Tim,
Isn't there an inefficiency here though? If my immutable had thirty fields
and I wanted to change the second field for each instance in an array of 1M
elements, I would have to read/write the whole array instead of just
writing one out of every thirty elements.
Cédric
On Tue, May 10, 2016 at 9:42 PM, Tim Holy <tim.h...@gmail.com> wrote:
> The right way to think about it is that an immutable is like a number:
> you're
> used to being able to have an array of Float64 and replace those, right?
> You're not redefining the meaning of "5.2", you're overwriting the value
> stored
> in that slot. Arrays of immutables work exactly the same way: the
> container is
> not immutable, but the individual values stored in it are. That doesn't
> prevent you from replacing them.
>
> Best,
> --Tim
>
> On Tuesday, May 10, 2016 08:43:04 PM Boylan, Ross wrote:
> > But if I make an array of immutables I won't be able to change them
> > afterwards. Though I can replace them.
> >
> > With the old code except for
> >
> > immutable Stuff
> > a::Int
> > b::Int
> > end
> >
> > I now have
> > julia> v=Array(TT.Stuff, 2)
> > 2-element Array{TT.Stuff,1}:
> > TT.Stuff(140124021836848,140124000460928)
> > TT.Stuff(140124048730192,0)
> >
> > julia> v[1].a=44 # what I'd like to do
> > ERROR: type Stuff is immutable
> >
> > julia> v[1]=TT.Stuff(3, 4) # work-around
> > TT.Stuff(3,4)
> >
> > !julia> v
> > 2-element Array{TT.Stuff,1}:
> > TT.Stuff(3,4)
> > TT.Stuff(140124048730192,0)
> >
> > With immutable, is everything in the array laid out contiguous in memory?
> > It seems sort of odd that the type is immutable yet I can overwrite data
> of
> > that type. I suppose that would only be prohibited if the array were
> > immutable.
> >
> > Thanks for the tip about using Type to get the zero to work.
> > Ross
> >
> > ________________________________
> > From: julia-users@googlegroups.com [julia-users@googlegroups.com] on
> behalf
> > of Cedric St-Jean [cedric.stj...@gmail.com] Sent: Tuesday, May 10, 2016
> > 1:20 PM
> > To: julia-users
> > Subject: [julia-users] Re: newbie questions (was undefined reference
> error)
> >
> > "normal" types are by definition heap-allocated, and are always
> manipulated
> > them through pointers. What you want is
> > immutables<
> http://docs.julialang.org/en/release-0.4/manual/types/#immutable
> > -composite-types>
> >
> > immutable Stuff
> > a::Int
> > b::Int
> > end
> >
> > # Also, for zeros to work,
> >
> > function zero(x::Type{Stuff})
> > Stuff(0, 0)
> > end
> >
> >
> > On Tuesday, May 10, 2016 at 4:04:37 PM UTC-4, Boylan, Ross wrote:
> > I'm puzzled that a type consisting only of 2 integers doesn't qualify
> as
> > "bitstype". Further experiment shows that the array seems to be an array
> > of references, I don't know how to implement zero, and generally that
> I'm a
> > bit lost :) My goal is to get a densely packed array of data. I assume
> > that will use less memory and generate faster code; if not, maybe I
> should
> > change my goal.
> >
> > BTW, my real use has a type more heterogeneous than 2 Int's, so a
> solution
> > that uses a 2D array doesn't really generalize appropriately for me.
> >
> > module TT
> > import Base.zero
> >
> > type Stuff
> > a::Int
> > b::Int
> > end
> >
> > function zero(x::Stuff)
> > Stuff(0, 0)
> > end
> >
> > end
> >
> > julia> v=Array(TT.Stuff, 3) #as before
> > 3-element Array{TT.Stuff,1}:
> > #undef
> > #undef
> > #undef
> >
> > julia> s=TT.Stuff(3, 5)
> > TT.Stuff(3,5)
> >
> > julia> v=fill(s, 2)
> > 2-element Array{TT.Stuff,1}:
> > TT.Stuff(3,5)
> > TT.Stuff(3,5)
> >
> > julia> s.a=900
> > 900
> >
> > julia> v ###OOPS: every array element points to the same instance
> > 2-element Array{TT.Stuff,1}:
> > TT.Stuff(900,5)
> > TT.Stuff(900,5)
> >
> > julia> zero(TT.Stuff) # This is probably what needs to work for
> zeros() to
> > work ERROR: MethodError: `zero` has no method matching
> > zero(::Type{TT.Stuff})
> >
> > !julia> zero(TT.Stuff(1, 1)) # this at least calls the right c'tor
> > TT.Stuff(0,0)
> >
> > Ross
> > ________________________________________
> > From: julia...@googlegroups.com<UrlBlockedError.aspx>
> > [julia...@googlegroups.com<UrlBlockedError.aspx>] on behalf of Lutfullah
> > Tomak [tomak...@gmail.com<UrlBlockedError.aspx>] Sent: Tuesday, May 10,
> > 2016 12:04 AM
> > To: julia-users
> > Subject: [julia-users] undefined reference error
> >
> > You need to initialize array entries if you don't have eltype as
> bitstype.
> > Here, undefined reference means you had not initialize the entry. And,
> full
> > type assignment works because it initializes the entry.
>
>
