I have a very large matrix (with millions of rows) which obviously uses a lot of memory. Each of its entry it is either 0, 1 or 2. At the moment, the matrix has type Array{Int8,2} but 2 bits per entry would suffice, and would also reduce the memory allocation.

I tried to define a new type:

primitive type UInt2 <: Unsigned 2 end

but it throws an error. Is 8 bits is the minimum one can use?

Indeed. A `DiBitVector`

might be useful to you: DiBitVector Â· DataStructures.jl

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Thereâ€™s also `BitArray`

s: https://docs.julialang.org/en/v1/base/arrays/#Base.BitArray

edit: oops, I misread the OP, thought they just needed two values instead of three.

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Yes, 8 bits is the minimum one can use for a primitive type.

However, you can still implement bit-packed data structures like `BitArray`

or `DBitVector`

as linked above. The way these works is to have arrays of e.g. `UInt64`

â€śchunksâ€ť (64-bit quantities) under the hood, but to provide accessor functions (e.g. `getindex`

) that extract individual 1-bit or 2-bit elements from the bits of these â€śchunksâ€ť, respectively.

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This ended up being the best solution for me. I did not want to define the matrix using a vector of DiBiVectors, and I preferred defining a BitArray matrix with twice the number of columns, storing the 2 bits information I needed in each 1x2 block of the matrix

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