CODEWITHSUNDEEP
arraysindexingjulia
coolsv
coolsv

Reputation: 781

Accessing elements of array based on indices given by another array

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.

Upvotes: 0

Views: 188

Answers (1)

phipsgabler
phipsgabler

Reputation: 20950

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.

Upvotes: 2

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