First let me start with a recommendation, what to do.
It is best to create your matrices in general in the following way:
julia> Array{Union{Nothing,String},2}(nothing,5,5)
5×5 Matrix{Union{Nothing, String}}:
nothing nothing nothing nothing nothing
nothing nothing nothing nothing nothing
nothing nothing nothing nothing nothing
nothing nothing nothing nothing nothing
nothing nothing nothing nothing nothing
In this way you ensure that they are properly initialized.
Now to explain what you observe. #undef as opposed to nothing is not a value. It means that the given cell in a matrix is not connected to any value. You cannot read from such a cell. You must first write to it before you can read it:
julia> x = Vector{String}(undef, 3)
3-element Vector{String}:
#undef
#undef
#undef
julia> x[1]
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex(A::Vector{String}, i1::Int64)
@ Base .\array.jl:801
[2] top-level scope
@ REPL[6]:1
julia> x[1] = "a"
"a"
julia> x
3-element Vector{String}:
"a"
#undef
#undef
julia> x[1]
"a"
You might ask why for the case of Union{Bool, Nothing} element type you get a value in an array, while in the case of Union{String, Nothing} you do get #undef, i.e. no value that you can read.
The answer is that in Julia there are two kinds of types:
- bits type, for which
isbitstype function returns true; such data are immutable and do not contain references to other values; an example of such data is Bool
- non-bits type, which either are mutable, or contain references; an example of such data is
String (as technically string is represented as a reference to some location in memory where the contents of the string is stored)
As you can see here:
julia> isbitstype(Bool)
true
julia> isbitstype(String)
false
Now - if your array is to store a bits type (or their union) then it stores it directly, so there is always some value (there is no guarantee what value it would be but you know you will get a value), e.g.:
julia> Matrix{Int}(undef, 5, 5)
5×5 Matrix{Int64}:
260255120 260384864 260235344 260254240 0
260254240 260235344 260235344 260255120 0
261849744 260235344 260235344 260235344 0
260465792 260465440 260464224 260235344 0
260235344 260235344 260235344 260235344 0
and as you can see we have some values stored in it, but it is undefined what the values would be.
On the other hand if your array is to store a non-bits type it actually stores references to the values. Which means that when you create an array of such values without initializing it you get #undef - which means that in this cell there is no reference to a valid value.
Just to show you that it has nothing to do directly with strings, let me show you e.g. the String7 type (it is exported e.g. by CSV.jl), which is a fixed width string (maximum width of 7 bytes) that is a bits type. Observe the difference:
julia> using CSV
julia> isbitstype(String7)
true
julia> Matrix{String7}(undef, 5, 5)
5×5 Matrix{String7}:
"\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" … "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0"
"\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0"
"\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0"
"\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0"
"\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0" "\0\0\0\0\x0f\x86_\x10\0\0\0\0\0\0\0\0"
julia> Matrix{String}(undef, 5, 5)
5×5 Matrix{String}:
#undef #undef #undef #undef #undef
#undef #undef #undef #undef #undef
#undef #undef #undef #undef #undef
#undef #undef #undef #undef #undef
#undef #undef #undef #undef #undef
In the first case you got a matrix of String7 values that is initialized, but the values it is initialized to are undefined (it is some garbage).
In the second case you got a matrix of String values, and since they are non-bits the matrix is uninitialized - it does not hold any values yet. You first have to assign some values before you will be able to read them.
Finally there is a isassigned function that allows you to check if the container has a value associated with some index (i.e. to check if it is not #undef or the index is out of bounds). Here is an example:
julia> x = Vector{String}(undef, 3)
3-element Vector{String}:
#undef
#undef
#undef
julia> x[1] = "a"
"a"
julia> isassigned(x, 1) # we have a value here
true
julia> isassigned(x, 2) # no value
false
julia> isassigned(x, 3) # no value
false
julia> isassigned(x, 4) # out of bounds
false
If anything is unclear please comment and I can expand the answer.
