Accessing elements of array based on indices given by another array

Question

Suppose I have the following 4x4 Array in Julia:

julia> A=rand(4,4)
4×4 Array{Float64,2}:
 0.00624508  0.624399  0.458094  0.341848
 0.303817    0.269487  0.580949  0.534655
 0.748525    0.419411  0.469938  0.659914
 0.730659    0.191461  0.996144  0.74772

And I have another 6x2 Array where each row represents a row-column pair:

julia> B=[ 1 1; 1 3; 2 2; 2 4; 3 1; 3 3]
6×2 Array{Int64,2}:
 1  1
 1  3
 2  2
 2  4
 3  1
 3  3

The first row of B represents the element [1,1] of A, the second row of B represents the element [1,3] of A, and so on. I want to access the elements of A based on the coordinates given by each row of B. In R for example, the command A[B] gives exactly what I want: 0.00624508 0.458094 0.269487 0.534655 0.748525 0.469938 but in Julia, the same command gives

julia> A[B]
6×2 Array{Float64,2}:
 0.00624508  0.00624508
 0.00624508  0.748525
 0.303817    0.303817
 0.303817    0.730659
 0.748525    0.00624508
 0.748525    0.748525

and this is not what I want. Is there a similar way of coding A[B] in Julia, so that I obtain what I obtain in R? Must be applicable to any Array.

phipsgabler · Accepted Answer · 2019-03-23 17:47:55Z

2

Construct an array of CartesianIndex out of your index pairs:

julia> A[CartesianIndex.(B[:, 1], B[:, 2])]
6-element Array{Float64,1}:
 0.987200021334854    
 0.5261639427155012   
 0.8430528192705655   
 0.5300778835366697   
 0.5044387593056074   
 0.0025132074927423087

This is necessary to distinguish such "point indexing" from "shape indexing", as you observed it (I made those terms up).

Ideally, you would already construct B as such an array, but that is not always feasible, of course.

answered Mar 23, 2019 at 17:47

phipsgabler

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1 Comment

coolsv Over a year ago

Oh I see! The secret was the CartesianIndex ! I was reading the Julia webpage concerning indexing and saw it but didn't see how I could use it. Thank you!

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