Suppose we have a composite type:
mutable struct MyType{TF<:AbstractFloat, TI<:Integer}
a::TF
b::TF
end
We define a constructor
function MyType(a; b = 1.0)
return MyType(a, b)
end
I can broadcast MyType over an array of a's, but how can I do that for b's?
I tried to do
MyType.([1.0, 2.0, 3.0]; [:b, 1.0, :b, 2.0, :b, 3.0,])
But, this does not work.
Note that the above example is totally artificial. In reality, I have a composite type that takes in many fields, many of which are constructed using keyword arguments, and I only want to change a few of them into different values stored in an array.
I don't think you can do this with dot-notation, however, you can manually construct the broadcast call:
julia> struct Foo
a::Int
b::Int
Foo(a; b = 1) = new(a, b)
end
julia> broadcast((x, y) -> Foo(x, b = y), [1,2,3], [4,5,6])
3-element Array{Foo,1}:
Foo(1, 4)
Foo(2, 5)
Foo(3, 6)
julia> broadcast((x, y) -> Foo(x; y), [1,2,3], [:b=>4,:b=>5,:b=>6])
3-element Array{Foo,1}:
Foo(1, 4)
Foo(2, 5)
Foo(3, 6)
Related
I'm learning Haskell and have some problems with list comprehension.
If I define a function to get a list of the divisors of a given number, I get an error.
check n = [x | x <- [1..(floor (n/2))], mod n x == 0]
I don't get why it's causing an error. If I want to generate a list from 1 to n/2 I can do it with [1..(floor (n/2))], but not if I do it in the list comprehension.
I tried another way but I get also an error (in this code I want to get all so called "perfect numbers")
f n = [1..(floor (n/2))]
main = print $ filter (\t -> foldr (+) 0 (f t) == t) [2..100]
Usually it is better to start writing a signature. While signatures are often not required, it makes it easier to debug a single function.
The signature of your check function is:
check :: (RealFrac a, Integral a) => a -> [a]
The type of input (and output) a thus needs to be both a RealFrac and an Integral. While technically speaking we can make such type, it does not make much sense.
The reason this happens is because of the use of mod :: Integral a => a -> a -> a this requires x and n to be both of the same type, and a should be a member of the Integral typeclass.
Another problem is the use of n/2, since (/) :: Fractional a => a -> a -> a requires that n and 2 have the same type as n / 2, and n should also be of a type that is a member of Fractional. To make matters even worse, we use floor :: (RealFrac a, Integral b) => a -> b which enforces that n (and thus x as well) have a type that is a member of the RealFrac typeclass.
We can prevent the Fractional and RealFrac type constaints by making use of div :: Integral a => a -> a -> a instead. Since mod already required n to have a type that is a member of the Integral typeclass, this thus will not restrict the types further:
check n = [x | x <- [1 .. div n 2], mod n x == 0]
This for example prints:
Prelude> print (check 5)
[1]
Prelude> print (check 17)
[1]
Prelude> print (check 18)
[1,2,3,6,9]
I would like to create a function that deals with missing values. However, when I tried to specify the missing type Array{Missing, 1}, it errors.
function f(x::Array{<:Number, 1})
# do something complicated
println("no missings.")
println(sum(x))
end
function f(x::Array{Missing, 1})
x = collect(skipmissing(x))
# do something complicated
println("removed missings.")
f(x)
end
f([2, 3, 5])
f([2, 3, 5, missing])
I understand that my type is not Missing but Array{Union{Missing, Int64},1}
When I specify this type, it works in the case above. However, I would like to work with all types (strings, floats etc., not only Int64).
I tried
function f(x::Array{Missing, 1})
...
end
But it errors again... Saying that
f (generic function with 1 method)
ERROR: LoadError: MethodError: no method matching f(::Array{Union{Missing, Int64},1})
Closest candidates are:
f(::Array{Any,1}) at ...
How can I say that I wand the type to be union missings with whatever?
EDIT (reformulation)
Let's have these 4 vectors and two functions dealing with strings and numbers.
x1 = [1, 2, 3]
x2 = [1, 2, 3, missing]
x3 = ["1", "2", "3"]
x4 = ["1", "2", "3", missing]
function f(x::Array{<:Number,1})
println(sum(x))
end
function f(x::Array{String,1})
println(join(x))
end
f(x) doesn't work for x2 and x3, because they are of type Array{Union{Missing, Int64},1} and Array{Union{Missing, String},1}, respectively.
It is possible to have only one function that detects whether the vector contains missings, removes them and then deals appropriately with it.
for instance:
function f(x::Array{Any, 1})
x = collect(skipmissing(x))
print("removed missings")
f(x)
end
But this doesn't work because Any indicates a mixed type (e.g., strings and nums) and does not mean string OR numbers or whatever.
EDIT 2 Partial fix
This works:
function f(x::Array)
x = collect(skipmissing(x))
print("removed missings")
f(x)
end
[But how, then, to specify the shape (number of dimensions) of the array...? (this might be an unrelated topic though)]
You can do it in the following way:
function f(x::Vector{<:Number})
# do something complicated
println("no missings.")
println(sum(x))
end
function f(x::Vector{Union{Missing,T}}) where {T<:Number}
x = collect(skipmissing(x))
# do something complicated
println("removed missings.")
f(x)
end
and now it works:
julia> f([2, 3, 5])
no missings.
10
julia> f([2, 3, 5, missing])
removed missings.
no missings.
10
EDIT:
I will try to answer the questions raised (if I miss something please add a comment).
First Vector{Union{Missing, <:Number}} is the same as Vector{Union{Missing, Number}} because of the scoping rules as tibL indicated as Vector{Union{Missing, <:Number}} translates to Array{Union{Missing, T} where T<:Number,1} and where clause is inside Array.
Second (here I am not sure if this is what you want). I understand you want the following behavior:
julia> g(x::Array{>:Missing,1}) = "$(eltype(x)) allows missing"
g (generic function with 2 methods)
julia> g(x::Array{T,1}) where T = "$(eltype(x)) does not allow missing"
g (generic function with 2 methods)
julia> g([1,2,3])
"Int64 does not allow missing"
julia> g([1,2,missing])
"Union{Missing, Int64} allows missing"
julia> g(["a",'a'])
"Any allows missing"
julia> g(Union{String,Char}["a",'a'])
"Union{Char, String} does not allow missing"
Note the last two line - although ["a", 'a'] does not contain missing the array has Any element type so it might contain missing. The last case excludes it.
Also you can see that you could change the second parameter of Array{T,N} to something else to get a different dimensionality.
Also this example works because the first method, as more specific, catches all cases that allow Missing and a second method, as more general, catches what is left (i.e. essentially what does not allow Missing).
I'm having an issue with type in functions, I've managed to write the minimal code that explains the problem:
immutable Inner{B<:Real, C<:Real}
a::B
c::C
end
immutable Outer{T}
a::T
end
function g(a::Outer{Inner})
println("Naaa")
end
inner = Inner(1, 1)
outer = Outer(inner)
g(outer)
Will lead to the method error
MethodError: no method matching g(::Outer{Inner{Int64,Int64}})
So basically, I don't want to have to say what the types of Inner are, I just want the function to make sure that it's an Outer{Inner} and not Outer{Float64} or something.
Any help would be appreciated
The type Inner{Int64,Int64} is a concrete Inner type and it is not a subtype of
Inner{Real, Real}, since different concrete types of Inner (Int64 or Float64)
can have different representations in memory.
According to the documentation, function g should be defined as:
function g(a::Outer{<:Inner})
println("Naaa")
end
so it can accept all arguments of type Inner.
Some examples, after define g with <::
# -- With Float32 --
julia> innerf32 = Inner(1.0f0, 1.0f0)
Inner{Float32,Float32}(1.0f0, 1.0f0)
julia> outerf32 = Outer(innerf32)
Outer{Inner{Float32,Float32}}(Inner{Float32,Float32}(1.0f0, 1.0f0))
julia> g(outerf32)
Naaa
# -- With Float64 --
julia> innerf64 = Inner(1.0, 1.0)
Inner{Float64,Float64}(1.0, 1.0)
julia> outerf64 = Outer(innerf64)
Outer{Inner{Float64,Float64}}(Inner{Float64,Float64}(1.0, 1.0))
julia> g(outerf64)
Naaa
# -- With Int64 --
julia> inneri64 = Inner(1, 1)
Inner{Int64,Int64}(1, 1)
julia> outeri64 = Outer(inneri64)
Outer{Inner{Int64,Int64}}(Inner{Int64,Int64}(1, 1))
julia> g(outeri64)
Naaa
More details at the documentation: Parametric Composite Type
Update: The way to declare an immutable composite type (as in the original question), have changed to:
struct Inner{B<:Real, C<:Real}
a::B
c::C
end
struct Outer{T}
a::T
end
Furthermore, function g could be declared with a parametric type:
function g(a::T) where T Outer{<:Inner}
println(a)
println(a.a)
println(a.c)
end
And hence, there is no need to create an instance of Outer before calling the function.
julia> ft64 = Inner(1.1, 2.2)
Inner{Float64,Float64}(1.1, 2.2)
julia> g(ft64)
Inner{Float64,Float64}(1.1, 2.2)
1.1
2.2
julia> i64 = Inner(3, 4)
Inner{Int64,Int64}(3, 4)
julia> g(i64)
Inner{Int64,Int64}(3, 4)
3
4
I'm working my way through Dave's upcoming book on Elixir, and in one exercise I would like to dynamically construct a function reference to Kernel.+/2, Kernel.-/2 etc, based on the contents of one character of a string, '+', '-' and so on.
Based on another SO question I expected to be able to call apply/3 passing Kernel, :+ and the two numbers like this:
apply(Kernel, :+, [5, 7])
This doesn't work because (if I understand right) Kernel.+/2 is a macro, not a function. I looked up the source code, and + is defined in terms of __op__, and I can call it from iex:
__op__(:+, 5, 7)
This works until I put the :+ into a variable:
iex(17)> h = list_to_atom('+')
:+
iex(18)> __op__(h, 5, 7)
** (CompileError) iex:18: undefined function __op__/3
src/elixir.erl:151: :elixir.quoted_to_erl/3
src/elixir.erl:134: :elixir.eval_forms/4
And I'm guessing there's no way to call __op__ using apply/3.
Of course, the brute-force method gets the job done.
defp _fn(?+), do: &Kernel.+/2
defp _fn(?-), do: &Kernel.-/2
defp _fn(?*), do: &Kernel.*/2
# defp _fn(?/), do: &Kernel.//2 # Nope, guess again
defp _fn(?/), do: &div/2 # or &(&1 / &2) or ("#{div &1, &2} remainder #{rem &1, &2}")
But is there something more concise and dynamic?
José Valim nailed it with his answer below. Here's the code in context:
def calculate(str) do
{x, op, y} = _parse(str, {0, :op, 0})
apply :erlang, list_to_atom(op), [x, y]
end
defp _parse([] , acc ) , do: acc
defp _parse([h | t], {a, b, c}) when h in ?0..?9, do: _parse(t, {a, b, c * 10 + h - ?0})
defp _parse([h | t], {_, _, c}) when h in '+-*/', do: _parse(t, {c, [h], 0})
defp _parse([_ | t], acc ) , do: _parse(t, acc)
You can just use the Erlang one:
apply :erlang, :+, [1,2]
We are aware this is confusing and we are studying ways to make it or more explicit or more transparent.
UPDATE: Since Elixir 1.0, you can dispatch directly to Kernel (apply Kernel, :+, [1, 2]) or even use the syntax the OP first attempted (&Kernel.+/2).
val m = scala.collection.mutable.Map[String, Int]()
// this doesn't work
m += ("foo", 2)
// this does work
m += (("foo", 2))
// this works too
val barpair = ("bar", 3)
m += barpair
So what's the deal with m += ("foo" , 2) not working? Scala gives the type error:
error: type mismatch;
found : java.lang.String("foo")
required: (String, Int)
m += ("foo", 2)
^
Apparently Scala thinks that I am trying to call += with two arguments, instead of one tuple argument. Why? Isn't it unambiguous, since I am not using m.+= ?
Unfortunately a b (c, d, e, ..) desugars to a.b(c, d, e, ..). Hence the error.
Isn't it unambiguous, since I am not using m.+= ?
No, it isn't, because parenthesis can always be used when there are multiple arguments. For example:
List(1, 2, 3) mkString ("<", ", ", ">")
So you might ask, what multiple parameters? Well, the Scala API doc is your friend (or mine, at least), so I present you with:
scala> val m = scala.collection.mutable.Map[String, Int]()
m: scala.collection.mutable.Map[String,Int] = Map()
scala> m += (("foo", 2), ("bar", 3))
res0: m.type = Map(bar -> 3, foo -> 2)
In other words, += takes a vararg.
The preferred way to define map entries is to use the -> method. Like
m += ("foo" -> 2)
Which constructs a tuple. a -> b gets desugared to a.->(b). Every object in scala has a -> method.