Accessing values in a matrix for a column before the first?

General Usage
,
1

I am trying to access elements of the previous columns than first column.

For example in python if i try to access elements before the first column 0 it will return an empty array.

import numpy as np
x = np.matrix([[ 1,  2, 10],
 [ 1,  4, 2],
 [ 2,  3,  2],
 [ 2,  3,  6]])
a = x[:, 0]
print('\n a is \n', a)
b = x[:, :0]
print('\n b is \n',b)

Output: -->

 a is 
 [[1]
 [1]
 [2]
 [2]]

 b is 
 []

But julia does not behave in such manner.

My trial in julia:

array = [1 2 3;3 4 5;2 3 4;1 2 3]
a = array[:, 1]
b = array[:, 1-1]

Output :-->

BoundsError: attempt to access 4×3 Array{Int64,2} at index [1:4, 0]

Stacktrace:
 [1] throw_boundserror(::Array{Int64,2}, ::Tuple{Base.Slice{Base.OneTo{Int64}},Int64}) at .\abstractarray.jl:541
 [2] checkbounds at .\abstractarray.jl:506 [inlined]
 [3] _getindex at .\multidimensional.jl:742 [inlined]
 [4] getindex(::Array{Int64,2}, ::Function, ::Int64) at .\abstractarray.jl:1060
 [5] top-level scope at In[1166]:1
 [6] include_string(::Function, ::Module, ::String, ::String) at .\loading.jl:1091

However, to replicate such operation I tried to implement try and catch method and use it over the loop to see what happens when i try accessing different columns. The output is always an empty array.

for i in 1:3
    try
       b = array[:, :i-1] 
    catch 
        b = []
    end
    println("\b b in $i is $b")
end

Output :--->

 b in 1 is Any[]
 b in 2 is Any[]
 b in 3 is Any[]

May I know how can I access row elements from first_column - 1 to last_column - 1, where first_column - 1 returns an empty array but rest returns data from the matrix.

2

Is this what you want?

julia> array = [1 2 3;3 4 5;2 3 4;1 2 3]
4×3 Matrix{Int64}:
 1  2  3
 3  4  5
 2  3  4
 1  2  3

julia> array[:, 1:0]
4×0 Matrix{Int64}
2 Likes
3

Thanks @ettersi , yes this is amazing.

4

@ettersi, the empty column output seems to be just the result of indexing with an empty array and not with a previous non-existing column index.
Empty columns are thus produced by:

array = [1 2 3;3 4 5;2 3 4;1 2 3]

array[:, 1:0]
array[:, 2:1]
array[:, []]

# above empty indexing always produces:
4×0 Matrix{Int64}
1 Like
5

If understanding this correctly, the following might be closer to OP’s question?

array = [1 2 3;3 4 5;2 3 4;1 2 3]
ix = 1:size(array,2)

i = 2
array[:, (i-1 in ix) ? i-1 : []]
4-element Vector{Int64}:
 1
 3
 2
 1

i = 1
array[:, (i-1 in ix) ? i-1 : []]
4×0 Matrix{Int64}
1 Like