Le mardi 07 avril 2015 à 10:56 -0700, David Gold a écrit : > Comprehensions may be what you are looking for: > http://docs.julialang.org/en/latest/manual/arrays/#comprehensions. > > For your example, > > julia> v = [1, 2, 3, 4] # Set the vector v > 4-element Array{Int64,1}: > 1 > 2 > 3 > 4 > > julia> A = [ j.*v[i] for j in 1:4, i in 1:length(v) ] # Create a 2d > array A where A[i, j] is given by j times the ith element of v > 4x4 Array{Any,2}: > 1 2 3 4 > 2 4 6 8 > 3 6 9 12 > 4 8 12 16 > > > > > I believe that, by default, arrays created using comprehensions are of > type Any. Actually, comprehensions use type inference to choose the type of the array. But it doesn't work well when non-const global variables are involved. If you declare v as const you get an Array{Int}. You could also wrap this inside a function and make v an argument.
Anyway, even inside functions, you're safer specifying the element type. For details, see https://github.com/JuliaLang/julia/issues/7258 Regards > If you want an array of a specific type -- say, integer in this case > -- you can do > > A = Int[ j.*v[i] for j in 1:4, i in 1:length(v) ] > 4x4 Array{Int64,2}: > 1 2 3 4 > 2 4 6 8 > 3 6 9 12 > 4 8 12 16 > > > > Is this what you're looking for? > > On Saturday, April 4, 2015 at 11:04:26 AM UTC-4, Tamas Papp wrote: > Hi, > > Suppose I have a function that maps an atom of type T, eg > Float64, into > Vector{T}, and takes another argument n that determines its > length. > > What is the idiomatic/fast way of collecting the values in the > columns > of a matrix? > > Currently I am using this: > > @doc """Map elements of `x` into columns of a matrix using > `f`. > Result is assumed to have the same element type as `x`.""" -> > function maptocols{T}(f,x::Vector{T},n) > k = length(x) > b = Array(T, n, k) > for j = 1:k > b[:,j] = f(x[j],n) > end > b > end > > Eg > > julia> maptocols((x,n) -> x.*[1:n;], [1,2,3,4], 4) > 4x4 Array{Int64,2}: > 1 2 3 4 > 2 4 6 8 > 3 6 9 12 > 4 8 12 16 > > which is OK but of course I would prefer a one-liner if there > is one > provided by the language (could not find it though). > > Best, > > Tamas
