mpnum | Matrix Product Representation library for Python | Machine Learning library

You created an Array with your MPData{T} struct as its element type and allocated memory for TotalNumMP elements, but no actual instances of the struct were created. That's called an uninitialized array, indicated by the undef.

If the struct was immutable and uses no pointers (isbitstype), you could still index the array, but all the values would be random garbage.

However, you had a mutable struct which necessarily uses a pointer, so it is not an isbitstype. In that case, Julia does not allow you to access that element as if there were an instance there (MPDataArray[MPNum]), because otherwise you would have a dangling pointer, something so dangerous they won't even let you have it as random garbage. The only thing you're allowed to do with an uninitialized element is assign a proper instance: