?eachindex

A = rand(3, 4)

for i in eachindex(A)
    @show i, A[i]
end

?CartesianIndex

?CartesianIndices

function f!(A::AbstractMatrix)
    for CI in CartesianIndices(A)
        i, j = CI.I
        @show CI, i, j, A[CI]
        A[CI] = i * j
    end
end

f!(A)

A