JULIA : How to permute randomly a vector in julia?

问题

l have a vector of random numbers that l want to permute randomly using randperm() function as follows but it's not working.

X=rand(100000) # a vector of 100000 random elements
Y=randperm(X) # want to permute randomly the vector x

the returned error is : ERROR: MethodError: no method matching randperm(::Array{Float64,1}) in eval(::Module, ::Any) at ./boot.jl:237

Thank you

回答1:

Based on the docs the randperm() accepts an integer n and gives a permutation of length n. You can use this ordering to then reorder your original vector:

julia> X = collect(1:5)
5-element Array{Int64,1}:
 1
 2
 3
 4
 5

julia> Y = X[randperm(length(X))]
5-element Array{Int64,1}:
 3
 4
 1
 2
 5

You can always check the docs by typing ?function_name in the REPL.

If your only goal is to randomly permute the vector, you can also use shuffle():

julia> shuffle(X)
5-element Array{Int64,1}:
 5
 4
 1
 2
 3

回答2:

To piggyback on the second point in the answer by @niczky12, if you want to randomly permute the vector X directly then it is actually more efficient to call shuffle!(X) instead of shuffle(X):

# precompile @time
@time 1+1

# create random vector
p = 10_000
X = collect(1:p)

# reproducible shuffles
srand(2016)

shuffle(X)
@time shuffle(X)

shuffle!(X)
@time shuffle!(X)

Output on my machine:

  0.000004 seconds (148 allocations: 10.151 KB)
  0.000331 seconds (6 allocations: 78.344 KB)
  0.000309 seconds (4 allocations: 160 bytes)

The call to shuffle! allocates substantially less memory (160 bytes v. 78 Kb) and thus will scale better with p.

来源：https://stackoverflow.com/questions/38010052/julia-how-to-permute-randomly-a-vector-in-julia

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julia

permute