Different ways to create a vector of strings in Julia

[Charles Novaes de Santana]

Dear all,

I was trying to create a vector of strings in Julia and I didn't understand why the following ways give me different results.

Everything is fine If I try the following two approaches:

names = String[];

push!(names,"word1");

names = Array(String,0);

push!(names,"word1");

However, I supposed this other way should work too, but it didn't:

names = Array{String};

push!(names,"word2");

It gives me the following error: "ERROR: `push!` has no method matching push!(::Type{Array{String,N}}, ::ASCIIString)"

Why is String[] and Array(String,0) different from Array{String}?

Thanks for any help! 

Best,

Charles

-- 

Um axé! :)

--
Charles Novaes de Santana, PhD
http://www.imedea.uib-csic.es/~charles

[Tamas Papp]

Hi Charles,

Array{String} is a type, not an array:

julia> isa(Array{String}, Type)
true

julia> typeof(Array{String})
DataType

See http://docs.julialang.org/en/release-0.3/manual/types/ .

The function Array(eltype, dimensions...) can be used to instantiate an
object of this type, which is the convention in Julia, so IMO you should
just keep using that.

Best,

Tamas

[Mauro]

> However, I supposed this other way should work too, but it didn't:
> names = Array{String};
> push!(names,"word2");
>
> It gives me the following error: "ERROR: `push!` has no method matching
> push!(::Type{Array{String,N}}, ::ASCIIString)"
>
> Why is String[] and Array(String,0) different from Array{String}?

`Array{String}` is a type (abstract at that), but you want an instance
of that type. So, what you would want to do is `Array{String,1}()`.
That ought to construct a array of strings with dimension 1 but doesn't.
At least that is what most constructors do, e.g. `Dict{Int,String}()`.
However, there are no array constructors which look like this, you
construct them with the syntax you used in the first two examples:

> names = String[];
> push!(names,"word1");

> names = Array(String,0);
> push!(names,"word1");

Have a look at the Construtors section of the manual

[Ivar Nesje]

So, what you would want to do is `Array{String,1}()`.
That ought to construct a array of strings with dimension 1 but doesn't.

But in 0.4 you can use Array{String,1}(0) to create a 1d array with 0 elements. Note that you have to provide the size of the array, and 0 is not default (, but maybe it should be?)

[Tamas Papp]

Array{T}(dims...)

is equivalent to

Array{T,length(dims)}(dims...)

while the latter has a redundant parameter. So instead of providing a
default for dims..., wouldn't

Array{T}(0)

be an idiomatic solution for creating an empty vector of eltype T? Has
one less character than

Array{T,1}(0)

:D

Best,

Tamas

[Charles Novaes de Santana]

Thank you all for the explanation! We don't stop learning in this list!

Best,

Charles

[Ismael VC]

@Tammas Array{T}(0) works in 0.4-dev, but I think that's why there is also a Vector{T} typealias for Array{T,1}, so it could be Vector{T}(0) which is slightly better IMHO.

julia> Vector{Int}(0)

0-element Array{Int64,1}

julia> Array{Int,1}(0)

0-element Array{Int64,1}

julia> Array{Int}(0)

0-element Array{Int64,1}