discourse.julialang.org/t/when-function-shouldnt-return-anything/4836/4 "2017-07-13T13:00:56Z")

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@bernhard Thank you very much for suggesting to use constructor itself as that approach seems better and in pointing out the convention of using ! cheers.