Creating an empty 2D array and append values to it

[Fred]

Hi !

The questions I have is so trivial that I spend a long time trying to find an example on the doc and on the web but I did not manage to do exactly what I want :

I want to create an empty 2D array  :  the function readdlm create exactly the kind of array I want if I read a CSV file with 2 columns. For example :

julia> m
1913x2 Array{Float64,2}:
  201.07     6597.95    
  202.092   17835.7      
  202.944    9667.59    
  204.015   11983.4      
  204.811    7586.52    
  205.803       1.1506
....

I tried to create exactly the same type of array and append values to it :

julia> a = Array{Float64,2}
Array{Float64,2}


push!(a, 10.0,3.0)
ERROR: MethodError: `push!` has no method matching push!(::Type{Array{Float64,2}}, ::Float64)
Closest candidates are:
  push!(::Any, ::Any, ::Any)
  push!(::Any, ::Any, ::Any, ::Any...)
  push!(::Array{Any,1}, ::ANY)


 push!(a, [10.0,3.0])
ERROR: MethodError: `push!` has no method matching push!(::Type{Array{Float64,2}}, ::Array{Float64,1})
Closest candidates are:
  push!(::Any, ::Any, ::Any)
  push!(::Any, ::Any, ::Any, ::Any...)
  push!(::Array{Any,1}, ::ANY)


append!(a, [10.0,3.0])
ERROR: MethodError: `append!` has no method matching append!(::Type{Array{Float64,2}}, ::Array{Float64,1})
Closest candidates are:
  append!{T}(::Array{T,1}, ::AbstractArray{T,1})
  append!{T}(::StatsBase.AbstractHistogram{T,1,E}, ::AbstractArray{T,1})
  append!{T}(::StatsBase.AbstractHistogram{T,1,E}, ::AbstractArray{T,1}, ::StatsBase.WeightVec{W,Vec<:AbstractArray{T<:Real,1}})
  ...

etc... I tried every possible syntax to append values to an empty 2D array and I did not find the solution and the correct syntax :)

Thank you for your help !

[Yichao Yu]

It's not possible without creating a new one.

You can create a 1d one and resize it.

[Fred]

Thank you very much Yichao ! This explain all the problems I have... But it still unclear :

I cannot create an empty 1D array and resize it :

julia> b = Array[]
0-element Array{Array{T,N},1}


julia> b = Array[]
0-element Array{Array{T,N},1}


julia> reshape(b,2,2)
ERROR: DimensionMismatch("new dimensions (2,2) must be consistent with array size 0")
 in reshape at array.jl:146
 in reshape at abstractarray.jl:215

if I create a non empty 2D array I still have problems to append values to it :

julia> b = [1 2; 3 4]
2x2 Array{Int64,2}:
 1  2
 3  4


julia> push!(b,[5,6])
ERROR: MethodError: `push!` has no method matching push!(::Array{Int64,2}, ::Array{Int64,1})


julia> append!(b,[5,6])
ERROR: MethodError: `append!` has no method matching append!(::Array{Int64,2}, ::Array{Int64,1})

even if possible, the values 1234 used to create the array will create mistakes in my calculations ;)

The only possibility I found is to create a non empty 1D array :

julia> c = [1 2 3 4]
1x4 Array{Int64,2}:
 1  2  3  4

An then reshape it :

julia> reshape(c,2,2)
2x2 Array{Int64,2}:
 1  3
 2  4

But that means, that the array c cannot grow in size using push!() , is that true ?

[Tim Holy]

Note that in `a = Array{Float64,2}`, `a` is a *type*, not an *instance*. You
presumably mean `a = Array(Float64, 0, 0)`.

But Yichao is right that you can't grow a 2d array, only a 1d one.

Best,
--Tim

[Fred]

Thank you very much Tim !

In fact I want to create an X,Y array so if I create a 1D array, I can only append to it (x1,y1) then (x2,y2)... (xn, yn), because I calculate x1 before x2...

julia> d = ["x1", "y1", "x2", "y2", "x3", "y3"]

6-element Array{ASCIIString,1}:

 "x1"

 "y1"

 "x2"

 "y2"

 "x3"

 "y3"

julia> reshape(d,3,2)

3x2 Array{ASCIIString,2}:

 "x1"  "y2"

 "y1"  "x3"

 "x2"  "y3"

you see the problem ? instead I would like to have :

x1 y1

x2 y2

..

xn yn 

because I want to be able to work on columns and line ... of course another easy solution is to use dataframes, but I tried with arrays because the code should be faster... :)

[Matt Bauman]

It'd probably be fastest if you can pre-allocate your array:

A = Array(ASCIIString, 3, 2)

for i=1:size(A, 1)

    A[i, 1] = string('w'+i, 1)

    A[i, 2] = string('w'+i, 2)

end

If you don't know how many elements you'll have, you can use two column vectors, `c1` and `c2`, and push to them independently.  Then horizontally concatenate them together to get your resulting `A = [c1 c2]`.

[Yichao Yu]

On Tue, Apr 12, 2016 at 10:58 AM, Matt Bauman <mba...@gmail.com> wrote:
> It'd probably be fastest if you can pre-allocate your array:
>
> A = Array(ASCIIString, 3, 2)
> for i=1:size(A, 1)
> A[i, 1] = string('w'+i, 1)
> A[i, 2] = string('w'+i, 2)
> end
>
> If you don't know how many elements you'll have, you can use two column
> vectors, `c1` and `c2`, and push to them independently. Then horizontally
> concatenate them together to get your resulting `A = [c1 c2]`.

I was assuming that the growing is in the continuous direction (i.e.
doesn't require moving all previous elements around) since this is the
case allowing growing a 2D array might make some sense.

If it is not the case then Matt's solution is much better.

[Fred]

Thank you very much Matt. In fact I will work with floats, but the idea to concatenate 2 columns is good !

[Fred]

Thank you ! I think I will use this solution :

julia> a = []
0-element Array{Any,1}

julia> b = []
0-element Array{Any,1}

julia> push!(a, 1)
1-element Array{Any,1}:
 1

julia> push!(a, 2)
2-element Array{Any,1}:
 1
 2

julia> a
4-element Array{Any,1}:
 1
 2
 3
 4

julia> b
4-element Array{Any,1}:
 10
 20
 30
 40

julia> C = [a b]
4x2 Array{Any,2}:
 1  10
 2  20
 3  30
 4  40

[Greg Plowman]

julia> reshape(d,3,2)

3x2 Array{ASCIIString,2}:

 "x1"  "y2"

 "y1"  "x3"

 "x2"  "y3"

This is because of Julia's column-major ordering.

you see the problem ? instead I would like to have :

x1 y1

x2 y2

..

xn yn 

In this case, you could use:

julia> reshape(d,2,3)'
3x2 Array{ASCIIString,2}:

 "x1"  "y1"
 "x2"  "y2"
 "x3"  "y3"