In the Julia manual metaprogramming section, we see the following example for how to use interpolation in an expression:
julia> ex = :(a in $:((1,2,3)) )
:($(Expr(:in, :a, :((1,2,3)))))
Given that I can create another expression
julia> ex1 = :(a in (1,2,3) )
:($(Expr(:in, :a, :((1,2,3)))))
julia> ex == ex1
true
Is there any reason for the extra $:(...) in the manual's example? Or is it just to indicate possibilities of this construction -- that $ and :(...) are "inverses"?
Related
I am creating a non-standard string literal with a macro with something like that:
macro R13_str(p)
rotate(13, p)
end
and it works. I can call it as:
R13"abc"
But I would like to declare the macro to work with any integer, like:
R1"abc"
or
R244"abc"
Let's say the function rotate() is:
function rotate(shift_amount::Int64, s::String)
# Ensure the shift is no bigger than the string
shift = shift_amount ≤ length(s) ? shift_amount : shift_amount % length(s)
# Circular shift
return s[end-shift+1:end] * s[1:end-shift]
end
How can I do that? I have checked all the docs, but it's not clear to me.
Can't see how to achieve exactly what is required. But the following might be good enough:
julia> macro R_str(p,flag)
rotate(flag, p)
end
#R_str (macro with 1 method)
julia> R"hello"3
"llohe"
julia> R"abc"1
"cab"
julia> R"abc"244
"cab"
See https://docs.julialang.org/en/v1/manual/metaprogramming/#meta-non-standard-string-literals
Trying to conform to OP call format:
julia> macro rework(expr)
if expr.head != :macrocall return expr ; end
r = String(expr.args[1])
rr = parse(Int, r[3:findfirst('_',r)-1])
:(rotate($rr, $(expr.args[3])))
end
#rework (macro with 1 method)
julia> #rework R13"hello"
"llohe"
This macro could help to read the prepared test cases??
I have found a solution:
for n in 0:244
#eval macro $(Symbol(:R, n, :_str))(s)
rotate($n, s)
end
end
While I believe that using flags is the better approach, with this for loop I can generate all the macros that I need.
Julia> R1"abc"
Julia> R24"acb"
Julia> R56"abc"
simply work.
For example in the code below, x is defining the domain, but why is there the double dot between 0 and 4pi?
using ApproxFun
x=Fun(identity,0..4π)
.. is an operator (like e.g. +) but it does not have a default definition. You can define it to to whatever you want:
julia> ..(a, b) = println(a, ", ", b)
.. (generic function with 1 method)
julia> "hello" .. "world"
hello, world
The Julia package IntervalArithmetic uses it to construct an interval, e.g.
julia> using IntervalArithmetic
julia> 4..5
[4, 5]
julia> typeof(4..5)
Interval{Float64}
and I suspect this is what it is used for in your code example.
.. is not part of Julia, rather part of the packages used by ApproxFun.
It is used to represent intervals, see the code below
julia> u = 1..3
1..3
julia> dump(u)
Interval{:closed,:closed,Int64}
left: Int64 1
right: Int64 3
So this is just a convenience constructor for the Interval object, see:
julia> 1..3 === Interval{:closed,:closed,Int64}(1,3)
true
I would like to form expressions like
julia> ex = :(a[$i,$j] + b[$i,$j])
because I am interested in writting a program that it is able to write a system of equations in which the user only gives the structure of them and then that julia writes explicitly the chunk of code involved. More generally, I would like to embed functions in a expression, something like
julia> ex = :(a[$myfunc(i,j),$j] + b[$i,$j])
Upon evaluating the first line of code, I get
ERROR: UndefVarError: i not defined
in eval(::Module, ::Any) at ./boot.jl:226
On the other hand, if I write
julia> ex = :(a[i,j] + b[i,j])
:(a[i,j] + b[i,j])
julia> for i in 1:2
for j in 1:2
println(eval(ex))
end
end
ERROR: UndefVarError: i not defined
in eval(::Module, ::Any) at ./boot.jl:226
[inlined code] from ./boot.jl:225
in anonymous at ./<no file>:4294967295
in eval(::Module, ::Any) at ./boot.jl:226
How is interpolation done in this case?
eval works at global scope, so it cannot reference any local variables. What you can do, however, is interpolate that expression into a larger chunk of code that's intended to be executed at the global scope:
julia> a = [1 2; 3 4]
b = [5 6; 7 8]
ex = :(a[i,j] + b[i,j]);
julia> #eval for i in 1:2
for j in 1:2
println($ex)
end
end
6
8
10
12
The #eval macro is a simple shorthand for quoting the passed syntax and passing it along to the eval() function. It'd probably even be better to define a custom function:
julia> #eval function f()
for i in 1:2
for j in 1:2
println($ex)
end
end
end
f (generic function with 1 method)
julia> f()
6
8
10
12
The usual caveats about eval apply: any passed code can be maliciously crafted to do nefarious or surprising things. Or just crash the program.
Hello i trying create converter method from Disct to Vector in Julia language.
But i receive error, with i can't understand
ERROR: TypeError: Tuple: in parameter, expected Type{T}, got Dict{AbstractString,Int64}
My code
type Family
name::UTF8String
value::Int
end
function convertToVector(a1::Dict{AbstractString, Int64}())
A::Vector{Node}
for k in sort(collect(keys(a1)))
push!(A, Family(a1[k] , k))
end
return A
end
Any idea hot to change convertToVector method ?
There were several typos in the above code, but I think this should work:
# No () after the type of a1
# Also, see comment, better to parameterize function, use concrete type for Dict
function convertToVector{T<:AbstractString}(a1::Dict{T, Int64})
# This is how you create an empty vector to hold Family objects
A = Vector{Family}()
for k in sort(collect(keys(a1)))
# The values passed to the Family constructor were backwards
push!(A, Family(k, a1[k]))
end
A
end
Another way (probably not very quick):
julia> dict = Dict("fred" => 3, "jim" => 4)
Dict{ASCIIString,Int64} with 2 entries:
"fred" => 3
"jim" => 4
julia> Vector{Family}(map(f -> Family(f...), map(x -> collect(x), dict)))
2-element Array{Family,1}:
Family("fred",3)
Family("jim",4)
Perhaps I've been using too much Lisp recently...
I have a function, f. I want to add a method that takes any container of Strings. For example, I want to write a method that generates the following when needed:
f(xs::Array{String, 1}) = ...
f(xs::DataArray{String, 1}) = ...
f(xs::ITERABLE{String}) = ...
Is this possible to do in Julia's type system? Right now, I'm using a macro to write a specialized method when I need it.
#make_f(Array{String, 1})
#make_f(DataArray{String, 1})
This keeps things DRY, but it feels...wrong.
Can't you just use duck typing? I.e., just assume that you're feeding the function an object of the right type and throw an error if at some point e.g. you don't have a string in your iterable.
This should improve once you can really talk about iterables using traits; currently there is no iterable type. Scott's answer, for example, will not work with a tuple of strings, even though that is iterable.
E.g.
julia> f(x) = string(x...) # just concatenate the strings
f (generic function with 1 method)
julia> f(("a", "á"))
"aá"
julia> f(["a", "á"])
"aá"
julia> f(["a" "b"; "c" "d"]) # a matrix of strings!
"acbd"
At least in Julia 0.4, the following should work:
julia> abstract Iterable{T} <: AbstractVector{T}
julia> f{T<:Union{Vector{String},Iterable{String}}}(xs::T) = 1
f (generic function with 1 method)
julia> x = String["a", "é"]
2-element Array{AbstractString,1}:
"a"
"é"
julia> f(x)
1