intuitive and more concise than a for loop. Thanks Dan.
On Sunday, December 20, 2015 at 1:30:43 AM UTC-8, Dan wrote:
>
> Is the following red expression a good Julia equivalent:
>
>
> julia> a = [ -1 2; 3 -4 ]
>
> ⋮
>
> julia> y = [ 1, 2 ]
>
> ⋮
>
> julia> [a[i,y[i]] for i=1:2]
> 2-element Array{Any,1}:
> -1
> -4
>
> (it's also fast)
>
> On Sunday, December 20, 2015 at 2:44:20 AM UTC+2, Lex wrote:
>>
>> I am expecting [-1 -4] in the Julia example.
>>
>> On Saturday, December 19, 2015 at 4:42:07 PM UTC-8, Lex wrote:
>>>
>>> Hi
>>>
>>> In Python, I am able to select values like this:
>>>
>>> >>> a = np.matrix([[-1,2],[3,-4]])
>>> >>> y = np.matrix([1,0])
>>> >>> a[range(2),y]
>>> matrix([[2, 3]])
>>> >>>
>>>
>>> Is there any equivalent in Julia or using a loop is the only way?
>>>
>>> julia> a
>>> 2x2 Array{Int64,2}:
>>> -1 2
>>> 3 -4
>>>
>>> julia> y
>>> 1x2 Array{Int64,2}:
>>> 1 2
>>>
>>> julia> a[:, y]
>>> ERROR: MethodError: `index_shape_dim` has no method matching
>>> index_shape_dim(::Array{Int64,2}, ::Int64, ::Array{Int64,2})
>>>
>>> You might have used a 2d row vector where a 1d column vector was
>>> required.
>>> Note the difference between 1d column vector [1,2,3] and 2d row vector
>>> [1 2 3].
>>> You can convert to a column vector with the vec() function.
>>> Closest candidates are:
>>> index_shape_dim(::Any, ::Any, ::Real...)
>>> index_shape_dim(::Any, ::Any, ::Colon)
>>> index_shape_dim(::Any, ::Any, ::Colon, ::Any, ::Any...)
>>> ...
>>> in getindex at abstractarray.jl:488
>>>
>>> julia> a[:, y[]]
>>> 2-element Array{Int64,1}:
>>> -1
>>> 3
>>>
>>> julia> a[:, y[:]]
>>> 2x2 Array{Int64,2}:
>>> -1 2
>>> 3 -4
>>>
>>> julia>
>>>
>>>
>>>
