There is no way to delete columns here and there and in general end up with
a valid dense matrix.
If you want a view on some of the columns of the matrix you can use slice.
julia> x = reshape(1:100, (10,10));
julia> slice(x, :, 1:3:10)
10x4 SubArray{Int64,2,Array{Int64,2},Tuple{Colon,StepRange{Int64,Int64}},1}:
1 31 61 91
2 32 62 92
3 33 63 93
4 34 64 94
5 35 65 95
6 36 66 96
7 37 67 97
8 38 68 98
9 39 69 99
10 40 70 100
If you want a new "first order" matrix then I don't think you will get
something better than
x = x[:, 1:3:10]
What's your use case?
On Thursday, October 22, 2015 at 10:48:21 AM UTC+2, Glen O wrote:
>
> The reason why it's not working is that you're re-assigning x, rather than
> editing it. By running "x=x[....]", you're creating a copy of x with the
> appropriate column removed, and then assigning x to point to the copy,
> rather than to the original array.
>
> If you wanted to edit an array's values, you'd do something like
> "x[:]=x[end:-1:1]" to reverse them in-place - this creates a reversed copy,
> and then copies the values from that reversed copy into the original array.
> Unfortunately, for removing columns or values, it's not quite so simple,
> because you have to deal with changing the size of the matrix.
>
> To get a feel for what is involved, have a look at this:
>
> https://github.com/JuliaLang/julia/blob/master/base/array.jl
>
> That file has the definitions of the functions that you're trying to
> mimic. As you'll see, they end up using "ccall" functions, which are used
> to call C and Fortran libraries. I believe these are necessary if you wish
> to alter the dimensions of an array in-place.
>
> On Thursday, 22 October 2015 09:55:55 UTC+10, Thuener Silva wrote:
>>
>> I want to change a matrix inside a function. Example( just to illustrate):
>>
>> julia> x = ones(2,3)
>> 2x3 Array{Float64,2}:
>> 1.0 1.0 1.0
>> 1.0 1.0 1.0
>>
>> julia> function deletecolumns(x,i)
>> x = x[:,1:size(x,2) .!= i]
>> end
>> deletecolumns (generic function with 1 method)
>>
>> julia> deletecolumns(x,1)
>> 2x2 Array{Float64,2}:
>> 1.0 1.0
>> 1.0 1.0
>>
>>
>>
>> julia> x
>> 2x3 Array{Float64,2}:
>>
>> 1.0 1.0 1.0
>>
>> 1.0 1.0 1.0
>>
>> The best way is to do "x = deletecolumns(x,1)" ? I want to make something
>> more like delete!(x,1).
>>
>>
>> Thanks in advance,
>> Thuener Silva
>>
>
