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eachindex

eachindex(A...)

Creates an iterable object for visiting each index of an AbstractArray A in an efficient manner. For array types that have opted into fast linear indexing (like Array), this is simply the range 1:length(A). For other array types, this returns a specialized Cartesian range to efficiently index into the array with indices specified for every dimension. For other iterables, including strings and dictionaries, this returns an iterator object supporting arbitrary index types (e.g. unevenly spaced or non-integer indices).

Example for a sparse 2-d array:

julia> A = sprand(2, 3, 0.5)
2x3 sparse matrix with 4 Float64 entries:
    [1, 1]  =  0.598888
    [1, 2]  =  0.0230247
    [1, 3]  =  0.486499
    [2, 3]  =  0.809041

julia> for iter in eachindex(A)
           @show iter.I_1, iter.I_2
           @show A[iter]
       end
(iter.I_1,iter.I_2) = (1,1)
A[iter] = 0.5988881393454597
(iter.I_1,iter.I_2) = (2,1)
A[iter] = 0.0
(iter.I_1,iter.I_2) = (1,2)
A[iter] = 0.02302469881746183
(iter.I_1,iter.I_2) = (2,2)
A[iter] = 0.0
(iter.I_1,iter.I_2) = (1,3)
A[iter] = 0.4864987874354343
(iter.I_1,iter.I_2) = (2,3)
A[iter] = 0.8090413606455655

If you supply more than one AbstractArray argument, eachindex will create an iterable object that is fast for all arguments (a UnitRange if all inputs have fast linear indexing, a CartesianRange otherwise). If the arrays have different sizes and/or dimensionalities, eachindex returns an iterable that spans the largest range along each dimension.

Examples

A = sprand(2, 3, 0.5)
for iter in eachindex(A)
    @show iter.I_1, iter.I_2
    @show A[iter]
end

This example demonstrates the usage of eachindex function with a sparse 2-dimensional array A. It iterates over each index of A efficiently.

Output:

(iter.I_1, iter.I_2) = (1, 1)
A[iter] = 0.5988881393454597
(iter.I_1, iter.I_2) = (2, 1)
A[iter] = 0.0
(iter.I_1, iter.I_2) = (1, 2)
A[iter] = 0.02302469881746183
(iter.I_1, iter.I_2) = (2, 2)
A[iter] = 0.0
(iter.I_1, iter.I_2) = (1, 3)
A[iter] = 0.4864987874354343
(iter.I_1, iter.I_2) = (2, 3)
A[iter] = 0.8090413606455655

In this example, eachindex provides an iterable object that allows efficient access to each index of the sparse matrix A along with its corresponding value. The iteration is performed with the for loop, and iter.I_1 and iter.I_2 represent the indices of the current element. A[iter] retrieves the value at the current index.

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