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Comparisons in Julia are not automatically applied elementwise, unlike in MATLAB. In Julia, just add a dot:
julia> A = [1 2 3; 4 3 8; 12 2 5; 7 4 9];
julia> ind = A[:,1] .> 6
4-element BitVector:
0
0
1
1
julia> A[ind,:]
2×3 Matrix{Int64}:
12 2 5
7 4 9