The use of "..." in julia, interesting!

For example, input = [["a","b","c"],[1,2,3],["e","f","g"],...], the length inside is not limited; Now we need to form an array format data with a tuple as an element for each element in it. The format inside does not destroy the format of the original element.

when input

input = [["a","b","c"],[1,2,3],["e","f","g"]]，

output

27-element Vector{Tuple{Char, Int64, String}}:
 ('a', 1, "e")
 ('a', 1, "f")
 ('a', 1, "g")
 ('a', 2, "e")
 ('a', 2, "f")
 ('a', 2, "g")
 ⋮
 ('c', 2, "f")
 ('c', 2, "g")
 ('c', 3, "e")
 ('c', 3, "f")
 ('c', 3, "g")

the code is as bellow:

function f(input)
    output =[]
    len = length(input)
    if len == 1
        return input
    elseif len ==0
        error("input is empty!")
    end
    for i = 1:len-1
        if i == 1
            output = g(input[1],input[i+1])
        else
            output = g(output,input[i+1])
        end
    end
    return output
end

function g(a,b)
    return [(i...,j) for i in a for j in b]
end

The above code is very general, but I think the usage of … is very good. If there is no …, it will become:

julia> f()
27-element Vector{Tuple{Tuple{String, Int64}, String}}:
 (("a", 1), "e")
 (("a", 1), "f")
 (("a", 1), "g")
 (("a", 2), "e")
 (("a", 2), "f")
 (("a", 2), "g")
 ⋮
 (("c", 2), "f")
 (("c", 2), "g")
 (("c", 3), "e")
 (("c", 3), "f")
 (("c", 3), "g")

Obviously, this is not what we want. So, because of the addition of... so little, the result is amazing!

Further, we make the input more complex and test the results:

julia>  arr_data = [["a","b","c"],[1,2,3],["e","f","g"],[1.0,2.0,5.0,6.0,7.0,8.0,9.0],["hello","world"],[["we"],["are"],["together!"]]]
6-element Vector{Vector{T} where T}:
 ["a", "b", "c"]
 [1, 2, 3]      
 ["e", "f", "g"]
 [1.0, 2.0, 5.0, 6.0, 7.0, 8.0, 9.0]
 ["hello", "world"]
 [["we"], ["are"], ["together!"]]

julia> f(arr_data)
1134-element Vector{Tuple{Char, Int64, String, Float64, String, Vector{String}}}:
 ('a', 1, "e", 1.0, "hello", ["we"])
 ('a', 1, "e", 1.0, "hello", ["are"])      
 ('a', 1, "e", 1.0, "hello", ["together!"])
 ('a', 1, "e", 1.0, "world", ["we"])       
 ('a', 1, "e", 1.0, "world", ["are"])      
 ('a', 1, "e", 1.0, "world", ["together!"])
 ('a', 1, "e", 2.0, "hello", ["we"])       
 ('a', 1, "e", 2.0, "hello", ["are"])      
 ('a', 1, "e", 2.0, "hello", ["together!"])
 ('a', 1, "e", 2.0, "world", ["we"])       
 ('a', 1, "e", 2.0, "world", ["are"])      
 ('a', 1, "e", 2.0, "world", ["together!"])
 ('a', 1, "e", 5.0, "hello", ["we"])       
 ⋮
 ('c', 3, "g", 7.0, "world", ["together!"])
 ('c', 3, "g", 8.0, "hello", ["we"])
 ('c', 3, "g", 8.0, "hello", ["are"])
 ('c', 3, "g", 8.0, "hello", ["together!"])
 ('c', 3, "g", 8.0, "world", ["we"])
 ('c', 3, "g", 8.0, "world", ["are"])
 ('c', 3, "g", 8.0, "world", ["together!"])
 ('c', 3, "g", 9.0, "hello", ["we"])
 ('c', 3, "g", 9.0, "hello", ["are"])
 ('c', 3, "g", 9.0, "hello", ["together!"])
 ('c', 3, "g", 9.0, "world", ["we"])
 ('c', 3, "g", 9.0, "world", ["are"])
 ('c', 3, "g", 9.0, "world", ["together!"])